ABCG2 is a homodimer with each monomer containing two domains, the nucleotide binding domain <scene name='83/832932/Overall_structure_nbd_unbound/1'>(NBD)</scene> and the transmembrane domain <scene name='83/832932/Overall_structure_tmd_unbound/2'>(TMD)</scene>, which are fused together as a single peptide chain.<ref name="Taylor"/> 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). [[Image:membrane domains.png|250 px|right|thumb|Figure 1. Orientation of ABCG2 in relation to the cell membrane. [https://www.rcsb.org/structure/5NJ3 (5NJ3)]]]
ABCG2 is a homodimer with each monomer containing two domains, the nucleotide binding domain <scene name='83/832932/Overall_structure_nbd_unbound/6'>(NBD)</scene> and the transmembrane domain <scene name='83/832932/Overall_structure_tmd_unbound/5'>(TMD)</scene>, which are fused together as a single peptide chain.<ref name="Taylor"/> 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). [[Image:membrane domains.png|250 px|right|thumb|Figure 1. Orientation of ABCG2 in relation to the cell membrane. [https://www.rcsb.org/structure/5NJ3 (5NJ3)]]]
===ATP Bound and Unbound Conformations===
===ATP Bound and Unbound Conformations===
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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: 5NJ36HBU6HCO6FFC)
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 (NBD) and the transmembrane domain (TMD), 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 ATP binds each NBD (2 molecules of ATP total) causing a conformational change of the overall structure from an inward-facing conformation to an outward-facing conformation. 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]
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). Cavity 1 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 open to the cytosol in order to recruit substrates for transport. Cavity 1 is narrow and full of nonpolar, hydrophobic residues and, as a result, prefers nonpolar, hydrophobic substrates, particularly flat, polycyclic molecules. Substrates, such as estrone sulfate, form hydrogen bonds and stacking interactions 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 collapse of Cavity 1 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]Cavity 2, which is occluded when the protein in is 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]Disulfide bonds in the external loops near the exit of Cavity 2 also help promote substrate release.[2] Once Cavity 2 is empty, the protein reverts to the inward-facing conformation via hydrolysis of ATP.
Cavities 1 and 2 are separated by a leucine plug which likely acts as a substrate check-point during transport; changes to either of these leucine residues 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 leucine plug opens 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 has been found to correlate with a 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 for 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]
Inhibitors
ABCG2 inhibitors that bind Cavity 1, acting as competitive inhibitors against ABCG2 substrates. Depending on the size of the inhibitor, one or two molecules can accommodate binding to the cavity and form hydrogen bonds, van der Waals, and stacking interactions 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]
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↑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:https://dx.doi.org/10.1038/nature22345
↑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:https://dx.doi.org/10.1038/s41586-018-0680-3
↑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:https://dx.doi.org/10.1038/s41568-018-0005-8
↑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:https://dx.doi.org/10.1038/s41594-018-0049-1
