Abstract
ABC transporters are a superfamily of enzyme pumps that hydrolyse ATP in exchange for translocation of substrates across cellular membranes. Architecturally, ABC transporters are a dimer of transmembrane domains coupled to a dimer of nucleotide binding domains (NBDs): the NBD dimer contains two ATP-binding sites at the intersubunit interface. A current controversy is whether the protomers of the NBD dimer separate during ATP hydrolysis cycling, or remain in constant contact. In order to investigate the ABC ATPase catalytic mechanism, MD simulations using the recent structure of the ADP+Pi-bound MJ0796 isolated NBD dimer were performed. In three independent simulations of the ADP+Pi/apo state, comprising a total of >0.5 µs, significant opening of the apo (empty) active site was observed; occurring by way of intrasubunit rotations between the core and helical subdomains within both NBD monomers. In contrast, in three equivalent simulations of the ATP/apo state, the NBD dimer remained close to the crystal structure, and no opening of either active site occurred. The results thus showed allosteric coupling between the active sites, mediated by intrasubunit conformational changes. Opening of the apo site is exquisitely tuned to the nature of the ligand, and thus to the stage of the reaction cycle, in the opposite site. In addition to this, in also showing how one active site can open, sufficient to bind nucleotide, while the opposite site remains occluded and bound to the hydrolysis products ADP+Pi, the results are consistent with a Constant Contact Model. Conversely, they show how there may be no requirement for the NBD protomers to separate to complete the catalytic cycle.
Highlights
ABC transporters comprise the major superfamily of primary active membrane translocases
We have proposed a Constant Contact Model in which a closed/open asymmetry between the active sites is mediated by relative rotations between the core and helical subdomains [30,36]
Rotation of the helical subdomain engaging the empty active site contributed toward its opening
Summary
ABC transporters comprise the major superfamily of primary active membrane translocases. They are found in all kingdoms of life, and are engaged in multifarious cellular processes [1,2,3]. A subgroup of ABC exporters mediate resistance to structurally diverse chemotherapeutic drugs; a phenomenon known as multidrug resistance (MDR) [4,5,6]. Other ABC transporters are involved in serious and prevalent genetic diseases in humans, such as cystic fibrosis [7,8]. An understanding of the molecular mechanism of ABC transporters is critical to the development of treatments for a wide range of pathological conditions affecting human health, and productivity in agriculture
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