3 NEW STRUCTURES OF THE CORE DOMAIN OF HIV-1 INTEGRASE - AN ACTIVE-SITE THAT BINDS MAGNESIUM

HIV-1 integrase is an essential enzyme in the life cycle of the virus, responsible for catalyzing the insertion of the viral genome into the host cell chromosome; it provides an attractive target for antiviral drug design. The previously reported crystal structure of the HIV-1 in tegrase core domain revealed that this domain belongs to the superfami ly of polynucleotidyltransferases. However, the position of the conser ved catalytic carboxylic acids differed from those observed in other e nzymes of the class, and attempts to crystallize in the presence of th e cofactor, Mg2+, were unsuccessful. We report here three additional c rystal structures of the core domain of HIV-1 integrase mutants, cryst allized in the presence and absence of cacodylate, as well as complexe d with Mg2+. These three crystal forms, containing between them seven independent core domain structures, demonstrate the unambiguous extens ion of the previously disordered helix alpha 4 toward the amino termin us from residue M154 and show that the catalytic E152 points in the ge neral direction of the two catalytic aspartates, D64 and D116, In the vicinity of the active site, the structure of the protein in the absen ce of cacodylate exhibits significant deviations from the previously r eported structures. These differences can be attributed to the modific ation of C65 and C130 by cacodylate, which was an essential component of the original crystallization mixture. We also demonstrate that in t he absence of cacodylate this protein will bind to Mg2+, and could pro vide a satisfactory platform for binding of inhibitors.