Molecular Basis of the “Alternate Access Model” in the Cation-Substrate Symporter Mhp1 from Computer Simulations and X-Ray Crystallography

Abstract

A common structural motif is emerging for a wide class of substrate-cation symporters. The fold of the amino acid-sodium symporter LeuT is shared by proteins unrelated by sequence identity such as the galactose-sodium symporter vSGLT, the nucleobase-cation symporter Mhp1, the betaine-transporting osmoregulator BetP, and amino acid-proton transporters AdiC, and ApcT. The “alternating access” model explains transport as cycling between at least three distinct conformational states that connect a central binding site to either the extracellular or the intracellular compartment. The crystal structures solved so far can be broadly categorized in these three conformations, outward facing (LeuT, Mhp1, BetP, AdiC), occluded (Mhp1, BetP, ApcT), and inward facing (vSGLT). We are currently studying the crystal structure of Mhp1 hydantoin transporter from Microbacterium liquefaciens in the inward facing open state. Together with the previous structures [1] a full picture of the conformational change occurring during transport emerges. Dynamic importance (DIMS) molecular dynamics (MD) simulations allow us to connect these three states with continuous transition trajectories. The combination of structural and simulation data puts the alternate access model on a firm structural basis and will facilitate future detailed studies of the energetics of cation-substrate coupled transport.We acknowledge support from the BBSRC and the EU EDICT consortium.[1] Weyand et al. (2008) Science 322, 709-713.