T-lymphocyte–tropic HIV-1 infection of CXCR4-expressing cells.

(A) The gp120 subunit of an env oligomer binds to CD4, inducing conformational changes including increased exposure of the V3 loop. This enables env to interact with the appropriate coreceptor, in this case CXCR4 (B). This interaction is then proposed to trigger the final conformational changes in env, including the formation of a coiled–coil structure in gp41 that ensures that the fusion peptide inserts into the cell membrane (C).