Multiple Membrane-Compatible Conformations of an Substrate-Binding Component of ECF Transporters

Abstract

Energy-coupling factor (ECF) transporters compose of a unique subfamily of the vast ATP-binding cassette (ABC) transporters. Unlike other ABC transporters, the basic architecture of ECF transporters is not symmetric, that their membrane sections include two structurally unrelated units: an energy-coupling component EcfT and a substrate-binding component EcfS. The distinctive composition of ECF transporters is likely to associate with a completely novel transport mechanism. Although several crystal structures of different isolated EcfS have been resolved, the structure of a full ECF transporter complex was available only very recently. Surprisingly, the EcfS orientation in the full ECF transporter is almost perpendicular to an ideal orientation should the EcfS exist as an isolated transmembrane protein. Based on the crystal structures, it is proposed that the transport mechanism of ECF transporters involves the reorientation of EcfS in membrane. Starting with the crystal structure of the folate ECF transporter, we have performed a series of MD simulations to evaluate the stability of EcfS with different orientations in membrane, either isolated or within the full ECF transporter. It is found that the substrate-free EcfS is able to maintain the membrane-parallel orientation regardless of the EcfT binding. That is, in addition to the ideal orientation for an isolated EcfS, the membrane-parallel orientation can be stabilized by the partition of hydrophilic residues near the membrane surface, as well as the accommodation of several charged residues near the membrane center by an enclosed lipid head group in the putative substrate binding site. The tilt angles of the two identified stable orientations of EcfS differ by ∼50 degrees, and the membrane-parallel form shows slightly higher structural deviations due to repartitioned N- and C-terminal helices near the membrane surface.