1q94

Structural highlights

Disease

[B2MG_HUMAN] Defects in B2M are the cause of hypercatabolic hypoproteinemia (HYCATHYP) [MIM:241600]. Affected individuals show marked reduction in serum concentrations of immunoglobulin and albumin, probably due to rapid degradation.^[1] Note=Beta-2-microglobulin may adopt the fibrillar configuration of amyloid in certain pathologic states. The capacity to assemble into amyloid fibrils is concentration dependent. Persistently high beta(2)-microglobulin serum levels lead to amyloidosis in patients on long-term hemodialysis.^{[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]}

Function

[1A11_HUMAN] Involved in the presentation of foreign antigens to the immune system. [POL_HV1Z2] Gag-Pol polyprotein and Gag polyprotein may regulate their own translation, by the binding genomic RNA in the 5'-UTR. At low concentration, Gag-Pol and Gag would promote translation, whereas at high concentration, the polyproteins encapsidate genomic RNA and then shutt off translation (By similarity). Matrix protein p17 has two main functions: in infected cell, it targets Gag and Gag-pol polyproteins to the plasma membrane via a multipartite membrane-binding signal, that includes its myristoylated N-terminus. The second function is to play a role in nuclear localization of the viral genome at the very start of cell infection. Matrix protein is the part of the pre-integration complex. It binds in the cytoplasm the human BAF protein which prevent autointegration of the viral genome, and might be included in virions at the ration of zero to 3 BAF dimer per virion. The myristoylation signal and the NLS thus exert conflicting influences its subcellular localization. The key regulation of these motifs might be phosphorylation of a portion of MA molecules on the C-terminal tyrosine at the time of virus maturation, by virion-associated cellular tyrosine kinase. Implicated in the release from host cell mediated by Vpu (By similarity). Capsid protein p24 forms the conical core that encapsulates the genomic RNA-nucleocapsid complex in the virion. Most core are conical, with only 7% tubular. The core is constituted by capsid protein hexamer subunits. The core is disassembled soon after virion entry. Interaction with human PPIA/CYPA protects the virus from restriction by human TRIM5-alpha and from an unknown antiviral activity in human cells. This capsid restriction by TRIM5 is one of the factors which restricts HIV-1 to the human species (By similarity). Nucleocapsid protein p7 encapsulates and protects viral dimeric unspliced (genomic) RNA. Binds these RNAs through its zinc fingers. Facilitates rearangement of nucleic acid secondary structure during retrotranscription of genomic RNA. This capability is referred to as nucleic acid chaperone activity (By similarity). The aspartyl protease mediates proteolytic cleavages of Gag and Gag-Pol polyproteins during or shortly after the release of the virion from the plasma membrane. Cleavages take place as an ordered, step-wise cascade to yield mature proteins. This process is called maturation. Displays maximal activity during the budding process just prior to particle release from the cell. Also cleaves Nef and Vif, probably concomitantly with viral structural proteins on maturation of virus particles (By similarity). Reverse transcriptase/ribonuclease H (RT) is a multifunctional enzyme that converts the viral RNA genome into dsDNA in the cytoplasm, shortly after virus entry into the cell. This enzyme displays a DNA polymerase activity that can copy either DNA or RNA templates, and a ribonuclease H (RNase H) activity that cleaves the RNA strand of RNA-DNA heteroduplexes in a partially processive 3' to 5' endonucleasic mode. Conversion of viral genomic RNA into dsDNA requires many steps. A tRNA(3)-Lys binds to the primer-binding site (PBS) situated at the 5'-end of the viral RNA. RT uses the 3' end of the tRNA primer to perform a short round of RNA-dependent minus-strand DNA synthesis. The reading proceeds through the U5 region and ends after the repeated (R) region which is present at both ends of viral RNA. The portion of the RNA-DNA heteroduplex is digested by the RNase H, resulting in a ssDNA product attached to the tRNA primer. This ssDNA/tRNA hybridizes with the identical R region situated at the 3' end of viral RNA. This template exchange, known as minus-strand DNA strong stop transfer, can be either intra- or intermolecular. RT uses the 3' end of this newly synthesized short ssDNA to perform the RNA-dependent minus-strand DNA synthesis of the whole template. RNase H digests the RNA template except for two polypurine tracts (PPTs) situated at the 5'-end and near the center of the genome. It is not clear if both polymerase and RNase H activities are simultaneous. RNase H probably can proceed both in a polymerase-dependent (RNA cut into small fragments by the same RT performing DNA synthesis) and a polymerase-independent mode (cleavage of remaining RNA fragments by free RTs). Secondly, RT performs DNA-directed plus-strand DNA synthesis using the PPTs that have not been removed by RNase H as primers. PPTs and tRNA primers are then removed by RNase H. The 3' and 5' ssDNA PBS regions hybridize to form a circular dsDNA intermediate. Strand displacement synthesis by RT to the PBS and PPT ends produces a blunt ended, linear dsDNA copy of the viral genome that includes long terminal repeats (LTRs) at both ends (By similarity). Integrase catalyzes viral DNA integration into the host chromosome, by performing a series of DNA cutting and joining reactions. This enzyme activity takes place after virion entry into a cell and reverse transcription of the RNA genome in dsDNA. The first step in the integration process is 3' processing. This step requires a complex comprising the viral genome, matrix protein, Vpr and integrase. This complex is called the pre-integration complex (PIC). The integrase protein removes 2 nucleotides from each 3' end of the viral DNA, leaving recessed CA OH's at the 3' ends. In the second step, the PIC enters cell nucleus. This process is mediated through integrase and Vpr proteins, and allows the virus to infect a non dividing cell. This ability to enter the nucleus is specific of lentiviruses, other retroviruses cannot and rely on cell division to access cell chromosomes. In the third step, termed strand transfer, the integrase protein joins the previously processed 3' ends to the 5' ends of strands of target cellular DNA at the site of integration. The 5'-ends are produced by integrase-catalyzed staggered cuts, 5 bp apart. A Y-shaped, gapped, recombination intermediate results, with the 5'-ends of the viral DNA strands and the 3' ends of target DNA strands remaining unjoined, flanking a gap of 5 bp. The last step is viral DNA integration into host chromosome. This involves host DNA repair synthesis in which the 5 bp gaps between the unjoined strands are filled in and then ligated. Since this process occurs at both cuts flanking the HIV genome, a 5 bp duplication of host DNA is produced at the ends of HIV-1 integration. Alternatively, Integrase may catalyze the excision of viral DNA just after strand transfer, this is termed disintegration (By similarity). [B2MG_HUMAN] Component of the class I major histocompatibility complex (MHC). Involved in the presentation of peptide antigens to the immune system.

Evolutionary Conservation

Check, as determined by ConSurfDB. You may read the explanation of the method and the full data available from ConSurf.

See Also

• Beta-2 microglobulin
• Major histocompatibility complex

References

1. ↑ Wani MA, Haynes LD, Kim J, Bronson CL, Chaudhury C, Mohanty S, Waldmann TA, Robinson JM, Anderson CL. Familial hypercatabolic hypoproteinemia caused by deficiency of the neonatal Fc receptor, FcRn, due to a mutant beta2-microglobulin gene. Proc Natl Acad Sci U S A. 2006 Mar 28;103(13):5084-9. Epub 2006 Mar 20. PMID:16549777 doi:10.1073/pnas.0600548103
2. ↑ Gorevic PD, Munoz PC, Casey TT, DiRaimondo CR, Stone WJ, Prelli FC, Rodrigues MM, Poulik MD, Frangione B. Polymerization of intact beta 2-microglobulin in tissue causes amyloidosis in patients on chronic hemodialysis. Proc Natl Acad Sci U S A. 1986 Oct;83(20):7908-12. PMID:3532124
3. ↑ Argiles A, Derancourt J, Jauregui-Adell J, Mion C, Demaille JG. Biochemical characterization of serum and urinary beta 2 microglobulin in end-stage renal disease patients. Nephrol Dial Transplant. 1992;7(11):1106-10. PMID:1336137
4. ↑ Momoi T, Suzuki M, Titani K, Hisanaga S, Ogawa H, Saito A. Amino acid sequence of a modified beta 2-microglobulin in renal failure patient urine and long-term dialysis patient blood. Clin Chim Acta. 1995 May 15;236(2):135-44. PMID:7554280
5. ↑ Cunningham BA, Wang JL, Berggard I, Peterson PA. The complete amino acid sequence of beta 2-microglobulin. Biochemistry. 1973 Nov 20;12(24):4811-22. PMID:4586824
6. ↑ Haag-Weber M, Mai B, Horl WH. Isolation of a granulocyte inhibitory protein from uraemic patients with homology of beta 2-microglobulin. Nephrol Dial Transplant. 1994;9(4):382-8. PMID:8084451
7. ↑ Trinh CH, Smith DP, Kalverda AP, Phillips SE, Radford SE. Crystal structure of monomeric human beta-2-microglobulin reveals clues to its amyloidogenic properties. Proc Natl Acad Sci U S A. 2002 Jul 23;99(15):9771-6. Epub 2002 Jul 15. PMID:12119416 doi:10.1073/pnas.152337399
8. ↑ Stewart-Jones GB, McMichael AJ, Bell JI, Stuart DI, Jones EY. A structural basis for immunodominant human T cell receptor recognition. Nat Immunol. 2003 Jul;4(7):657-63. Epub 2003 Jun 8. PMID:12796775 doi:10.1038/ni942
9. ↑ Kihara M, Chatani E, Iwata K, Yamamoto K, Matsuura T, Nakagawa A, Naiki H, Goto Y. Conformation of amyloid fibrils of beta2-microglobulin probed by tryptophan mutagenesis. J Biol Chem. 2006 Oct 13;281(41):31061-9. Epub 2006 Aug 10. PMID:16901902 doi:10.1074/jbc.M605358200
10. ↑ Eakin CM, Berman AJ, Miranker AD. A native to amyloidogenic transition regulated by a backbone trigger. Nat Struct Mol Biol. 2006 Mar;13(3):202-8. Epub 2006 Feb 19. PMID:16491088 doi:10.1038/nsmb1068
11. ↑ Iwata K, Matsuura T, Sakurai K, Nakagawa A, Goto Y. High-resolution crystal structure of beta2-microglobulin formed at pH 7.0. J Biochem. 2007 Sep;142(3):413-9. Epub 2007 Jul 23. PMID:17646174 doi:10.1093/jb/mvm148
12. ↑ Ricagno S, Colombo M, de Rosa M, Sangiovanni E, Giorgetti S, Raimondi S, Bellotti V, Bolognesi M. DE loop mutations affect beta2-microglobulin stability and amyloid aggregation. Biochem Biophys Res Commun. 2008 Dec 5;377(1):146-50. Epub 2008 Oct 1. PMID:18835253 doi:S0006-291X(08)01866-4
13. ↑ Esposito G, Ricagno S, Corazza A, Rennella E, Gumral D, Mimmi MC, Betto E, Pucillo CE, Fogolari F, Viglino P, Raimondi S, Giorgetti S, Bolognesi B, Merlini G, Stoppini M, Bolognesi M, Bellotti V. The controlling roles of Trp60 and Trp95 in beta2-microglobulin function, folding and amyloid aggregation properties. J Mol Biol. 2008 May 9;378(4):887-97. Epub 2008 Mar 8. PMID:18395224 doi:10.1016/j.jmb.2008.03.002
14. ↑ Ricagno S, Raimondi S, Giorgetti S, Bellotti V, Bolognesi M. Human beta-2 microglobulin W60V mutant structure: Implications for stability and amyloid aggregation. Biochem Biophys Res Commun. 2009 Mar 13;380(3):543-7. Epub 2009 Jan 25. PMID:19284997 doi:10.1016/j.bbrc.2009.01.116
15. ↑ Li L, Bouvier M. Structures of HLA-A*1101 complexed with immunodominant nonamer and decamer HIV-1 epitopes clearly reveal the presence of a middle, secondary anchor residue. J Immunol. 2004 May 15;172(10):6175-84. PMID:15128805