Mechanism of ABC transporters: A molecular dynamics simulation of a well characterized nucleotide-binding subunit

ATP-binding cassette (ABC) transporters are membrane-bound molecular pumps that form one of the largest of all protein families. Several of them are central to phenomena of biomedical interest, including cystic fibrosis and resistance to chemotherapeutic drugs. ABC transporters share a common architecture comprising two hydrophilic nucleotide-binding domains (NBDs) and two hydrophobic transmembrane domains (TMDs) that form the substrate pathway across the membrane. The conformational changes in the NBDs induced by ATP hydrolysis and the means by which they are transmitted to the TMDs to effect substrate translocation remain largely unknown. We have performed a molecular dynamics simulation of HisP, the well studied NBD of the bacterial histidine permease, to identify hinges and switches of the NBD conformational transitions and subunit-subunit interfaces. This analysis reveals that the TMDs regulate ATP hydrolysis by controlling conformational transitions of the NBD helical domains, and identifies the conformational changes and the crucial TMD:NBD interface, by which the energy of ATP hydrolysis is transmitted to the TMDs. We also define the conformational transitions of the Q-loop, a key element of the NBD mechanism, and identify pathways by which Q-loop switching is coordinated with TMD and NBD conformational changes. We propose a model for the catalytic cycle of ABC transporters that shows how substrate-binding and transport by the TMDs may be coordinated and coupled with ATP binding and hydrolysis in the NBDs.