Protein-protein interaction surfaces can exhibit structural plasticity, a mechanism whereby an interface adapts to mutations as binding partners coevolve. The HIV-1 envelope glycoprotein gp120-gp41 complex, which is responsible for receptor attachment and membrane fusion, represents an extreme example of a coevolving complex as up to 35% amino acid sequence divergence has been observed in these proteins among HIV-1 isolates. In this study, the function of conserved gp120 contact residues, Leu593, Trp596, Gly597, Lys601, and Trp610 within the disulfide-bonded region of gp41, was examined in envelope glycoproteins derived from diverse HIV-1 isolates. We found that the gp120-gp41 association function of the disulfide-bonded region is conserved. However, the contribution of individual residues to gp41 folding and/or stability, gp120-gp41 association, membrane fusion function, and viral entry varied from isolate to isolate. In gp120-gp41 derived from the dual-tropic isolate, HIV-189.6, the importance of Trp596 for fusion function was dependent on the chemokine receptor utilized as a fusion cofactor. Thus, the engagement of alternative chemokine receptors may evoke distinct fusion-activation signals involving the site of gp120-gp41 association. An examination of chimeric glycoproteins revealed that the isolate-specific functional contributions of particular gp120-contact residues are influenced by the sequence of gp120 hypervariable regions 1, 2, and 3. These data indicate that the gp120-gp41 association site is structurally and functionally adaptable, perhaps to maintain a functional glycoprotein complex in a setting of host selective pressures driving the rapid coevolution of gp120 and gp41.