Picky ABCG5/G8 and promiscuous ABCG2 - a tale of fatty diets and drug toxicity

Abstract

Structural data on ABCG5/G8 and ABCG2 reveal a unique molecular architecture for subfamily G ATP-binding cassette (ABCG) transporters and disclose putative substrate-binding sites. ABCG5/G8 and ABCG2 appear to use several unique structural motifs to execute transport, including the triple helical bundles, the membrane-embedded polar relay, the re-entry helices, and a hydrophobic valve. Interestingly, ABCG2 shows extreme substrate promiscuity, whereas ABCG5/G8 transports only sterol molecules. ABCG2 structures suggest a large internal cavity, serving as a binding region for substrates and inhibitors, while mutational and pharmacological analyses support the notion of multiple binding sites. By contrast, ABCG5/G8 shows a collapsed cavity of insufficient size to hold substrates. Indeed, mutational analyses indicate a sterol-binding site at the hydrophobic interface between the transporter and the lipid bilayer. In this review, we highlight key differences and similarities between ABCG2 and ABCG5/G8 structures. We further discuss the relevance of distinct and shared structural features in the context of their physiological functions. Finally, we elaborate on how ABCG2 and ABCG5/G8 could pave the way for studies on other ABCG transporters.

Keywords: ABCG2; ABCG5; ABCG8; ATP-binding cassette; cholesterol efflux; membranes; multidrug resistance; polar relay; structural biology.

Fig. 1

Sequence alignment of mammalian ABCGs. (A) MSA was analyzed using ClustalX2. Conserved residues are highlighted with the conservation scale as the height of gray bars at the bottom of each residue. Conserved regions in the NBD are highlighted in the pink boxes, including A‐loop, Walker A, Q‐loop, hot spot helix, Signature loop, Pro‐loop, Walker B, D‐loop, and H‐loop, respectively. (B) Conserved regions in the TMD. The elbow connecting helix (blue box), TMH1‐TMH6 (yellow boxes), ICL1 coupling helix (light blue box), leucine valve (red box), re‐entry helix (green box), respectively. (C) The putative topology model of mammalian ABCGs. Colors are as in panel B. The putative substrate binding is highlighted as yellow in TMH2, TMH3, and TMH5.

Fig. 2

Common structural features and differences among ABCG2, ABCG5, and ABCG8. (A) Structural elements characteristic to the ABCG subfamily are indicated in ABCG2 (PDBID 5NJ3). The transporter is shown in side‐view as a functional dimer across the dimer interface as a monomer. (B) Sequence conservation among the human ABCG family members projected on the monomeric ABCG2 structure oriented as in panel A. Highly conserved regions are shown as thick purple tubes with a decreasing conservation toward yellow using the Clustal consensus score [112]. (C‐E) Zoom of the polar relay residues (acidic—red, basic—blue, polar—green) are shown as sticks on the monomeric cartoon representation of ABCG2 (C), ABCG5 (D), and ABCG8 (E). (F) Overlay of TMH5 and the immediately following loop that forms a hydrophobic valve across the transporter symmetry axes. Valve residues are shown as sticks and colored red for ABCG2, orange for ABCG5, and purple for ABCG8. (G) Overlay of ABCG2 and ABCG5/G8 fitted on TMH5, highlighting the hydrophobic valve residues from the extracellular side, color coded as in panel F.

Fig. 3

Localization of disease‐causing mutations and SNPs in ABCG2, ABCG5, and ABCG8. The positions of disorder‐related polymorphisms or mutations are highlighted as the colored spheres on the structures of ABCG5 (PDBID 5D07, chain A), ABCG8 (PDBID 5D07, chain B), and ABCG2 (PDBID 5NJ3). The color code is based on the structural motifs as shown in Fig. 2; NBD interface (orange sphere), elbow connecting helix (blue spheres), polar relay (purple spheres), and re‐entry helix (green sphere); otherwise, other residues are in black sphere, respectively. All details are indicated in Table 1.

Fig. 4

Proposed transport mechanism. ABCG transporters rest in the inward‐facing state. Substrate selectivity is determined by binding zone residues, which partially overlap with the polar relay in the core. Substrates (S) can enter either from the cytoplasm or from the inner leaflet of the lipid membrane. Substrate recognition may trigger ATP and substrate binding, followed by a transporter switch to the outward‐facing conformation. Substrate is extruded either into the extracellular medium or to the membrane outer leaflet. ATP hydrolysis resets the transporter to the inward‐facing state ready to bind either ATP or substrates if available.