HIV gp120 is specifically cleaved at a single site in the V, loop between Arg315 and Ala316 by thrombin. Previous observations by others have indicated that binding to CD4 enhances the rate of Vg loop cleavage, and that this cleavage is a prerequisite for HIV infection. Other observations also suggest that the cleavage site is in a type II B-turn centered at Pro313 -Gly314. However, our docking experiments indicate that this conformation cannot dock to thrombin and other trypsin-like serine proteases. Thus, based on the thrombin-bound conformation of peptide substrates, we propose that CD4 binding, at a site remote from the V, loop, induces and stabilizes a trans to cis isomerization of the highly conserved residue Pro"', and that this conformational shift is a prerequisite for cleavage by a 'thrombin-like' cellular protease and subsequent infection.