DIFFERENT OLIGOMERIC FORMS OF PROTEIN PHOSPHATASE-2A ACTIVATE AND INHIBIT SIMIAN-VIRUS 40 DNA-REPLICATION

The ability of simian virus 40 (SV40) large T antigen to catalyze the initiation of viral DNA replication is regulated by its phosphorylatio n state. Previous studies have identified the free catalytic subunit o f protein phosphatase 2A (PP2A(c)) as the cellular phosphatase which c an remove inhibitory phosphoryl groups from serines 120 and 123. The c atalytic C subunit exists in the cell complexed with a 65-kDa A subuni t and one of several B subunits. To determine if any of the holoenzyme s could activate T antigen, we tested the ability of the heterodimeric AC and two heterotrimeric ABC forms to stimulate T-antigen function i n unwinding the origin of SV40 DNA replication. Only free catalytic su bunit C and the heterotrimeric form with a 72-kDa B subunit (PP2A-T72) could stimulate T-antigen-dependent origin unwinding. Both the dimeri c form (PP2A-D) and the heterotrimer with a 55-kDa B subuuit (PP2A-T55 ) actively inhibited T-antigen function. We found that PP2A-T72 activa ted T antigen by dephosphorylating serines 120 and 123, while PP2A-D a nd PP2A-T55 inactivated T antigen by dephosphorylating the p34(cdc2) t arget site, threonine 124. Thus, alterations in the subunit compositio n of PP2A holoenzymes have significant functional consequences for the initiation of in vitro SV40 DNA replication. The regulatory B subunit s of PP2A may play a role in regulating SV40 DNA replication in infect ed cells as well.