X-ray crystal structures of Mhp1. (a) X-ray crystal structures of
Mhp1 in the inward-facing (Protein Data Bank (PDB) 2X79),46 outward-occluded (PDB 4D1B),47 and outward-facing
(PDB 2JLN)48 conformations. Helices are represented as cylinders.
The bundle46 helices (TMHs 1, 2, 6, and
7) are colored red, the hash46 motif helices
(TMHs 3, 4, 8, and 9) are colored yellow, the flexible helices (TMHs
5 and 10) are colored blue, the C-terminal helices (TMHs 11 and 12)
and the surface extracellular and cytoplasmic helices are colored
gray. The Cys residues are shown in cyan. (b, c) Location of Cys327
(cyan) in the (b) inward-facing and (c) outward-facing conformations
of Mhp1, showing that TMH9 protects Cys327 from solvent in the outward-facing
conformation. (d) Average side-chain solvent-accessible surface area
(SASA) values of the three Cys residues of Mhp1 in 20 ns MD simulations
started from the inward-facing (IF), outward-occluded (OOc), and outward-facing
(OF) crystal structures.