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. 2022 Aug 16;13:4812. doi: 10.1038/s41467-022-32437-3

Fig. 7. Structural features of ABCA1 and ABCA4 may contribute to their transporting substrates in opposite directions.

Fig. 7

Our model for substrate transport by ABCA1 proposes that the negatively charged phosphate group of PL in the outer leaflet of the membrane initially interacts with the positively charged residue K568. It then moves through the gateway (A, magenta) and into the elongated hydrophobic tunnel. The ATP-free structure of ABCA4 exhibits a structure analogous to the gateway (B, magenta). An extended loop, termed the S-loop (green, space-filling) because it binds N-retinylidene-phosphatidylethanolamine (B, yellow space-filling), resides adjacent to the gateway of ABCA4. The location of the S-loop in the outward-facing transmembrane cavity of ABCA4 may prevent movement of N-retinylidene-phosphatidylethanolamine into the gateway of ABCA4.