FIG 6.
Structure and function of the H stalk. (A, left) Fusion-support function of the Cys substitution mutants indicated on a schematic/structure-based model (24) of the H stalk. From the bottom, the membrane-proximal amino acids (59 to 74) are indicated as a cylinder. The 4HB homology model which covers residues 75 to 127 is shown next in surface representation. Residues 128 to 138 are shown again as a cylinder followed by the head-proximal residues 139 to 154, which bifurcate to form two dimeric branches. Different colors denote the fusion score: red, 0; orange, 1; blue, 2; and green, 3. (Right) The tetramer-to-dimer ratio for each Cys mutant along the length of the stalk is indicated by the length of the horizontal bar. The ability to restore fusion by disulfide bond reduction is indicated using the same fusion score color scheme as that for the left panel. Gray bars represent mutants that were not tested for rescue of function by disulfide bond reduction. (B) Fusion support function of Ser/Ala mutants of a part of the central fusion activation segment mapped on the MeV stalk model (31). Shading denotes the fusion score: black, 0; dark gray, 1; gray, 2; and white, 3. (C, top) Sequence conservation of the same selected morbillivirus stalk residues as those depicted in Fig. 1B, with the fusion function color coded as described above for panel A. (Bottom) Multiple-sequence alignment of four paramyxovirus stalks. The MeV F-activation domain is delineated by a red line, and the residues are colored red. Residues comprising the PIV5 and NDV F-activation domains are also colored red. HN-stalk residues that interact with the HN heads based on the PIV5 (22) and NDV (20) heads-down crystal structures are shaded light purple.