High-resolution structure of unbound human immunodeficiency virus 1 subtype C protease: implications of flap dynamics and drug resistance

The X-ray crystal structure of the unbound state of human immunodeficiency virus 1 (HIV-1) subtype C protease (C PR) has been determined to 1.20 Å resolution in the tetragonal space group P4₁2₁2, with one monomer per asymmetric unit and unit-cell parameters a = 46.7, c = 100.8 Å, allowing full anisotropic least-squares refinement. The refined model has a conventional R factor of 14.1% for all reflections and estimated standard deviations in bond lengths and angles for all main-chain non-H atoms of 0.014 Å and 0.030°, respectively. The structure is compared with three unbound subtype B proteases (B PRs) to identify structural changes arising from the naturally occurring polymorphisms and delineate their implications in antiretroviral drug resistance/susceptibility. The unbound C PR exhibits a larger distance between the tips of the flaps, a downward displacement of the 36-41 loop and an increased thermal stability of the 10s loop when compared with the B PR structures. The C PR structure presents the highest resolution of the unbound state of a non-subtype-B PR and adds to the understanding of flap dynamics and drug resistance.