Retroviral cDNA integration: Stimulation by HMG I family proteins

To replicate, a retrovirus must synthesize a cDNA copy of the viral RNA gen ome and integrate that cDNA into a chromosome of the host. We have investig ated the role of a host cell cofactor, HMG I(Y) protein, in integration of human immunodeficiency virus type 1 (HIV-1) and Moloney murine leukemia vir us (MoMLV) cDNA. Previously we reported that HMG I(Y) cofractionates with H IV-1 preintegration complexes (PICs) isolated from freshly infected cells. PICs depleted of required components by treatment with high concentrations of salt could be reconstituted by addition of purified HMG I(Y) in vitro. H ere we report studies using immunoprecipitation that indicate that HMG I(Y) is associated with MoMLV preintegration complexes. In mechanistic studies, we show for both HIV-1 and MoMLV that each HMG I (Y) monomer must contain multiple DNA binding domains to stimulate integration by HMG I(Y)-depleted PICs. We also find that HMG I(Y) can condense model HIV-1 or MoMLV cDNA in vitro as measured by stimulation of intermolecular ligation. This reaction, like reconstitution of integration, depends on the presence of multiple DN A binding domains in each HMG I(Y) monomer. These data suggest that binding of multivalent HMG I(Y) monomers to multiple cDNA sites compacts retrovira l cDNA, thereby promoting formation of active integrase-cDNA complexes.