A transient precursor of the HIV-1 protease. Isolation, characterization, and kinetics of maturation

Crystallographic analysis of human immunodeficiency virus 1 protease with an analog of the conserved CA-p2 substrate -- interactions with frequently occurring glutamic acid residue at P2' position of substrates

Human immunodeficiency virus type 1 (HIV-1) protease hydrolysis of the Gag CA-p2 cleavage site is crucial for virion maturation and is optimal at acidic pH. To understand the processing of the CA-p2 site, we have determined the structure of HIV-1 protease complexed with an analog of the CA-p2 site, the reduced peptide inhibitor Arg-Val-Leu-r-Phe-Glu-Ala-Ahx-NH2 [r denotes the reduced peptide bond and Ahx 2-aminohexanoic acid (norleucine), respectively]. The crystal structure was refined to an R-factor of 0.17 at 0.21-nm resolution. The crystals have nearly the same lattice as related complexes in P2(1)2(1)2(1) which have twofold disordered inhibitor, but are in space group P2(1). and the asymmetric unit contains two dimers of HIV-1 protease related by 180 degrees rotation. An approximate non-crystallographic symmetry has replaced the exact crystal symmetry resulting in well-ordered inhibitor structure. Each protease dimer binds one ordered inhibitor molecule, but in opposite orientations. The interactions of the inhibitor with the two dimers are very similar for the central P2 Val to P2' Glu residues, but show more variation for the distal P3 Arg and P4' Ahx residues. Importantly, the carboxylate oxygens of Glu at P2' in the inhibitor are within hydrogen-bonding distance of a carboxylate oxygen of Asp30 of the protease suggesting that the two side chains share a proton. This interaction suggests that the enzyme-substrate complex is additionally stabilized at lower pH. The importance of this interaction is emphasized by the absence of polymorphisms of Asp30 in the protease and variants of P2' Glu in the critical CA-p2 cleavage site.

Kinetic properties of saquinavir-resistant mutants of human immunodeficiency virus type 1 protease and their implications in drug resistance in vivo

In order to study the basis of resistance of human immunodeficiency virus, type 1 (HIV-1), to HIV-1 protease inhibitor saquinavir, the catalytic and inhibition properties of the wild-type HIV-1 protease and three saquinavir resistant mutants, G48V, L90M, and G48V/L90M, were compared. The kinetic parameter kcat/Km was determined for these proteases using eight peptide substrates whose sequences were derived from the natural processing site sequences of HIV-1. The kcat/Km values were determined using conventional steady-state kinetics as well as initial velocities of mixed substrate cleavages under the condition where the substrate concentrations [S]o << Km. The independently determined kcat and Km values for some of the substrates confirmed the accuracy of the mixed-substrate method and also permitted the calculation in all cases of true rather than relative kcat/Km values. The Ki values were also determined. Using a previously described kinetic model [Tang, J., & Hartsuck, J. A. (1995) FEBS Lett. 367, 112-116], the relative processing activities of HIV-1 protease variants were estimated in the saquinavir concentration range of 0-10(-7) M. Although the protease activity of G48V, L90M, and G48V/L90M are only about 10, 7, and 3% of that of the wild-type HIV-1 protease in the absence of inhibitor, the resistance tendencies of the three mutants are clearly manifest by relatively less activity loss as inhibitor concentration becomes higher. Also, the ratios of the activities of the four protease species at certain saquinavir concentrations appear to correlate with the population ratios of the four protease species at different time points of clinical trials. This correlation suggests that the population ratio of the protease species is driven by in vivo saquinavir concentration, which appears to be in the range 10(-10)-10(-9) M during the clinical trials.

Influence of flanking sequences on the dimer stability of human immunodeficiency virus type 1 protease

The maturation of the human immunodeficiency virus type 1 protease (PR) from the Gag-Pol polyprotein is dependent on the intrinsic proteolytic activity of the dimeric Gag-Pol. Herein, we report the kinetics and conformational stabilities of two unique fusion proteins of the protease. In one, X28-PR, a random sequences of 28 amino acids (X28) was linked to the N terminus of the mature protease. In the second construct, X28-delta TF*PR*delta Pol, X28 is fused to the protease which is flanked at both its termini by short sequences (delta) which correspond to the native sequences of the Gag-Pol precursor. Autoprocessing of the latter protein was prevented by inserting an Ala at the native protease cleavage sites. The measured kinetic parameters and the pH-rate profile of both enzymes are nearly identical to those of the mature protease. However, these fusion proteins are more sensitive to acid and urea denaturation than the mature protease. The decrease in the conformational stability of X28-PR and X28-delta TF*PR*delta Pol is reflected by increases in their apparent dissociation constants (Kd) from < 5 nM to approximately 180 and 25 nM, respectively. These results suggest that subunit interactions and hence the dimer stability of the protease domain in the Gag-Pol polyprotein differ from those of the mature protease. The high Kd of X28-PR further suggests that addition of non-native sequences to the N terminus of the protease destablizes the dimer.