Fundamental Difference of Transport Mechanisms Between ABC Importers and Exporters

Abstract

ABC transporters use the energy of ATP to drive the active transport of various substrates across the membrane. All members of this superfamily share a common machinery of two nucleotide binding domains (NBDs) at their cytoplasmic side, which close and open in response to ATP binding and hydrolysis. Despite highly homologous NBDs, substrates can be either imported or exported in different subfamilies. Although crystal structures of ABC transporters have been resolved in different conformational/functional states, it is still puzzling how a common set of motor domains can drive transport in opposite directions. We use comparative molecular dynamics simulations of an ABC-importer (maltose transporter) and an ABC-exporter (P-glycoprotein), to investigate functionally relevant dynamical differences. The two transporters exhibit very different structural flexibility and fluctuations in their inward-facing states, especially with regard to the separation of the NBDs. Comparing the dynamics of NBDs, it becomes evident that ABC importers fluctuate significantly lower than ABC exporters, suggesting that the conformational ensembel obtained for the former represents a deeper energy well. In contrast, the NBDs of P-glycoprotein are able to reach near dimerized conformations even in the absence of the nucleotide due to the overall higher structural flexibility of the protein architecture. The dynamical difference between the two ABC transporters appears to present a fundamental mechanistic difference between the subfamilies they belong to, which can be described as different energy landscapes along their pathways for NBD dimerization. The difference in energy landscapes is evidenced by their transport activities in that the NBDs of the maltose transporter dimerize only in the presence of its substrate, whereas P-glycoprotein possesses a remarkable substrate-independent ATPase activity.