SIMIAN-VIRUS-40 PREVENTS ACTIVATION OF M-PHASE-PROMOTING FACTOR DURING LYTIC INFECTION

Simian virus 40 (SV40) infection stimulates confluent cultures of monkey kidney cells into successive rounds of cellular DNA synthesis without intervening mitosis. As an initial step in defining the mechanisms responsible for viral inhibition of mitosis, M-phase-promoting factor (MPF) was examined in SV40-infected CV-1 cells passing from G(2) phase into a second S phase. MPF is a serine-threonine protein kinase that is essential for mitosis in eukaryotic cells. In SV40 infected cells exiting G(2) phase, there was a reduced amount of MPF-associated H1 kinase activity relative to that of uninfected cells passing through mitosis. Both subunits of MPF, cyclin B and the p34(cdc2) catalytic subunit, were present and in a complex in infected cells. In uninfected cultures, passage through mitosis was associated with the dephosphorylation of the p34(cdc2) subunit, which is characteristic of MPF activation. In contrast, the p34(cdc2) subunit remained in the tyrosine-phosphorylated, inactive form in SV40-infected cells passing from G(2) phase into a second S phase. These results suggest that although the MPF complex is assembled and modified normally, SV40 interferes with pathways leading to MPF activation.