HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 NUCLEOCAPSID PROTEIN SPECIFICALLYSTIMULATES MG2-DEPENDENT DNA INTEGRATION IN-VITRO()

The integrase (IN) protein of the human immunodeficiency virus mediate s integration of the viral DNA into the cellular genome. In vitro, thi s reaction can be mimicked by using purified recombinant IN and model DNA substrates. IN mediates two reactions: an endonucleolytic cleavage at each 3' end of the proviral DNA (terminal cleavage) and the joinin g of the linear viral DNA to 5' phosphates in the target DNA (strand t ransfer). Previous investigators have shown that purified IN requires Mn2+ or Mg2+ to promote strand transfer in vitro, although Mg2+ is the likely metal cofactor in vivo. IN activity in the presence of Mg2+ in vitro requires high IN concentrations and low concentrations of salt. Here, we show that the viral nucleocapsid protein NCp7 allows efficie nt IN-mediated strand transfer in the presence of Mg2+ at low enzyme c oncentrations. This potentiating effect appears to be unique to NCp7, as other smalt DNA-binding proteins, while capable of stimulating inte gration in the presence of Mn2+, all failed to stimulate strand transf er in the presence of Mg2+.