Lipid transport mechanisms in human ABCA family transporters: a structural perspective.

ATP-binding cassette (ABC) transporters are essential membrane proteins that couple ATP hydrolysis to move diverse substrates across lipid bilayers through large-scale conformational changes. In humans, 48 ABC transporters span seven subfamilies (A-G); within these, the ABCA subfamily mediates cellular lipid handling in contexts ranging from neural function to pulmonary surfactant production, and its dysfunction contributes to human disease from cardiovascular disorders to Alzheimer's. These diverse physiological roles all depend on precise lipid translocation within or across membrane systems, a shared principle that is often underemphasized in broad "lipid-transporter" classifications. This review summarizes the structural landscape of the ABCA family and re-examines the mechanistic insights that have emerged. We compare and contrast transport models derived from detergent-solubilized and lipid-embedded structures, with particular emphasis on lipid-embedded ABCA7, which supports a membrane-integrated mechanism in which the bilayer itself contributes to the transport pathway. We highlight shared rigid-body transitions, outline open questions surrounding transport directionality and protein-lipid coupling, and suggest that future models should treat the membrane not merely as a passive scaffold but as an integral component of the transport mechanism, while recognizing that membrane-integrated behavior is currently established structurally only for ABCA7 and remains a working hypothesis for other family members.