Structure of  the hydantoin permease Mhp1

We have recently solved the structure of Mhp1, the hydantoin permease from Microbacterium liquifaciens with and without substrate bound (Weyand et al. Science 322, 709-13, 2008). In the absence of substrate the protein contains a large cavity open to the outer face of the membrane. Upon substrate binding a conformational change in one of the helices blocks the entrance of the cavity. Comparison of these structures with those of other proteins with similar folds indicates a clear mechanism for transport utilising the internal symmetry of the protein and providing a structural basis for the alternating access model of membrane transport.

The alternating access mechanism of membrane transport is well established. In this model the substrate binding site has alternate access to the outside and the inside of the cell. The steps in the mechanism include the following conformations of the protein: 1) outward-facing open; 2) outward-facing occluded; 3) inward-facing occluded and 4) inward facing open. Mhp1 contains 12-transmembrane helices as shown in the diagram. Helices 1-5 (red to yellow) and helices 6-10 (green to blue) are inverted repeats of the same structural unit. The structure of Mhp1 without substrate bound is in conformation 1. When the substrate binds helix 10 folds over the ligand blocking its exit to the outside of the cell resulting in conformation 2. For the alternating access mechanism there then needs to be a larger conformational change to the protein to open up a channel towards the interior of the cell. The galactose transporter vSGLT contains a similar fold to Mhp1 but is in conformational state 3. Comparison of this structure with the two structures of Mhp1 suggests a mechanism for the transition to the inward-facing conformation utilising the internal repeat unit of the protein. This involves helices 3-5 and their equivalents in the second repeat unit 8-10. This is shown in the accompanying movies.

The movies have been made based on the two crystal structures of Mhp1 and a model of Mhp1 in an inward conformation based on the structure of vSGLT. They first show the benzyl-hydantoin entering the protein from the outside of the cell. Helix 10 then packs over the substrate before the transition is made to the inward conformation. Note that helix 8 (turquoise), 9 (light blue) and 10 (blue, split) of the second repeat are equivalent to helices 3 (orange), 4 (yellow/orange), and 5 (yellow) of the first.