Fig. 1.

Structure of M2TM′. (A) The backbone of three monomers is drawn in light gray, and the pore-lining side chain groups are shown as sticks. From the N-terminus (viral exterior) to the C-terminus (viral interior), these are: the Val27 valve (blue), Ser31 (dark gray), the His-box (orange), the Trp-basket (purple), and the Asp/Arg-box (light blue). Water molecules belonging to the entry, bridging, and exit clusters are represented by red spheres. Black lines indicate the observed water–protein H-bonds. The uppermost dimer of waters in the entry cluster exists in two nearly equally occupied configurations in the crystal lattice, related by a 90° rotation down the central axis of the channel. Only one of the two orientations is shown in the figure. In the exit cluster the fifth water molecule (showing a high Debye–Waller factor) is drawn transparent and with a larger radius. The wireframe represents the peak region of the diffuse electron density detected right under the Val27 valve. (B) A closer view of the His-box surrounded by the entry and bridging clusters. (C) A closer view of the exit cluster.