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This Sandbox is Reserved from Jan 13 through September 1, 2020 for use in the course CH462 Biochemistry II taught by R. Jeremy Johnson at the Butler University, Indianapolis, USA. This reservation includes Sandbox Reserved 1598 through Sandbox Reserved 1627.

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ABCG2 Multidrug Transporter

ABCG2 Multidrug Transporter. Green represents residues in monomer A; Purple represents residues in monomer B. Blue is used to highlight areas of interest in select scenes. (PDB Codes: 5NJ3 6HBU 6HCO 6FFC)

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1 Background
2 Structural highlights
3 Disease
- 3.1 Cancer
- 3.2 ABCG2 Inhibitors

Background

The ABCG2 multidrug transporter is a membrane protein from the ATP-Binding Cassette (ABC) transporter family, specifically the G-subfamily. Also know as the breast cancer resistance protein (BCRP), ABCG2 has physiological roles in various tissue cells including the mammary gland and the blood-brain, blood-testis, and maternal-fetal barriers.^[1] ABCG2 protects cells by exporting xenobiotic molecules out of the cell using ATP hydrolysis. ABCG2 also affects the pharmacokinetics of many drugs and contributes to multidrug resistance.^[2]

Structural highlights

Overall Structure

ABCG2 is a homodimer with each monomer containing two domains, the nucleotide binding domain and the transmembrane domain , which are fused together as a single peptide chain.^[1] The NBD binds and processes ATP and is located inside of the cell where it is exposed to the cytosol. The TMD is responsible for binding and transporting any foreign substrates and is embedded in the cell membrane and extends into the extracellular region. (Figure 1).

Figure 1. Orientation of ABCG2 in relation to the cell membrane. (5NJ3)

ATP Bound and Unbound Conformations

As an ABC Transporter, ABCG2 exhibits ATPase activity, using the energy of ATP hydrolysis to facilitate transport. After substrate bind in the TMD, one molecule of (2 molecules of ATP total) causing a conformational change of the overall structure from an to an . One molecule of ATP is hydrolyzed to transport substrates across the cell membrane while the second molecule of ATP is hydrolyzed to reset the transporter to its inward-facing conformation.^[3]

When ATP binds, α-helices in the NBD approximately 35° relative to the . This shift in the NBD causes slight shifts of α-helices in the TMD; these helices are relative to the . The overall shift from inward-facing to outward-facing promotes the transport of substrates through the transporter.^[2]

Cavities and Leucine Plug

Figure 2. Locations of Cavities 1 and 2 and the Leucine Plug in ABCG2. The protein is in the inward-facing conformation with Cavity 1 open to the cytosol for substrate recruitment, the Leucine Plug is intact, and Cavity 2 is completely occluded. (5NJ3)

Substrates are transported through ABCG2 via two cavities separated by a leucine plug (Figure 2). acts as a multidrug binding pocket and is formed by helices at the interface of the monomers in the TMD. When ATP is not bound to the NBDs, Cavity 1 is in order to recruit substrates for transport. Cavity 1 is and full of nonpolar, hydrophobic residues and, as a result, prefers nonpolar, hydrophobic substrates, particularly flat, polycyclic molecules. Substrates, such as estrone sulfate, with residues from each subunit in Cavity 1.^[1]

After substrates bind in Cavity 1, ATP binds each NBD leading to the transporter shifting from inward-facing to outward-facing. The outward-facing conformation results in the in the TMD which closes Cavity 1 to the cytosol and forces the substrate to move forward to Cavity 2 as there is no longer room in Cavity 1 to accommodate substrates.^[2] , which is occluded when in the inward-facing conformation, is now open to the extracellular space and able to release the substrate. Cavity 2 contains a less hydrophobic environment and, as a result, substrates are released due to hydrophobic mismatch.^[1] in the external loop near the exit of Cavity 2 also help promote substrate release.^[2] Once Cavity 2 is empty, the protein can be converted back to the inward-facing conformation via hydrolysis of ATP.

Cavities 1 and 2 are separated by a which likely acts as a substrate check-point during transport; removal or mutations of this plug have exhibited an increase in transport and a decrease in substrate specificity.^[2] After the substrate binds Cavity 1 and ATP molecules bind each NBD, the to allow the substrate to enter Cavity 2. Once the substrate enters Cavity 2, the plug is able to reform and promote substrate release and conversion to the inward-facing conformation.

Disease

Dysfunctions in ABCG2 are linked to hyperuricemia which can lead to gout, kidney disease, and hypertension, all of which are thought to be the result of impaired transport of uric acid. Additionally, the expression of ABCG2 correlates to poor prognosis and treatment outcome of certain cancers such as breast, ovarian, and lung.^[4]

Cancer

ABCG2 hinders cancer treatment by contributing to multidrug resistance in tumor cells. ABCG2 exports xenbiotics, including vital anti-cancer drugs, which results in the inability to treat cancer cells. The inhibition of ABCG2 would stop the transport of anti-cancer drugs out of cancer cells. Due to the potential of ABCG2 inhibition to aid in cancer treatment, efforts have been made to develop specific inhibitors of ABCG2 and other ABC transporters. Some promising inhibitors were derived from fungal toxin fumitremorgin C; however, many of those derivatives developed have neurotoxic effects.^[4]

ABCG2 Inhibitors

, acting as competitive inhibitors against ABCG2 substrates. Depending on the size of the inhibitor, one or two molecules are required to bind to the cavity and form within the binding site.^[4] Many inhibitors are too big to be transported via the leucine plug resulting in the "clogging" of the transporter. With inhibitors acting as wedges, ABCG2 is locked in the inward-facing conformation and unable to transport molecules out of the cell.^[2]

References

^[1] ^[2] ^[3] ^[4]

↑ ^1.0 ^1.1 ^1.2 ^1.3 ^1.4 Taylor NMI, Manolaridis I, Jackson SM, Kowal J, Stahlberg H, Locher KP. Structure of the human multidrug transporter ABCG2. Nature. 2017 Jun 22;546(7659):504-509. doi: 10.1038/nature22345. Epub 2017 May, 29. PMID:28554189 doi:http://dx.doi.org/10.1038/nature22345
↑ ^2.0 ^2.1 ^2.2 ^2.3 ^2.4 ^2.5 ^2.6 Manolaridis I, Jackson SM, Taylor NMI, Kowal J, Stahlberg H, Locher KP. Cryo-EM structures of a human ABCG2 mutant trapped in ATP-bound and substrate-bound states. Nature. 2018 Nov;563(7731):426-430. doi: 10.1038/s41586-018-0680-3. Epub 2018 Nov, 7. PMID:30405239 doi:http://dx.doi.org/10.1038/s41586-018-0680-3
↑ ^3.0 ^3.1 Robey RW, Pluchino KM, Hall MD, Fojo AT, Bates SE, Gottesman MM. Revisiting the role of ABC transporters in multidrug-resistant cancer. Nat Rev Cancer. 2018 Jul;18(7):452-464. doi: 10.1038/s41568-018-0005-8. PMID:29643473 doi:http://dx.doi.org/10.1038/s41568-018-0005-8
↑ ^4.0 ^4.1 ^4.2 ^4.3 Jackson SM, Manolaridis I, Kowal J, Zechner M, Taylor NMI, Bause M, Bauer S, Bartholomaeus R, Bernhardt G, Koenig B, Buschauer A, Stahlberg H, Altmann KH, Locher KP. Structural basis of small-molecule inhibition of human multidrug transporter ABCG2. Nat Struct Mol Biol. 2018 Apr;25(4):333-340. doi: 10.1038/s41594-018-0049-1. Epub, 2018 Apr 2. PMID:29610494 doi:http://dx.doi.org/10.1038/s41594-018-0049-1

Student Contributors

Julia Pomeroy

Retrieved from "http://52.214.119.220/wiki/index.php/Sandbox_Reserved_1606"

@@ Line 20: / Line 20: @@
 ===Cavities and Leucine Plug===
 [[Image:Map_leucine_plug.png|250 px|right|thumb|Figure 2. Locations of Cavities 1 and 2 and the Leucine Plug in ABCG2. The protein is in the inward-facing conformation with Cavity 1 open to the cytosol for substrate recruitment, the Leucine Plug is intact, and Cavity 2 is completely occluded. [https://www.rcsb.org/structure/5NJ3 (5NJ3)]]]
-Substrates are transported through ABCG2 via two cavities separated by a leucine plug (Figure 2).  <scene name='83/832932/Cavity_1_zoom_out/2'>Cavity 1</scene> acts as a multidrug binding pocket and is formed by helices at the interface of the monomers in the TMD. When ATP is not bound to the NBDs, Cavity 1 is <scene name='83/832932/Overall_structure_cavity_1hel/2'>open to the cytosol</scene> in order to recruit substrates for transport. Cavity 1 is <scene name='83/832932/Cavity_1_narrow_surface/3'>narrow</scene> and full of nonpolar, hydrophobic residues and, as a result, prefers nonpolar, hydrophobic substrates, particularly flat, polycyclic molecules. Substrates, such as estrone sulfate, <scene name='83/832932/Cavity_1_-_use2/3'>form hydrogen bonds and stacking interactions</scene> with residues from each subunit in Cavity 1.<ref name="Taylor"/>
+Substrates are transported through ABCG2 via two cavities separated by a leucine plug (Figure 2).  <scene name='83/832932/Cavity_1_zoom_out/2'>Cavity 1</scene> acts as a multidrug binding pocket and is formed by helices at the interface of the monomers in the TMD. When ATP is not bound to the NBDs, Cavity 1 is <scene name='83/832932/Overall_structure_cavity_1hel/3'>open to the cytosol</scene> in order to recruit substrates for transport. Cavity 1 is <scene name='83/832932/Cavity_1_narrow_surface/4'>narrow</scene> and full of nonpolar, hydrophobic residues and, as a result, prefers nonpolar, hydrophobic substrates, particularly flat, polycyclic molecules. Substrates, such as estrone sulfate, <scene name='83/832932/Cavity_1_-_use2/4'>form hydrogen bonds and stacking interactions</scene> with residues from each subunit in Cavity 1.<ref name="Taylor"/>
 After substrates bind in Cavity 1, ATP binds each NBD leading to the transporter shifting from inward-facing to outward-facing. The outward-facing conformation results in the <scene name='83/832932/Atp_bound_cavity_2/2'>collapse of Cavity 1</scene> in the TMD which closes Cavity 1 to the cytosol and forces the substrate to move forward to Cavity 2 as there is no longer room in Cavity 1 to accommodate substrates.<ref name="Manolaridis"/> <scene name='83/832932/Atp_bound_use_cav_2/1'>Cavity 2</scene>, which is occluded when in the inward-facing conformation, is now open to the extracellular space and able to release the substrate.  Cavity 2 contains a less hydrophobic environment and, as a result, substrates are released due to hydrophobic mismatch.<ref name="Taylor"/> <scene name='83/832932/Atp_bound_use_el_disulfides/2'>Disulfide bonds</scene> in the external loop near the exit of Cavity 2 also help promote substrate release.<ref name="Manolaridis"/> Once Cavity 2 is empty, the protein can be converted back to the inward-facing conformation via hydrolysis of ATP.