Proton-coupled sugar transport in the prototypical major facilitator superfamily protein XylE

Abstract

The major facilitator superfamily is the largest collection of structurally related membrane proteins that transport a wide array of substrates. The proton-coupled sugar transporter XylE is the first member of the MFS that has been structurally characterized in multiple transporting conformations including both the outward and inward facing states. Here we report the crystal structure of XylE in a new inward-facing open conformation, allowing us to visualize the rocker-switch movement of the N-domain against the C-domain during the transport cycle. Using molecular dynamics simulation, and functional transport assays, we describe the movement of XylE that facilitates sugar translocation across a lipid membrane and identify the likely candidate proton coupling residues as the conserved Asp27 and Arg133. This study addresses the structural basis for proton-coupled substrate transport and release mechanism for the sugar porter family of proteins.

Structural Biology of the Major Facilitator Superfamily Transporters

“…Until 2013, none of the proteins had been captured in more than one conformational state (Figure 2) (115). Since then, however, structures for more than one conformational state of the D-xylose:proton symporter XylE (84,110), LacY (60), the nitrate:nitrite antiporter NarU (114), and the melibiose:cation symporter MelB (19) have been obtained ( Table 2).…”

Shared Molecular Mechanisms of Membrane Transporters

“…The first crystal structures describing the MFS fold were those of the lactose-H + symporter LacY and the glycerol-3-phosphate-phosphate antiporter GlpT (56,57). Since then, the structures of MFS transporters that recognize sugars (5,54,(58)(59)(60)(61)(62), oligopeptides (63)(64)(65)(66)(67), nitrate (68)(69)(70)(71)(72), phosphate (73), and xenobiotics (74,75) have been determined in one or several distinct conformations. The symmetrically related bundles of six TM segments form the characteristic V-and -like structures in, respectively, the outward-and inward-facing conformations (29) (Figure 3b).…”

Protonation of Glu 135 Facilitates the Outward-to-Inward Structural Transition of Fucose Transporter

“…According to the alternating access model (17), transporters require both outward-open and inward-open conformations as well as some intermediate states (e.g., the occluded conformations) to fulfill a complete cycle of substrate transport. Until now, crystal structures from several orthologs have been successively determined, and the full transport cycle of MFS transporters could be presumably assembled from these homologous structures (6,8,9,(12)(13)(14)(15)(16)(24)(25)(26).…”