Sequential deletion of the integrase (Gag-Pol) carboxyl terminus reveals distinct phenotypic classes of defective HIV-1.

A requisite step in the life cycle of human immunodeficiency virus type 1 (HIV-1) is the insertion of the viral genome into that of the host cell, a process catalyzed by the 288-amino-acid (32-kDa) viral integrase (IN). IN recognizes and cleaves the ends of reverse-transcribed viral DNA and directs its insertion into the chromosomal DNA of the target cell. IN function, however, is not limited to integration, as the protein is required for other aspects of viral replication, including assembly, virion maturation, and reverse transcription. Previous studies demonstrated that IN is comprised of three domains: the N-terminal domain (NTD), catalytic core domain (CCD), and C-terminal domain (CTD). Whereas the CCD is mainly responsible for providing the structural framework for catalysis, the roles of the other two domains remain enigmatic. This study aimed to elucidate the primary and subsidiary roles that the CTD has in protein function. To this end, we generated and tested a nested set of IN C-terminal deletion mutants in measurable assays of virologic function. We discovered that removal of up to 15 residues (IN 273) resulted in incremental diminution of enzymatic function and infectivity and that removal of the next three residues resulted in a loss of infectivity. However, replication competency was surprisingly reestablished with one further truncation, corresponding to IN 269 and coinciding with partial restoration of integration activity, but it was lost permanently for all truncations extending N terminal to this position. Our analyses of these replication-competent and -incompetent truncation mutants suggest potential roles for the IN CTD in precursor protein processing, reverse transcription, integration, and IN multimerization.