An Inhibitory Monoclonal Antibody Binds at the Turn of the Helix-Turn-Helix Motif in the N-terminal Domain of HIV-1 Integrase*

With the increase in our understanding of its structure and enzymatic mechanism, HIV-1 integrase (IN) has become a promising target for designing drugs to treat patients with AIDS. To investigate the structure and function of IN, a panel of monoclonal antibodies (mAbs) directed against HIV-1 IN was raised and characterized previously in this laboratory. Among them, mAbs17, -4, and -33 were found to inhibit IN activity in vitro. In this study, we investigated the interaction of N-terminal-specific mAb17 and its isolated Fab fragment with full-length HIV-1 IN(1–288) and its isolated N-terminal, Zn2+-binding domain IN(1–49). Our results show that binding of Zn2+ to IN(1–49) stabilizes the mAb17-IN complex and that dimer dissociation is not required for binding of the Fab. To identify the epitope recognized by mAb17, we developed a protein footprinting technique based on controlled proteolysis and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Binding was mapped to a region within amino acids Asp25–Glu35. This peptide corresponds to the end of a helix-turn-helix motif in the IN(1–55) NMR structure and contributes to the dimerization of the N-terminal domain. Antibody binding also appears to destabilize the N-terminal helix in this domain. A molecular model of the [IN(1–49)]2·(Fab)1 complex shows Fab binding across the dimer protein and suggests a potential target for drug design. These data also suggest that mAb17 inhibits integrase activity by blocking critical protein-protein interactions and/or by distorting the orientation of the N-terminal α-helix. The relevance of our results to an understanding of IN function is discussed.