A cooperative folding unit in HIV-1 protease. Implications for protein stability and occurrence of drug-induced mutations.

We investigated the HIV-1 protease molecule for the occurrence of cooperative folding units, i.e. structural units that exhibit a relatively stronger protection against unfolding than do other parts of the molecule. Calculated unfolding penalties are used to delineate folding units. This procedure identifies a folding core in HIV-1 protease, based on an ensemble of denatured states derived from native structures, comprising a spatially close unit of residues 84-91, 74-78 and 22-32, the last of which contains the active site residues D25, T26 and G27. Observed enzyme mutations of HIV-1 protease, either naturally occurring or induced by drug therapy, are found in regions that are not structurally designed to withstand unfolding. These mutations are especially likely to occur in the flap region, a part of the protein which is not essential for the stability of the protein, but does contribute significantly to the stability of protease-drug complexes. A similar avoidance of structurally protected regions in the reverse transcriptase enzyme is also observed.