Cig2, a B-Type Cyclin, Promotes the Onset of S in Schizosaccharomyces pombe

Abstract

Cdc2, a catalytic subunit of cyclin-dependent kinases, is required for both the G 1 -to-S and G 2 -to-M transitions in the fission yeast Schizosaccharomyces pombe. Cdc13, a B-type cyclin, is required for the M-phase induction function of Cdc2. Two additional B-type cyclins, Cig1 and Cig2, have been identified in S. pombe, but none of the B-type cyclins are individually required for the onset of S. We report that Cdc13 is important for DNA replication in a strain lacking Cig2. Unlike ⌬cdc13 cells, double-mutant ⌬cdc13 ⌬cig2 cells are defective in undergoing multiple rounds of DNA replication. The conclusion that Cig2 promotes S is further supported by the finding that Cig2 protein and Cig2-associated kinase activity appear soon after the completion of M and peak during S, as well as the observation that S is delayed in ⌬cig2 cells as they recover from a G 1 arrest induced by nitrogen starvation. These studies indicate that Cig2 is the primary S-phase-promoting cyclin in S. pombe but that Cdc13 can effectively substitute for Cig2 in ⌬cig2 cells. These observations also suggest that the gradual increase in the activity of Cdc2-Cdc13 kinase can be sufficient for the correct temporal ordering of S and M phases in ⌬cig2 cells.A key aim of cell cycle studies is to understand how orderly cell cycle progression is regulated. One productive investigative approach has been to identify fission yeast mutants that have uncoupled the DNA replication (S) and nuclear division (M) phases (32). In the fission yeast Schizosaccharomyces pombe, a single cyclin-dependent kinase complex, consisting of the Cdc2 catalytic subunit bound to the Cdc13 cyclin B subunit, is essential for the induction of mitosis (4, 28). Like B-type cyclins of animal cells, Cdc13 accumulates during interphase and is rapidly degraded upon exit from M. Interestingly, ⌬cdc13 cells undergo multiple rounds of S without intervening M phases (18). This leads to the formation of gigantic cells that each have a huge nucleus. Studies have also shown that high overexpression of Cdc2 and Cdc13 can force the induction of mitosis in G 1 cells (18). These observations have led to the hypothesis that the presence of Cdc2-Cdc13 complex defines a cell as being in G 2 and effectively gives the cell no choice other than to initiate M. The acquisition of the opportunity to undergo DNA replication depends on the destruction of Cdc13 that normally occurs at the end of M.Cdc2 is required for both the G 1 -to-S and G 2 -to-M transitions, whereas Cdc13 is essential only for the onset of M (33). Since cyclin-dependent kinases are inactive unless bound to a cyclin, there must be at least one cyclin that is involved in promoting the onset of S in S. pombe. Genes encoding five additional cyclin-related proteins have been identified in S. pombe. Two of the genes, mcs2 ϩ and pch1 ϩ , encode cyclin C-like proteins having essential functions that do not appear to be directly related to cell cycle control (14,26). A third gene, puc1 ϩ , encodes an unusual type of cyclin that has no ascribed fu...

Negative Regulation of Cdc18 DNA Replication Protein by Cdc2

“…Our new findings raise the question of whether the Rum1 effect is due to inhibition of Cdc2-Cdc13 or Cdc2-Cig2. We favor the latter possibility, largely because Cdc2-Cig2 is more active than Cdc2-Cdc13 during S phase, at least when both complexes are assayed as histone H1 kinases in vitro (Mondesert et al, 1996). We propose that there is a rather brief window during S phase in which reduction of Cdc2 kinase activity will rescue cdc18-K9, and it is during this window that Cdc2-Cig2 kinase is most active.…”

“…However, previous studies indicated that Cdc2-Cig2 kinase is exclusively involved in promoting the onset of S phase from G 1 phase (Fisher and Nurse, 1996;Martin-Castellanos et al, 1996;Mondesert et al, 1996). Therefore, the association involving Cdc18 and Cig2 was inconsistent with the hypothesis that phosphorylation of Cdc18 catalyzed by Cdc2 has a purely inhibitory role.…”

“…The G 1 -S activity is predominantly provided by Cdc2-Cig2, whereas Cdc2-Cdc13 is required for the initiation of mitosis. However, in cells lacking the cig2 ϩ gene, Cdc2-Cdc13 is sufficient to promote the onset of S phase with only a modest delay relative to wild-type cells (Fisher and Nurse, 1996;Martin-Castellanos et al, 1996;Mondesert et al, 1996). Cdc2 is also implicated in the mechanism that prevents reinitiation of S phase from G 2 phase.…”

Fission Yeast Ste9, a Homolog of Hct1/Cdh1 and Fizzy-related, Is a Novel Negative Regulator of Cell Cycle Progression during G1-Phase

“…It was demonstrated that the Cig2 cyclin was a major partner of the Cdc2 kinase in the G 1 /S-phase in fission yeast (Fisher and Nurse, 1996;Martín-Castellanos et al, 1996;Mondesert et al, 1996). Cells harboring the deleted cig2 allele are viable but exhibit a delay in progression through G 1 to the S-phase in some situations, e.g., small cells generated by the wee mutations or nitrogen starvation.…”

Regulation of Schizosaccharomyces pombeWee1 Tyrosine Kinase

“…In animal cells this phosphorylation is carried out by Wee1 and Myt1 kinases, with the latter enzyme also phosphorylating the preceding threonine residue (7,(12)(13)(14)(15)(16)(17). Dephosphorylation of these residues and consequent activation of Cdc2 is largely carried out by Cdc25 dual specificity phosphatase (18 -24), although other phosphatases may weakly contribute to the activation of Cdc2-cyclin B (25,26).…”