When a quiescent monolayer of CV-1 (African green monkey kidney) cells is infected with simian virus 40 (SV40), the cells are induced into the cell cycle. The infected cells progress through G 1 , S, and G 2 phases but do not enter mitosis (17,35,58). Rather, the DNA content of the infected cells increases beyond 4C (defined as ϾG 2 phase) because of replication of both viral and cellular DNA. By 48 h postinfection (hpi), the total DNA content per cell has increased to 10C to 12C. The majority of viral DNA is replicated in ϾG 2 phase and accounts for 20 to 30% of the total cellular DNA content (34). Progression into ϾG 2 involves both of the early gene products of SV40. Large-tumor antigen (large T) is essential for viral DNA replication (65) and for progression into ϾG 2 phase (11, 12). Small-tumor antigen (small t) is not required for SV40 DNA replication, but the rate of viral DNA replication (8,16,60) and the rate of entry into ϾG 2 are slower in the absence of small t (70).One goal in understanding the altered cell cycle regulation during SV40 lytic infection is to define the mechanisms responsible for the absence of mitosis in infected cells. Mitosis in uninfected proliferating cells is controlled by mitosis-promoting factor (MPF), a heterodimer of cyclin B and the cyclindependent kinase Cdc2 (62, 64). The ability of MPF to induce mitosis is regulated by the nuclear-cytoplasmic localization of the cyclin B subunit and phosphorylation of Cdc2. During interphase, cyclin B is actively transported out of the nucleus by a CRM1-mediated export mechanism while the Wee1 and Myt1 kinases phosphorylate the T14 and Y15 residues of Cdc2 to inhibit catalytic activity. During mitotic initiation, cyclin B1 is phosphorylated by Plk1 and localizes in the nucleus during prophase. MPF is activated when the T14 and Y15 residues of Cdc2 are dephosphorylated by the dual-specificity phosphatases Cdc25C and Cdc25B.

Simian virus 40 (SV40) infection of quiescent monkey kidney cells stimulates two successive rounds of cellular DNA synthesis without an intervening mitosis. This uncoupling of S phase and mitosis indicates that SV40 modulates pathways regulating the G2‐to‐M phase transition. To examine the integrity of mitotic initiation pathways in infected cells that have bypassed mitosis, SV40‐infected CV‐1 cells were treated with okadaic acid (OA), a known inducer of premature mitosis in other cell types. OA treatment triggered the appearance of the mitotic marker MPM‐2 in SV40‐infected CV‐1 cells progressing through either the first (diploid) or second (tetraploid) S phases. These results demonstrate that a subset of mitotic pathways are intact but inactive in SV40‐infected cells that have bypassed mitosis and initiated tetraploid S phase. Cytometry 31:260–264, 1998. © 1998 Wiley‐Liss, Inc.

Simian virus 40 (SV40) infection of quiescent monkey kidney cells stimulates two successive rounds of cellular DNA synthesis without an intervening mitosis. This uncoupling of S phase and mitosis indicates that SV40 modulates pathways regulating the G2-to-M phase transition. To examine the integrity of mitotic initiation pathways in infected cells that have bypassed mitosis, SV40-infected CV-1 cells were treated with okadaic acid (OA), a known inducer of premature mitosis in other cell types. OA treatment triggered the appearance of the mitotic marker MPM-2 in SV40-infected CV-1 cells progressing through either the first (diploid) or second (tetraploid) S phases. These results demonstrate that a subset of mitotic pathways are intact but inactive in SV40-infected cells that have bypassed mitosis and initiated tetraploid S phase.