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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

Figure 1. Domains of ABCG2 Multidrug transporter. The above protein is in the inward-facing conformation

ATP Bound and Unbound Conformations

As an ABC Transporter, ABCG2 exhibits ATPase activity in which the protein binds and hydrolyzes ATP. After the binding of a substrate, one molecule of causing a conformational change of the overall structure from an with no ATP bound 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 each NBD approximately 35° relative to their . This shift in the NBD causes slight shifts of α-helices in each TMD; these helices are relative to their . 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 above protein is in the inward-facing conformation

Substrates are transported through ABCG2 via two cavities separated by a leucine plug. acts as the multidrug binding pocket and is formed by helices at the interface of the subunits 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.^[1] Substrates, such as estrone sulfate, with residues from each subunit in Cavity 1.

After substrates bind in Cavity 1, ATP binds each NBD leading to the shift from inward-facing to outward-facing. The outward-facing conformation results in the in the TMD. This collapse 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 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 of this plug has exhibited an increase in transport and a decrease in substrate specificity.^[2] After the substrate binds Cavity 1 and ATP molecules bind the NBDs, the to allow the substrate to enter Cavity 2. Once the substrate enters Cavity 2, the plug is able to reform and promotes substrate release and conversion to the inward-facing conformation.

Disease

Dysfunctions in ABCG2 is 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 the inhibition of ABCG2 to aid in cancer treatment, efforts have gone towards the development of specific inhibitors of ABCG2 and other ABC transporters. Inhibitors were derived from fungal toxin fumitremorgin C; however, many of those developed have neurotoxic effects.^[4]

and act as competitive inhibitors against ABCG2 substrates. Depending on the size of the inhibitor, one or two molecules can bind to "wedge and clog" the transporter locking it in the inward-facing conformation. Inhibitors form many of the same binding interactions as substrates in Cavity 1.