STRUCTURAL ROLE OF THE 30S LOOP IN DETERMINING THE LIGAND SPECIFICITYOF THE HUMAN-IMMUNODEFICIENCY-VIRUS PROTEASE

The structural basis of ligand specificity in human immunodeficiency v irus (HIV) protease has been investigated by determining the crystal s tructures of three chimeric HIV proteases complexed with SB203386, a t ripeptide analogue inhibitor. The chimeras are constructed by substitu ting amino acid residues in the HIV type 1 (HIV-1) protease sequence w ith the corresponding residues from HIV type 2 (HIV-2) in the region s panning residues 31-37 and in the active site cavity. SB203386 is a po tent inhibitor of HIV-1 protease (K-i = 18 nM) but has a decreased aff inity for HIV-2 protease (K-i = 1280 nM). Crystallographic analysis re veals that substitution of residues 31-37 (30's loop) with those of HI V-2 protease renders the chimera similar to HIV-2 protease in both the inhibitor binding affinity and mode of binding (two inhibitor molecul es per protease dimer). However, further substitution of active site r esidues 47 and 82 has a compensatory effect which restores the HIV-1-l ike inhibitor binding mode (one inhibitor molecule in the center of th e protease active site) and partially restores the affinity. Compariso n of the three chimeric protease structures with those of HIV-1 and SI V proteases complexed with the same inhibitor reveals structural chang es in the flap regions and the 80's loops, as well as changes in the d imensions of the active site cavity, The study provides structural evi dence of the role of the 30's loop in conferring inhibitor specificity in HIV proteases.