Figure 2.

Structure of gp21. (a) Ribbon diagram of the gp21 monomer with the coiled coil-forming helix magenta, the disulfide-bonded domain CX₆CC green, the C-terminal helix cyan, and the rest gray. The side chains of Met-338, Gly-343, Leu-346, and Val-350 with unusual packing (yellow), the a and d residues in the coiled coil (magenta), the hydrophobic residues in the base of the coiled coil (gray), and the cysteine residues (green) are shown. (b) As in a but showing trimer. (c) Ribbon diagram of MoMLV TM trimer (8) with the coiled coil-forming helix magenta and the disulfide-bonded domain CX₆CC green. The side chains of the a and d residues in the coiled coil (magenta), the hydrophobic residues in the base of the coiled coil (gray), and the cysteine residues (green) are shown. (d) Ribbon diagram of HIV-1 gp41 trimer (10) with the coiled coil-forming helix magenta and the C-terminal helix cyan. The side chains of Gln-41 in the glutamine-rich layer (yellow), and the a and d residues in the coiled coil (magenta) are shown. GCN4 region is not shown. (e) Ribbon diagram of the TBHA₂ trimer (26) with the coiled coil-forming helix magenta, the C-terminal helix cyan, the HA1 residues blue, and the rest gray. The side chains of Thr-59 with x-type packing in a region where loop-to-helix transition occurs at low pH (yellow), and the a and d residues in the coiled coil (magenta) are shown. (f) Molecular surface (probe radius 1.4 Å) of the trimeric coiled coil of gp21 (residues 338–387) color-coded according to surface complementarity (50) with the C-terminal segment (residues 390–421). Red, S_c (shape correlation statistic) > 0.76; yellow, 0.76 > S_c > 0.3; green, 0.3 > S_c > −0.3; light blue, S_c < −0.3. The overall S_c equals 0.65. The C-terminal segment is shown in magenta, with the interacting residues (contacts < 4 Å) in white. Drawn by grasp (51).