Principles of Energy Transduction and Alternating Access of ABC Exporters

Abstract

ATP binding cassette (ABC) transporters to drive the energetically uphill translocation of substrates including the export of amphiphilic molecules out of the cell by harnessing the energy of ATP hydrolysis. Dedicated motor domains, the nucleotide binding domains (NBDs), bind and turn ATP over to power conformational rearrangements in the transmembrane domain (TMD) which binds and transports substrates. Although numerous investigations have defined the architecture and described the ATP-coupled conformational rearrangements for a number of ABC transporters, a general model of the transduction of ATP energy to the work of substrate remains elusive. We utilized double electron-electron resonance (DEER) and molecular dynamics (MD) simulations to describe the ATP- and substrate-coupled conformational cycle of the mammalian ABC efflux transporter P-glycoprotein (Pgp), or ABCB1, which has been extensively studied owing to its role in the clearance of xenobiotics and clinical implications in cancer resistance to chemotherapy. Pairs of spin labels were introduced at residues selected to track the putative inward-facing (IF) to outward-facing (OF) transition. Our findings illuminate how ATP energy is harnessed in the NBDs in a two-stroke cycle and elucidate the consequent conformational motion that reconfigures the TMD, two critical aspects of Pgp transport mechanism. Along with a novel, fully atomistic model of the OF conformation in membrane, the insight into Pgp conformational dynamics harmonizes mechanistic and structural data into a novel perspective on ATP-coupled transport. When considered in the context of previous studies of ABC exporters, these findings reveal mechanistic divergence within the efflux class of ABC transporters.