Elucidating the structural features of ABCA1 in its heterogeneous membrane environment

Abstract

ABCA1 plays an integral part in Reverse Cholesterol Transport (RCT) and is critical for maintaining lipid homeostasis. One theory of lipid efflux by the transporter (alternating access) proposes that ABCA1 harbors two different conformations that provide alternate access for lipid binding and release, leading to sequestration via a direct interaction between ABCA1 and its partner, ApoA1. The alternative theory (lateral access) proposes that ABCA1 obtains lipids laterally from the membrane to form a temporary extracellular reservoir containing an isolated pressurized lipid monolayer caused by the net accumulation of lipids in the exofacial leaflet. Recently, a full-length Cryo-EM structure of this 2,261-residue transmembrane protein showed its discreetly folded domains and conformations, as well as detected the presence of a tunnel enclosed within ECDs. While the tunnel was wide enough at the proximal end for accommodating passage of lipids, the distal end displayed substantial narrowing, making it inaccessible for ApoA1. Therefore, this structure was hypothesized to substantiate the lateral access theory, whereby ApoA1 obtained lipids from the proximal end. Utilizing long time-scale multiple replica atomistic molecular dynamics simulations (MDS), we simulated the structure in a heterogeneous lipid environment and found that along with several large conformational changes, the protein widens enough at the distal end of its ECD tunnel to now enable lipid accommodation. In this study we have characterized ABCA1 and the lipid dynamics along with the protein-lipid interactions in the heterogeneous environment, providing novel insights into understanding ABCA1 conformation at an atomistic level.