FIG 4.

Comparison of wt V1/V2 loops and the V2 core mutant. The crystal structure of the V1/V2 loops from gp120 of strain ZM109 reveals that the V2 loop assumes a Greek key fold (PDB accession number 3U2S) (78). The Greek key fold is comprised of four antiparallel β-strands (designated a to d) connected by the V1 loop (dashed) and two small loops (designated L1 and L2), which are part of the V2 loop (black). Based upon this crystal structure, the V2 core mutant was constructed by using gp120 from the BaL strain (the same strain upon which all of the envelope mutants in this study are based). The V2 core mutant was constructed by removing most of the V1 loop and replacing the parts of the V2 loop that do not participate in the Greek fold (loop L2) with a Ser-Gly linker. Numbering refers to adjacent cysteine residues and is based on the BaL strain, with disulfide bonds shown as black bars. Shown are the V1 loop (dashed), the V2 loop (black), and disulfide bonds (black bars between cysteine residues). (A) Linearized schematic of the wt V1/V2 loop fold. (B) Schematic of the fold of the wt V1/V2 loops. (C) Schematic of the fold of the V2 core mutant. (D) Sequence of the V2 core mutant.