Rebecca Martin/Sandbox1 - Revision history

Jaime Prilusky at 11:34, 17 March 2013

2013-03-17T11:34:23Z

←Older revision		Revision as of 11:34, 17 March 2013
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	:: TCR: Crystal Structure of the G17E/A52V/S54N/Q72H/E80V/L81S/T87S/G96V variant of the murine T cell receptor V beta 8.2 domain [[2apv]]		:: TCR: Crystal Structure of the G17E/A52V/S54N/Q72H/E80V/L81S/T87S/G96V variant of the murine T cell receptor V beta 8.2 domain [[2apv]]
	* V-type immunoglobulin examples		* V-type immunoglobulin examples
-	:: Crystal Structure of a Ligand-Binding Domain of the Human Polymeric Ig Receptor, pIgR [[~~1XED~~]]	+	:: Crystal Structure of a Ligand-Binding Domain of the Human Polymeric Ig Receptor, pIgR [[1xed]]
	:: Crystal structure of human FcaRI [[10vz]]		:: Crystal structure of human FcaRI [[10vz]]
	:: Influenza virus hemagglutinin complexed with a neutralizing antibody [[1QFU]]		:: Influenza virus hemagglutinin complexed with a neutralizing antibody [[1QFU]]

Rebecca Martin at 23:40, 1 May 2009

2009-05-01T23:40:04Z

←Older revision		Revision as of 23:40, 1 May 2009
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	'''Conclusions on Function'''		'''Conclusions on Function'''
	:The secretory component interacts with either dimeric IgA1 or IgA2 to form a '''functional unit''', structurally adapted to the harsh mucosal environment and to control potentially pathogenic mucosal flora primarily through neutralization. Inflammation is controlled by limiting the available binding sites on the Fc portion of IgA, effectively preventing FcαR clustering through a 1:1 stoichiometric binding. Differences in structure and resulting function allows the two isoforms fill unique niches in mucosal immune responses, suggesting selective advantages for each. Whereas IgA1 specializes in protein detection, IgA2 tends to bind LPS and polysaccharide antigen. So, structure arms the IgA secretory unit with specific advantages suited for its environmental, maintains balance between inflammation and mucosal barrier protection by limiting effector responses, and imparts unique functional roles to IgA isoforms. Together, structure and function determine the immune niches filled by IgA1 and IgA2.		:The secretory component interacts with either dimeric IgA1 or IgA2 to form a '''functional unit''', structurally adapted to the harsh mucosal environment and to control potentially pathogenic mucosal flora primarily through neutralization. Inflammation is controlled by limiting the available binding sites on the Fc portion of IgA, effectively preventing FcαR clustering through a 1:1 stoichiometric binding. Differences in structure and resulting function allows the two isoforms fill unique niches in mucosal immune responses, suggesting selective advantages for each. Whereas IgA1 specializes in protein detection, IgA2 tends to bind LPS and polysaccharide antigen. So, structure arms the IgA secretory unit with specific advantages suited for its environmental, maintains balance between inflammation and mucosal barrier protection by limiting effector responses, and imparts unique functional roles to IgA isoforms. Together, structure and function determine the immune niches filled by IgA1 and IgA2.
		+


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	:In other areas of science, studying mouse models of pathologies involving IgA1 introduces an added variable since IgA1 is found in higher apes only <ref name="ten"/>. Such complications in the experimental model must be taken into account when interpreting results.		:In other areas of science, studying mouse models of pathologies involving IgA1 introduces an added variable since IgA1 is found in higher apes only <ref name="ten"/>. Such complications in the experimental model must be taken into account when interpreting results.
		+
		+

	== Limitations of the Current Studies ==		== Limitations of the Current Studies ==
	:Because IgA has a high amount of glycosylation and a relatively large amount of flexibility, it has proven particularly difficult to crystallize in its intact form. Similarly, glycosylation and long linker regions between domains poses challenges to the crystallization of the secretory component. Alternative techniques employed in these studies included x-ray, neutron scattering analysis, analytical ultracentrifugation, and constrained modeling. Details provided in crystallographic studies – like disulfide bond, glycosylation residues and sites, detailed visualization of binding interaction – are absent in these results. Because of the limiting resolution of these models, many details concerning the binding residues and residue interactions are left unknown. Therefore, numerous questions are left unanswered, some of which are listed below. <ref name="eight"/>, <ref name="ten"/>, <ref name="five"/>, <ref name="seven" />		:Because IgA has a high amount of glycosylation and a relatively large amount of flexibility, it has proven particularly difficult to crystallize in its intact form. Similarly, glycosylation and long linker regions between domains poses challenges to the crystallization of the secretory component. Alternative techniques employed in these studies included x-ray, neutron scattering analysis, analytical ultracentrifugation, and constrained modeling. Details provided in crystallographic studies – like disulfide bond, glycosylation residues and sites, detailed visualization of binding interaction – are absent in these results. Because of the limiting resolution of these models, many details concerning the binding residues and residue interactions are left unknown. Therefore, numerous questions are left unanswered, some of which are listed below. <ref name="eight"/>, <ref name="ten"/>, <ref name="five"/>, <ref name="seven" />
		+
		+

	== Questions Unanswered (a few of many)==		== Questions Unanswered (a few of many)==
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	:What is the structure of IgA involved in IgA nephropathy? <ref name="eight"/>		:What is the structure of IgA involved in IgA nephropathy? <ref name="eight"/>
	:Crystallographic structure will yield further insights into the structure of IgA, the interactions between IgA and other molecules.		:Crystallographic structure will yield further insights into the structure of IgA, the interactions between IgA and other molecules.
		+

Rebecca Martin at 23:30, 1 May 2009

2009-05-01T23:30:27Z

(Difference between revisions)

Rebecca Martin: /* Implications in Science and Medicine */

2009-05-01T23:07:54Z

Implications in Science and Medicine

←Older revision		Revision as of 23:07, 1 May 2009
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	== Implications in Science and Medicine ==		== Implications in Science and Medicine ==
	[[Image:IgA_IFA.jpg\|thumb\|Immunofluorescence detecting IgA in IgA glomerulonephritis. From http://www.unckidneycenter.org/images/IgA_IFA.jpg.]]		[[Image:IgA_IFA.jpg\|thumb\|Immunofluorescence detecting IgA in IgA glomerulonephritis. From http://www.unckidneycenter.org/images/IgA_IFA.jpg.]]
-	:IgA nephropathy is the most prebvalent cause of chronic glomerulonephritis in the world and is caused by polymeric IgA1 deposited @ kidney glomeruli <ref name="eight"/>. Notably, 90% of serum IgA is IgA1, mostly in the monomeric form.The observation that individuals with IgA myeloma [http://en.wikipedia.org/wiki/Multiple_myeloma] lack nephropathy suggests an abnormality in IgA structure, leading to an abnormal amount of polymerization. Steric hindrance of the fab segments normally limits the amount of polymerization of IgA. Bonner, et al proposes that a disturbance in the hinge region or an absence of fab. Similarly, decreased O-glycosylation might could destabilize the hinge region, allowing IgA to self associate. Likewise, destabilizing this region might make IgA susceptable to cleavage of fab fragments by bacterial proteases, leading to self aggregation and renal pathology.	+	:IgA nephropathy is the most prebvalent cause of chronic glomerulonephritis in the world and is caused by polymeric IgA1 deposited @ kidney glomeruli <ref name="eight"/>. Notably, 90% of serum IgA is IgA1, mostly in the monomeric form.The observation that individuals with IgA myeloma [http://en.wikipedia.org/wiki/Multiple_myeloma] lack nephropathy suggests an abnormality in IgA structure, leading to an abnormal amount of polymerization. Steric hindrance of the fab segments normally limits the amount of polymerization of IgA. Bonner, et al proposes that a disturbance in the hinge region or an absence of fab. Similarly, decreased O-glycosylation might could destabilize the hinge region, allowing IgA to self associate. Likewise, destabilizing this region might make IgA susceptable to cleavage of fab fragments by bacterial proteases, leading to self aggregation and renal pathology. For more information on IgA nephropathy: [http://http://www.unckidneycenter.org/contact.html].

	:In other areas of science, studying mouse models of pathologies involving IgA1 introduces an added variable since IgA1 is found in higher apes only <ref name="ten"/>. Such complications in the experimental model must be taken into account when interpreting results.		:In other areas of science, studying mouse models of pathologies involving IgA1 introduces an added variable since IgA1 is found in higher apes only <ref name="ten"/>. Such complications in the experimental model must be taken into account when interpreting results.
-

	== Limitations of the Current Studies ==		== Limitations of the Current Studies ==

Rebecca Martin at 22:23, 1 May 2009

2009-05-01T22:23:46Z

←Older revision		Revision as of 22:23, 1 May 2009
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	<references />		<references />
		+
		+
		+	--[[User:Rebecca Martin\|Rebecca Martin]] 01:23, 2 May 2009 (IDT)

Rebecca Martin at 22:22, 1 May 2009

2009-05-01T22:22:45Z

Rebecca Martin at 22:20, 1 May 2009

2009-05-01T22:20:31Z

←Older revision		Revision as of 22:20, 1 May 2009
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	:Related structures		:Related structures
	::Proteins containing the classic immunoglobulin-like domain are found predominantly in the immune system <ref name="att" />. In fact, the antibody's closest related structires are those that recognize antigen: MHC and TCRs.		::Proteins containing the classic immunoglobulin-like domain are found predominantly in the immune system <ref name="att" />. In fact, the antibody's closest related structires are those that recognize antigen: MHC and TCRs.
-	::The V-type domain is found in a wider variety of proteins, including the Ig-binding molecules, such as the pIgR and the ~~FcalphaR~~ <ref name="att" />. Viral hemagluttinin is yet another example.	+	::The V-type domain is found in a wider variety of proteins, including the Ig-binding molecules, such as the pIgR and the FcαR <ref name="att" />. Viral hemagluttinin is yet another example.


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	:When IgA forms dimers, the Fc regions align end to end without overlap <ref name="eight"/>. The J chain lies within a fold in the bent Fc region. This conformation may allow the J chain access to the Secretory Component of the pIgR, which allows translocation across the mucosal epithelia to the luminal surface. Of note, in the image the J chains the J chains are extending from the dimer, which does not match with the described interaction of the J chain with the Fc portions of the anitbody (see Limitations of the Current Studies).		:When IgA forms dimers, the Fc regions align end to end without overlap <ref name="eight"/>. The J chain lies within a fold in the bent Fc region. This conformation may allow the J chain access to the Secretory Component of the pIgR, which allows translocation across the mucosal epithelia to the luminal surface. Of note, in the image the J chains the J chains are extending from the dimer, which does not match with the described interaction of the J chain with the Fc portions of the anitbody (see Limitations of the Current Studies).
-
	==Secretory Component==		==Secretory Component==
	:IgA is secreted as a dimer when it binds to the pIgR and is transported across the cell membrane <ref name ="ten" />. Upon IgA binding, the receptor-antibody complex is transocytosed to the lumenal side, where native proteases cleave the pIgR, releasing the secretory IgA (sIgA) into the lumen. The region of the pIgR that remains attached to the IgA upon pIgR cleavage is known as the <scene name='Rebecca_Martin/Sandbox1/Sc/1'>Secretory Component</scene>.		:IgA is secreted as a dimer when it binds to the pIgR and is transported across the cell membrane <ref name ="ten" />. Upon IgA binding, the receptor-antibody complex is transocytosed to the lumenal side, where native proteases cleave the pIgR, releasing the secretory IgA (sIgA) into the lumen. The region of the pIgR that remains attached to the IgA upon pIgR cleavage is known as the <scene name='Rebecca_Martin/Sandbox1/Sc/1'>Secretory Component</scene>.
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	'''Conclusions on Function'''		'''Conclusions on Function'''
	:The secretory component interacts with either dimeric IgA1 or IgA2 to form a '''functional unit''', structurally adapted to the harsh mucosal environment and to control potentially pathogenic mucosal flora primarily through neutralization. Inflammation is controlled by limiting the available binding sites on the Fc portion of IgA, effectively preventing Fcalpha clustering through a 1:1 stoichiometric binding. Differences in structure and resulting function allows the two isoforms fill unique niches in mucosal immune responses, suggesting selective advantages for each. Whereas IgA1 specializes in protein detection, IgA2 tends to bind LPS and polysaccharide antigen. So, structure arms the IgA secretory unit with specific advantages suited for its environmental, maintains balance between inflamation and mucosal barrier protection by limiting effector responses, and imparts unique functional roles to IgA isoforms. Together, structure and function determine the immune niches filled by IgA1 and IgA2.		:The secretory component interacts with either dimeric IgA1 or IgA2 to form a '''functional unit''', structurally adapted to the harsh mucosal environment and to control potentially pathogenic mucosal flora primarily through neutralization. Inflammation is controlled by limiting the available binding sites on the Fc portion of IgA, effectively preventing Fcalpha clustering through a 1:1 stoichiometric binding. Differences in structure and resulting function allows the two isoforms fill unique niches in mucosal immune responses, suggesting selective advantages for each. Whereas IgA1 specializes in protein detection, IgA2 tends to bind LPS and polysaccharide antigen. So, structure arms the IgA secretory unit with specific advantages suited for its environmental, maintains balance between inflamation and mucosal barrier protection by limiting effector responses, and imparts unique functional roles to IgA isoforms. Together, structure and function determine the immune niches filled by IgA1 and IgA2.
		+

	== Implications in Science and Medicine ==		== Implications in Science and Medicine ==

Rebecca Martin at 22:14, 1 May 2009

2009-05-01T22:14:52Z

←Older revision		Revision as of 22:14, 1 May 2009
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	:: IgM: Solution structure of human Immunoglobulin M [[2rcj]]		:: IgM: Solution structure of human Immunoglobulin M [[2rcj]]
	:: IgG: Crystal structure of the intact human IgG B12 with broad and potent activity against primary HIV-1 isolates: a template for HIV vaccine design [[1hzh]]		:: IgG: Crystal structure of the intact human IgG B12 with broad and potent activity against primary HIV-1 isolates: a template for HIV vaccine design [[1hzh]]
-	:: IgG: Three=dimensional structure of a human immunoglobulin with a hinge deletion [[1mco]]	+	:: IgG: Three=dimensional structure of a human immunoglobulin with a hinge deletion [[1mco]]
	:: IgD: Semi-extended solution structure of human myeloma immunoglobulin D determined by constrained X-ray scattering [[1zvo]]		:: IgD: Semi-extended solution structure of human myeloma immunoglobulin D determined by constrained X-ray scattering [[1zvo]]
	:: IgE: Structure of the human ige-fc bound to its high affinity receptor fc(epsilon)ri(alpha) [[1f6a]]		:: IgE: Structure of the human ige-fc bound to its high affinity receptor fc(epsilon)ri(alpha) [[1f6a]]
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	:: TCR: Crystal Structure of the G17E/A52V/S54N/Q72H/E80V/L81S/T87S/G96V variant of the murine T cell receptor V beta 8.2 domain [[2apv]]		:: TCR: Crystal Structure of the G17E/A52V/S54N/Q72H/E80V/L81S/T87S/G96V variant of the murine T cell receptor V beta 8.2 domain [[2apv]]
	* V-type immunoglobulin examples		* V-type immunoglobulin examples
-	:: Crystal Structure of a Ligand-Binding Domain of the Human Polymeric Ig Receptor, pIgR [[1XED]]	+	:: Crystal Structure of a Ligand-Binding Domain of the Human Polymeric Ig Receptor, pIgR [[1XED]]
	:: Crystal structure of human FcaRI [[10vz]]		:: Crystal structure of human FcaRI [[10vz]]
	:: Influenza virus hemagglutinin complexed with a neutralizing antibody [[1QFU]]		:: Influenza virus hemagglutinin complexed with a neutralizing antibody [[1QFU]]

Rebecca Martin: /* Insights into Function */

2009-05-01T21:57:15Z

Insights into Function

←Older revision		Revision as of 21:57, 1 May 2009
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	:The Fc portion is more susceptible to intestinal proteases than other regions of the IgA. This region of increased susceptibility is the precise region to which the secretory component remains after the pIgR is cleaved <ref name="seven"/>. So, the secretory component offers the antibody additional '''protection against proteolytic cleavage'''. Binding to Fc region reduces flexibility at the hinge and between the 2 Fc regions. The loss in flexibility correlates with a decrease in the likelihood that the IgA will be in the correct conformation for cleavage to occur <ref name=" nineseven" /> and prevents large bacterial matrix metalloproteases from cleaving the Fc and hinge regions. So, the secretory component and dimeric IgA synergize to create a protected protein fit for the harsh mucosal environment.		:The Fc portion is more susceptible to intestinal proteases than other regions of the IgA. This region of increased susceptibility is the precise region to which the secretory component remains after the pIgR is cleaved <ref name="seven"/>. So, the secretory component offers the antibody additional '''protection against proteolytic cleavage'''. Binding to Fc region reduces flexibility at the hinge and between the 2 Fc regions. The loss in flexibility correlates with a decrease in the likelihood that the IgA will be in the correct conformation for cleavage to occur <ref name=" nineseven" /> and prevents large bacterial matrix metalloproteases from cleaving the Fc and hinge regions. So, the secretory component and dimeric IgA synergize to create a protected protein fit for the harsh mucosal environment.
-

	'''Limiting Effector Responses through Decreased FcalphaR Binding'''		'''Limiting Effector Responses through Decreased FcalphaR Binding'''
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	:The binding of IgA to the Fcalpha receptor does not elicit a structural change in the antibody <ref name="two"/>. Effector function is elicited when multiple receptors bind and resultant clustering triggers signaling events. The 1:1 stoichiometry greatly limits Fcalpha clustering and consequent effector functions by effectively '''limiting the concentration of available antibody binding sites''' in the local environment, favoring neutralization in the absence of cytotoxic and inflammatory responses upon antigen recognition. Additional modulation occurs through internal signaling events. For example, cytokines strigger changes in cytoskeletal arrangments that result in clustering of the FcalphaR at the cell surface. In effect, the interaction of the '''secretory component limits the effector and inflammatory responses''' upon antigen binding without limiting the ability of the antibody to neutralize pathogens or exclude commensals from breeching the mucosal barrier.		:The binding of IgA to the Fcalpha receptor does not elicit a structural change in the antibody <ref name="two"/>. Effector function is elicited when multiple receptors bind and resultant clustering triggers signaling events. The 1:1 stoichiometry greatly limits Fcalpha clustering and consequent effector functions by effectively '''limiting the concentration of available antibody binding sites''' in the local environment, favoring neutralization in the absence of cytotoxic and inflammatory responses upon antigen recognition. Additional modulation occurs through internal signaling events. For example, cytokines strigger changes in cytoskeletal arrangments that result in clustering of the FcalphaR at the cell surface. In effect, the interaction of the '''secretory component limits the effector and inflammatory responses''' upon antigen binding without limiting the ability of the antibody to neutralize pathogens or exclude commensals from breeching the mucosal barrier.
-

	'''Differences in Antigen Binding'''		'''Differences in Antigen Binding'''
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	:Since <scene name='Rebecca_Martin/Sandbox1/Siga1def/1'>secretory IgA1</scene> is planar and more flexible, this might lend to '''antigen binding''' on proteins, which are larger and more variable <ref name="nineten"/>. Flexibility allows IgA1 access to a more diverse array of orientations. Likewise the more compact, nonplanar <scene name='Rebecca_Martin/Sandbox1/Siga1/1'>secretory IgA2</scene> might preferentially bind repeating patterns on fixed surfaces, like bacteria coating intestinal mucosa. It is intersting to note that IgA2 tends to induce signaling more slowly than IgA1 upon bindig FcalphaR. So, differences in isoform structure correspond to different antigen specificities and consequent differences in the roles each isoform plays in elciting mucosal immune responses.		:Since <scene name='Rebecca_Martin/Sandbox1/Siga1def/1'>secretory IgA1</scene> is planar and more flexible, this might lend to '''antigen binding''' on proteins, which are larger and more variable <ref name="nineten"/>. Flexibility allows IgA1 access to a more diverse array of orientations. Likewise the more compact, nonplanar <scene name='Rebecca_Martin/Sandbox1/Siga1/1'>secretory IgA2</scene> might preferentially bind repeating patterns on fixed surfaces, like bacteria coating intestinal mucosa. It is intersting to note that IgA2 tends to induce signaling more slowly than IgA1 upon bindig FcalphaR. So, differences in isoform structure correspond to different antigen specificities and consequent differences in the roles each isoform plays in elciting mucosal immune responses.
-

	'''Conclusions on Function'''		'''Conclusions on Function'''
	:The secretory component interacts with either dimeric IgA1 or IgA2 to form a '''functional unit''', structurally adapted to the harsh mucosal environment and to control potentially pathogenic mucosal flora primarily through neutralization. Inflammation is controlled by limiting the available binding sites on the Fc portion of IgA, effectively preventing Fcalpha clustering through a 1:1 stoichiometric binding. Differences in structure and resulting function allows the two isoforms fill unique niches in mucosal immune responses, suggesting selective advantages for each. Whereas IgA1 specializes in protein detection, IgA2 tends to bind LPS and polysaccharide antigen. So, structure arms the IgA secretory unit with specific advantages suited for its environmental, maintains balance between inflamation and mucosal barrier protection by limiting effector responses, and imparts unique functional roles to IgA isoforms. Together, structure and function determine the immune niches filled by IgA1 and IgA2.		:The secretory component interacts with either dimeric IgA1 or IgA2 to form a '''functional unit''', structurally adapted to the harsh mucosal environment and to control potentially pathogenic mucosal flora primarily through neutralization. Inflammation is controlled by limiting the available binding sites on the Fc portion of IgA, effectively preventing Fcalpha clustering through a 1:1 stoichiometric binding. Differences in structure and resulting function allows the two isoforms fill unique niches in mucosal immune responses, suggesting selective advantages for each. Whereas IgA1 specializes in protein detection, IgA2 tends to bind LPS and polysaccharide antigen. So, structure arms the IgA secretory unit with specific advantages suited for its environmental, maintains balance between inflamation and mucosal barrier protection by limiting effector responses, and imparts unique functional roles to IgA isoforms. Together, structure and function determine the immune niches filled by IgA1 and IgA2.
-

	== Implications in Science and Medicine ==		== Implications in Science and Medicine ==

Rebecca Martin at 21:55, 1 May 2009

2009-05-01T21:55:45Z

←Older revision		Revision as of 22:22, 1 May 2009
Line 121:		Line 121:
	:The Fc portion is more susceptible to intestinal proteases than other regions of the IgA. This region of increased susceptibility is the precise region to which the secretory component remains after the pIgR is cleaved <ref name="seven"/>. So, the secretory component offers the antibody additional '''protection against proteolytic cleavage'''. Binding to Fc region reduces flexibility at the hinge and between the 2 Fc regions. The loss in flexibility correlates with a decrease in the likelihood that the IgA will be in the correct conformation for cleavage to occur <ref name=" nineseven" /> and prevents large bacterial matrix metalloproteases from cleaving the Fc and hinge regions. So, the secretory component and dimeric IgA synergize to create a protected protein fit for the harsh mucosal environment.		:The Fc portion is more susceptible to intestinal proteases than other regions of the IgA. This region of increased susceptibility is the precise region to which the secretory component remains after the pIgR is cleaved <ref name="seven"/>. So, the secretory component offers the antibody additional '''protection against proteolytic cleavage'''. Binding to Fc region reduces flexibility at the hinge and between the 2 Fc regions. The loss in flexibility correlates with a decrease in the likelihood that the IgA will be in the correct conformation for cleavage to occur <ref name=" nineseven" /> and prevents large bacterial matrix metalloproteases from cleaving the Fc and hinge regions. So, the secretory component and dimeric IgA synergize to create a protected protein fit for the harsh mucosal environment.

-	'''Limiting Effector Responses through Decreased ~~FcalphaR~~ Binding'''	+	'''Limiting Effector Responses through Decreased FcαR Binding'''
-	<applet load='1ow0' size='300' frame='true' align='right' caption='Fc portion of IgA bound to ~~FcalphaR~~' />	+	<applet load='1ow0' size='300' frame='true' align='right' caption='Fc portion of IgA bound to FcαR' />
-	:The ~~FcalphaR~~ binding sites are located one per heavy chain at each Ch2-Ch3 interface. Both both domains contribute one binding site. So, the stoichiometry between monomeric IgA and the ~~FcalphaR~~ is <scene name='Rebecca_Martin/Sandbox1/Fc/3'>2:1</scene> <ref name="five"/>. The Fc portion is shown in red, and the receptor is in blue. Dimerization would increase this stoichiometry 4:1; however, 2 of the binding sites will be <scene name='Rebecca_Martin/Sandbox1/Siga1_sites_covered/1'>covered by the secretory component</scene>. Because of <scene name='Rebecca_Martin/Sandbox1/Fc/4'>orientation</scene> constraints, only 1 of the 2 remaining binding sites will be available to bind receptor. Therefore, physiologic stoichiometry is 1:1.	+	:The FcαR binding sites are located one per heavy chain at each Ch2-Ch3 interface. Both both domains contribute one binding site. So, the stoichiometry between monomeric IgA and the FcαR is <scene name='Rebecca_Martin/Sandbox1/Fc/3'>2:1</scene> <ref name="five"/>. The Fc portion is shown in red, and the receptor is in blue. Dimerization would increase this stoichiometry 4:1; however, 2 of the binding sites will be <scene name='Rebecca_Martin/Sandbox1/Siga1_sites_covered/1'>covered by the secretory component</scene>. Because of <scene name='Rebecca_Martin/Sandbox1/Fc/4'>orientation</scene> constraints, only 1 of the 2 remaining binding sites will be available to bind receptor. Therefore, physiologic stoichiometry is 1:1.

-	:The binding of IgA to the ~~Fcalpha receptor~~ does not elicit a structural change in the antibody <ref name="two"/>. Effector function is elicited when multiple receptors bind and resultant clustering triggers signaling events. The 1:1 stoichiometry greatly limits ~~Fcalpha~~ clustering and consequent effector functions by effectively '''limiting the concentration of available antibody binding sites''' in the local environment, favoring neutralization in the absence of cytotoxic and inflammatory responses upon antigen recognition. Additional modulation occurs through internal signaling events. For example, cytokines strigger changes in cytoskeletal arrangments that result in clustering of the ~~FcalphaR~~ at the cell surface. In effect, the interaction of the '''secretory component limits the effector and inflammatory responses''' upon antigen binding without limiting the ability of the antibody to neutralize pathogens or exclude commensals from breeching the mucosal barrier.	+	:The binding of IgA to the FcαR does not elicit a structural change in the antibody <ref name="two"/>. Effector function is elicited when multiple receptors bind and resultant clustering triggers signaling events. The 1:1 stoichiometry greatly limits FcαR clustering and consequent effector functions by effectively '''limiting the concentration of available antibody binding sites''' in the local environment, favoring neutralization in the absence of cytotoxic and inflammatory responses upon antigen recognition. Additional modulation occurs through internal signaling events. For example, cytokines strigger changes in cytoskeletal arrangments that result in clustering of the FcαR at the cell surface. In effect, the interaction of the '''secretory component limits the effector and inflammatory responses''' upon antigen binding without limiting the ability of the antibody to neutralize pathogens or exclude commensals from breeching the mucosal barrier.

	'''Differences in Antigen Binding'''		'''Differences in Antigen Binding'''
	:While both IgA1 and IgA2 are able to bind polysaccharide, IgA1 preferentially binds protein antigen, while IgA2 preferentially binds lipopolysaccharide lipid A <ref name="nineten"/>. This difference in structure can be explained, at least in part, by structural differences. The bindng of the secretory component to IgA1 results in a planar antibody with a wide, rigid antigenic reach. In contrast, secretory component binding to IgA2 results in a compact nonplanar form.		:While both IgA1 and IgA2 are able to bind polysaccharide, IgA1 preferentially binds protein antigen, while IgA2 preferentially binds lipopolysaccharide lipid A <ref name="nineten"/>. This difference in structure can be explained, at least in part, by structural differences. The bindng of the secretory component to IgA1 results in a planar antibody with a wide, rigid antigenic reach. In contrast, secretory component binding to IgA2 results in a compact nonplanar form.

-	:Since <scene name='Rebecca_Martin/Sandbox1/Siga1def/1'>secretory IgA1</scene> is planar and more flexible, this might lend to '''antigen binding''' on proteins, which are larger and more variable <ref name="nineten"/>. Flexibility allows IgA1 access to a more diverse array of orientations. Likewise the more compact, nonplanar <scene name='Rebecca_Martin/Sandbox1/Siga1/1'>secretory IgA2</scene> might preferentially bind repeating patterns on fixed surfaces, like bacteria coating intestinal mucosa. It is intersting to note that IgA2 tends to induce signaling more slowly than IgA1 upon bindig ~~FcalphaR~~. So, differences in isoform structure correspond to different antigen specificities and consequent differences in the roles each isoform plays in elciting mucosal immune responses.	+	:Since <scene name='Rebecca_Martin/Sandbox1/Siga1def/1'>secretory IgA1</scene> is planar and more flexible, this might lend to '''antigen binding''' on proteins, which are larger and more variable <ref name="nineten"/>. Flexibility allows IgA1 access to a more diverse array of orientations. Likewise the more compact, nonplanar <scene name='Rebecca_Martin/Sandbox1/Siga1/1'>secretory IgA2</scene> might preferentially bind repeating patterns on fixed surfaces, like bacteria coating intestinal mucosa. It is intersting to note that IgA2 tends to induce signaling more slowly than IgA1 upon bindig FcαR. So, differences in isoform structure correspond to different antigen specificities and consequent differences in the roles each isoform plays in elciting mucosal immune responses.

	'''Conclusions on Function'''		'''Conclusions on Function'''
-	:The secretory component interacts with either dimeric IgA1 or IgA2 to form a '''functional unit''', structurally adapted to the harsh mucosal environment and to control potentially pathogenic mucosal flora primarily through neutralization. Inflammation is controlled by limiting the available binding sites on the Fc portion of IgA, effectively preventing ~~Fcalpha~~ clustering through a 1:1 stoichiometric binding. Differences in structure and resulting function allows the two isoforms fill unique niches in mucosal immune responses, suggesting selective advantages for each. Whereas IgA1 specializes in protein detection, IgA2 tends to bind LPS and polysaccharide antigen. So, structure arms the IgA secretory unit with specific advantages suited for its environmental, maintains balance between inflamation and mucosal barrier protection by limiting effector responses, and imparts unique functional roles to IgA isoforms. Together, structure and function determine the immune niches filled by IgA1 and IgA2.	+	:The secretory component interacts with either dimeric IgA1 or IgA2 to form a '''functional unit''', structurally adapted to the harsh mucosal environment and to control potentially pathogenic mucosal flora primarily through neutralization. Inflammation is controlled by limiting the available binding sites on the Fc portion of IgA, effectively preventing FcαR clustering through a 1:1 stoichiometric binding. Differences in structure and resulting function allows the two isoforms fill unique niches in mucosal immune responses, suggesting selective advantages for each. Whereas IgA1 specializes in protein detection, IgA2 tends to bind LPS and polysaccharide antigen. So, structure arms the IgA secretory unit with specific advantages suited for its environmental, maintains balance between inflamation and mucosal barrier protection by limiting effector responses, and imparts unique functional roles to IgA isoforms. Together, structure and function determine the immune niches filled by IgA1 and IgA2.


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	:What residues on the secretory component are glycosylated?		:What residues on the secretory component are glycosylated?
	:What binding differences characterize IgA1 vs IgA2? <ref name="seven" />		:What binding differences characterize IgA1 vs IgA2? <ref name="seven" />
-	:Why does IgA2 lack as robust an effector function in binding to ~~Fcalpha~~?	+	:Why does IgA2 lack as robust an effector function in binding to FcαR?
	:What are the precise binding motifs of the secretory component and IgA1? <ref name="eight"/>		:What are the precise binding motifs of the secretory component and IgA1? <ref name="eight"/>
	:What is the structure of IgA involved in IgA nephropathy? <ref name="eight"/>		:What is the structure of IgA involved in IgA nephropathy? <ref name="eight"/>

←Older revision		Revision as of 21:55, 1 May 2009
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	At least two isotypes exist, termed IgA1 and IgA2. IgA2 can further be categorized into 2 allotypes: IgA2 m(1) and IgA2 m(2). While IgA2 is found in most mammalian species, IgA1 is found only in higher apes. An approximately equal ratio of secretory IgA1 (sIgA1) to secretory IgA2 (sIgA2) reside at the mucosal surface, with the exception of the colon, where the majority is sIgA2 <ref name=" nineten">PMID:19109255</ref>. In the serum, about 90% of the IgA is monomeric IgA1 <ref name ="ten" />. While both isoforms are able to bind polysaccharide, IgA1 preferentially binds protein antigen, while IgA2 preferentially binds lipopolysaccharide lipid A.		At least two isotypes exist, termed IgA1 and IgA2. IgA2 can further be categorized into 2 allotypes: IgA2 m(1) and IgA2 m(2). While IgA2 is found in most mammalian species, IgA1 is found only in higher apes. An approximately equal ratio of secretory IgA1 (sIgA1) to secretory IgA2 (sIgA2) reside at the mucosal surface, with the exception of the colon, where the majority is sIgA2 <ref name=" nineten">PMID:19109255</ref>. In the serum, about 90% of the IgA is monomeric IgA1 <ref name ="ten" />. While both isoforms are able to bind polysaccharide, IgA1 preferentially binds protein antigen, while IgA2 preferentially binds lipopolysaccharide lipid A.
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	The receptors for IgA include the Fcα Receptor (FcαRI; CD89) and the polyimmunologlobulin receptor (pIgR). When binding to FcαRI results in the dimerization, the consequent signaling results in effector functions, including respiratory burst, mucosal surface, phaocytosis, and eosinophil degranulation. Binding to the pIgR results in transoocytosis and IgA secretion <ref name="five" />. Unlike other antibody isotypes, IgA exists in mutiple oligomeric states <ref name="nineseven" />. The most common of which are the monomeric, dimeric, and secretory forms <ref name="ten" />, adding to the complexity of structural functions for IgA. Exploring IgA's structure and protein interactions illuminates the unique and critical function IgA plays in humoral immunity.		The receptors for IgA include the Fcα Receptor (FcαRI; CD89) and the polyimmunologlobulin receptor (pIgR). When binding to FcαRI results in the dimerization, the consequent signaling results in effector functions, including respiratory burst, mucosal surface, phaocytosis, and eosinophil degranulation. Binding to the pIgR results in transoocytosis and IgA secretion <ref name="five" />. Unlike other antibody isotypes, IgA exists in mutiple oligomeric states <ref name="nineseven" />. The most common of which are the monomeric, dimeric, and secretory forms <ref name="ten" />, adding to the complexity of structural functions for IgA. Exploring IgA's structure and protein interactions illuminates the unique and critical function IgA plays in humoral immunity.
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	::Proteins containing the classic immunoglobulin-like domain are found predominantly in the immune system <ref name="att" />. In fact, the antibody's closest related structires are those that recognize antigen: MHC and TCRs.		::Proteins containing the classic immunoglobulin-like domain are found predominantly in the immune system <ref name="att" />. In fact, the antibody's closest related structires are those that recognize antigen: MHC and TCRs.
	::The V-type domain is found in a wider variety of proteins, including the Ig-binding molecules, such as the pIgR and the FcalphaR <ref name="att" />. Viral hemagluttinin is yet another example.		::The V-type domain is found in a wider variety of proteins, including the Ig-binding molecules, such as the pIgR and the FcalphaR <ref name="att" />. Viral hemagluttinin is yet another example.
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	== IgA1 and IgA2: a Structural Comparison ==		== IgA1 and IgA2: a Structural Comparison ==
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	<scene name='Rebecca_Martin/Sandbox1/Igd/1'>IgD</scene> Hinge region is 64 amino acids in length. Note similarity to IgA1.		<scene name='Rebecca_Martin/Sandbox1/Igd/1'>IgD</scene> Hinge region is 64 amino acids in length. Note similarity to IgA1.
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	== The J Chain allows IgA to form Dimers==		== The J Chain allows IgA to form Dimers==
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	:The secretory compenent is the first 585 residues of the pIgR <ref name="seven"/>. The C terminal end of the secretory component is linked to the pIgR, but maintains no specific fold. The ability for the secretory to move freely facilitates its proteolytic cleavage and the secretion of sIgA. Structurally, the secretory component is comprised of 5 V-type immunoglobulin-like domains (D1-5) with 5-7 glycan chains, which increase the chains resistance to proteases. These glycosylation sites are located on one side of the protein and do not interfere with IgA binding. A long (10 amino acids) linker region exists between D3 and D4, so the D4 and D5 regions fold in on D2 and D3 in a compact J-shape. D1-3 are 12nm in length, while D4-5 are 10 nm long. Thus, D1 remains accessable. The one-sided glycans allow free access of D1's CDR regions and the Cys 502 at D5 to interact with IgA. It is thought that when D1 interacts with IgA's Fc region and the J chain, allowing the secretory component to unfold and disulfide formation between D5 C502 and IgA's Ch2 C311. While SC unfolds upon IgA binding, this binding imparts no change on the structure of IgA <ref name="nineseven"/>.		:The secretory compenent is the first 585 residues of the pIgR <ref name="seven"/>. The C terminal end of the secretory component is linked to the pIgR, but maintains no specific fold. The ability for the secretory to move freely facilitates its proteolytic cleavage and the secretion of sIgA. Structurally, the secretory component is comprised of 5 V-type immunoglobulin-like domains (D1-5) with 5-7 glycan chains, which increase the chains resistance to proteases. These glycosylation sites are located on one side of the protein and do not interfere with IgA binding. A long (10 amino acids) linker region exists between D3 and D4, so the D4 and D5 regions fold in on D2 and D3 in a compact J-shape. D1-3 are 12nm in length, while D4-5 are 10 nm long. Thus, D1 remains accessable. The one-sided glycans allow free access of D1's CDR regions and the Cys 502 at D5 to interact with IgA. It is thought that when D1 interacts with IgA's Fc region and the J chain, allowing the secretory component to unfold and disulfide formation between D5 C502 and IgA's Ch2 C311. While SC unfolds upon IgA binding, this binding imparts no change on the structure of IgA <ref name="nineseven"/>.
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	==sIgA1 and sIgA2==		==sIgA1 and sIgA2==
	[[Image:SIgA.jpg\|thumb\|Adapted from Bonner, et al 2009 and Bonner, et al 2008.]]		[[Image:SIgA.jpg\|thumb\|Adapted from Bonner, et al 2009 and Bonner, et al 2008.]]
	:Binding of the secretory component to the convex edge of the Fc region of dimeric IgA1 maintains <scene name='Rebecca_Martin/Sandbox1/Siga1def/1'>Secretory IgA1</scene> in a near planar conformation, <ref name="nineten" />, <ref name="eight" />. The Fc regions align end to end without overlap, and the fab fragments remain in alignment with the Fc plane. In contrast, <scene name='Rebecca_Martin/Sandbox1/Siga1/1'>Secretory IgA2</scene> fab fragments remain out of alignment with the Fc plane. Because the secretory component resides at the convex region of the Fc portion, the D1 and D5 impart steric hindrance on the fab fragments, which are forced out of alignment. Consequently, IgA2 assumes a nonplanar conformation. The longer hinge region of IgA1 allows it to maintain its planar conformation.		:Binding of the secretory component to the convex edge of the Fc region of dimeric IgA1 maintains <scene name='Rebecca_Martin/Sandbox1/Siga1def/1'>Secretory IgA1</scene> in a near planar conformation, <ref name="nineten" />, <ref name="eight" />. The Fc regions align end to end without overlap, and the fab fragments remain in alignment with the Fc plane. In contrast, <scene name='Rebecca_Martin/Sandbox1/Siga1/1'>Secretory IgA2</scene> fab fragments remain out of alignment with the Fc plane. Because the secretory component resides at the convex region of the Fc portion, the D1 and D5 impart steric hindrance on the fab fragments, which are forced out of alignment. Consequently, IgA2 assumes a nonplanar conformation. The longer hinge region of IgA1 allows it to maintain its planar conformation.
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	'''Structure and the Mucosal Environment'''		'''Structure and the Mucosal Environment'''
	:'''Glycosylation''' of the IgA, the J chain, and the secretory component lends to protection against proteolytic attack in the harsh mucosal environment. '''Dimerization''' allows transcytosis. In addition, glycosylation on the secretory chain is thought to assist with anchoring to the mucosa. Through steric hindrance, the secretory component assists in preventing the binding of microorganisms to gut mucosa, impeding their entry into the mucosa <ref name="ten"/>, <ref name="two">PMID: 12768205</ref>.		:'''Glycosylation''' of the IgA, the J chain, and the secretory component lends to protection against proteolytic attack in the harsh mucosal environment. '''Dimerization''' allows transcytosis. In addition, glycosylation on the secretory chain is thought to assist with anchoring to the mucosa. Through steric hindrance, the secretory component assists in preventing the binding of microorganisms to gut mucosa, impeding their entry into the mucosa <ref name="ten"/>, <ref name="two">PMID: 12768205</ref>.
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	:The Fc portion is more susceptible to intestinal proteases than other regions of the IgA. This region of increased susceptibility is the precise region to which the secretory component remains after the pIgR is cleaved <ref name="seven"/>. So, the secretory component offers the antibody additional '''protection against proteolytic cleavage'''. Binding to Fc region reduces flexibility at the hinge and between the 2 Fc regions. The loss in flexibility correlates with a decrease in the likelihood that the IgA will be in the correct conformation for cleavage to occur <ref name=" nineseven" /> and prevents large bacterial matrix metalloproteases from cleaving the Fc and hinge regions. So, the secretory component and dimeric IgA synergize to create a protected protein fit for the harsh mucosal environment.		:The Fc portion is more susceptible to intestinal proteases than other regions of the IgA. This region of increased susceptibility is the precise region to which the secretory component remains after the pIgR is cleaved <ref name="seven"/>. So, the secretory component offers the antibody additional '''protection against proteolytic cleavage'''. Binding to Fc region reduces flexibility at the hinge and between the 2 Fc regions. The loss in flexibility correlates with a decrease in the likelihood that the IgA will be in the correct conformation for cleavage to occur <ref name=" nineseven" /> and prevents large bacterial matrix metalloproteases from cleaving the Fc and hinge regions. So, the secretory component and dimeric IgA synergize to create a protected protein fit for the harsh mucosal environment.
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	<applet load='1ow0' size='300' frame='true' align='right' caption='Fc portion of IgA bound to FcalphaR' />		<applet load='1ow0' size='300' frame='true' align='right' caption='Fc portion of IgA bound to FcalphaR' />
	:The FcalphaR binding sites are located one per heavy chain at each Ch2-Ch3 interface. Both both domains contribute one binding site. So, the stoichiometry between monomeric IgA and the FcalphaR is <scene name='Rebecca_Martin/Sandbox1/Fc/3'>2:1</scene> <ref name="five"/>. The Fc portion is shown in red, and the receptor is in blue. Dimerization would increase this stoichiometry 4:1; however, 2 of the binding sites will be <scene name='Rebecca_Martin/Sandbox1/Siga1_sites_covered/1'>covered by the secretory component</scene>. Because of <scene name='Rebecca_Martin/Sandbox1/Fc/4'>orientation</scene> constraints, only 1 of the 2 remaining binding sites will be available to bind receptor. Therefore, physiologic stoichiometry is 1:1.		:The FcalphaR binding sites are located one per heavy chain at each Ch2-Ch3 interface. Both both domains contribute one binding site. So, the stoichiometry between monomeric IgA and the FcalphaR is <scene name='Rebecca_Martin/Sandbox1/Fc/3'>2:1</scene> <ref name="five"/>. The Fc portion is shown in red, and the receptor is in blue. Dimerization would increase this stoichiometry 4:1; however, 2 of the binding sites will be <scene name='Rebecca_Martin/Sandbox1/Siga1_sites_covered/1'>covered by the secretory component</scene>. Because of <scene name='Rebecca_Martin/Sandbox1/Fc/4'>orientation</scene> constraints, only 1 of the 2 remaining binding sites will be available to bind receptor. Therefore, physiologic stoichiometry is 1:1.
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	:The binding of IgA to the Fcalpha receptor does not elicit a structural change in the antibody <ref name="two"/>. Effector function is elicited when multiple receptors bind and resultant clustering triggers signaling events. The 1:1 stoichiometry greatly limits Fcalpha clustering and consequent effector functions by effectively '''limiting the concentration of available antibody binding sites''' in the local environment, favoring neutralization in the absence of cytotoxic and inflammatory responses upon antigen recognition. Additional modulation occurs through internal signaling events. For example, cytokines strigger changes in cytoskeletal arrangments that result in clustering of the FcalphaR at the cell surface. In effect, the interaction of the '''secretory component limits the effector and inflammatory responses''' upon antigen binding without limiting the ability of the antibody to neutralize pathogens or exclude commensals from breeching the mucosal barrier.		:The binding of IgA to the Fcalpha receptor does not elicit a structural change in the antibody <ref name="two"/>. Effector function is elicited when multiple receptors bind and resultant clustering triggers signaling events. The 1:1 stoichiometry greatly limits Fcalpha clustering and consequent effector functions by effectively '''limiting the concentration of available antibody binding sites''' in the local environment, favoring neutralization in the absence of cytotoxic and inflammatory responses upon antigen recognition. Additional modulation occurs through internal signaling events. For example, cytokines strigger changes in cytoskeletal arrangments that result in clustering of the FcalphaR at the cell surface. In effect, the interaction of the '''secretory component limits the effector and inflammatory responses''' upon antigen binding without limiting the ability of the antibody to neutralize pathogens or exclude commensals from breeching the mucosal barrier.