THE SOLUTION STRUCTURE OF THE AMINO-TERMINAL HHCC DOMAIN OF HIV-2 INTEGRASE - A 3-HELIX BUNDLE STABILIZED BY ZINC

Background: Integrase mediates a crucial step in the life cycle of the human immunodeficiency virus (HIV). The enzyme cleaves the viral DNA ends in a sequence-dependent manner and couples the newly generated hy droxyl groups to phosphates in the target DNA. Three domains have been identified in HIV integrase: an amino-terminal domain, a central cata lytic core and a carboxy-terminal DNA-binding domain. The amino-termin al region is the only domain with unknown structure thus far. This dom ain, which is known to bind zinc, contains a HHCC motif that is conser ved in retroviral integrases. Although the exact function of this doma in is unknown, it is required for cleavage and integration. Results: T he three-dimensional structure of the amino-terminal domain of HIV-2 i ntegrase has been determined using two-dimensional and three-dimension al nuclear magnetic resonance data. We obtained 20 final structures, c alculated using 693 nuclear Overhauser effects, which display a backbo ne root-mean square deviation versus the average of 0.25 Angstrom, for the well defined region. The structure consists of three alpha helice s and a helical turn. The zinc is coordinated with His12 via the N-eps ilon 2 atom, with His16 via the N-delta 1 atom and with the sulfur ato ms of Cys40 and Cys43. The alpha helices form a three-helix bundle tha t is stabilized by this zinc-binding unit. The helical arrangement is similar to that found in the DNA-binding domains of the trp repressor, the prd paired domain and Tc3A transposase. Conclusion: The amino-ter minal domain of HIV-2 integrase has a remarkable hybrid structure comb ining features of a three-helix bundle fold with a zinc-binding HHCC m otif. This structure shows no similarity with any of the known zinc-fi nger structures. The strictly conserved residues of the HHCC motif of retroviral integrases are involved in metal coordination, whereas many other well conserved hydrophobic residues are part of the protein cor e. (C) Current Biology Ltd ISSN 0960-9822.