A family of cyclin homologs that control the G1 phase in yeast.

Abstract

Two Saccharomyces cerevisiae genes were isolated based upon their dosage-dependent rescue of a temperature-sensitive mutation of the gene CDC28, which encodes a protein kinase involved in control of cell division. CLNI and CLN2 encode closely related proteins that also share homology with cyclins. Cyclins, characterized by a dramatic periodicity of abundance through the cell cycle, are thought to be involved in mitotic induction in animal cells. A dominant mutation in the CLN2 gene, CLN2-1, advances the G1-to S-phase transition in cycling cells and impairs the ability of cells to arrest in G1 phase in response to external signals, suggesting that the encoded protein is involved in G, control of the cell cycle in Saccharomyces.Cyclins were initially identified in the embryos of a number of marine invertebrates as proteins that undergo dramatic fluctuations in abundance as a function of cell cycle progression (1, 2). In clams and sea urchins, where early embryonic cleavages occur with a high degree of synchrony, cyclin levels can be seen to peak at the onset of mitosis. During mitosis, cyclins are rapidly degraded but begin to accumulate during the subsequent interphase. Periodicity of cyclin levels results entirely from posttranslational regulation, as the cyclin mRNAs are abundant maternal species that are translated at a constant rate during early development. Based on the kinetics of cyclin accumulation and turnover, it was proposed that these proteins might be involved in triggering mitosis. The first evidence, however, that cyclins are rate limiting for mitotic induction was the demonstration that injection of cyclin-encoding mRNA could induce maturation ofXenopus oocytes (3), a process analogous to mitosis in that cells resting in meiotic prophase are induced to enter into meiotic divisions.In this report we describe a class of proteins homologous to cyclins from the budding yeast, Saccharomyces cerevisiae. The genes encoding these cyclin homologs were identified by using a genetic screen in which elevated expression of a heterologous sequence was required for rescue of a temperature-sensitive mutation in the gene CDC28 (4). CDC28 encodes the catalytic subunit of a protein kinase complex required for cell cycle initiation in Saccharomyces (5-8). The observation that cyclin hyperexpression can rescue temperature-sensitive cdc28 mutations implies a role for these yeast cyclins in the maintenance or regulation of the Cdc28 protein kinase complex. Furthermore, we show that a mutation in one of the cyclin genes confers a dominant pleiotropic phenotype characterized by advance of the G1-to S-phase transition in cycling cells as well as by loss of G, control of cell division in response to nutrient limitation. These results suggest that in budding yeast, a class of cyclins is rate-limiting for G1-to S-phase transition rather than for the induction of mitosis. MATERIALS AND METHODSStrains and Medium. Suppressor plasmids were initially isolated and subcloned by using S. cerevisiae strain JF210-92: MATa, trpi...

Yeast homolog of human SAG/ROC2/Rbx2/Hrt2 is essential for cell growth, but not for germination: chip profiling implicates its role in cell cycle regulation

“…Through F-box protein Cdc4, SCF promotes ubiquitination and degradation of SIC1, a negative regulator of G1 progression, leading to activation of Cdc28P/cyclin B that trigger the initiation of DNA synthesis Verma et al, 1997). On the other hand, through F box protein Grr1, SCF induces ubiquitination of G1 cyclin CLN1/2 to limit their accumulation (Deshaies et al, 1995; Hadwiger et al, 1989). Thus, as a newly identi®ed activating component of SCF ubiquitin ligase, SAG/ROC/Rbx/ Hrt could promote G1?S progression Tyers and Willems, 1999).…”

Human cDNAs encoding homologs of the small p34Cdc28/Cdc2-associated protein of Saccharomyces cerevisiae and Schizosaccharomyces pombe.

“…A property of the S. pombe Sucl protein and the S. cerevisiae Cksl protein is that they bind to CdcZ/Cdc28 Draetta et al 1987;Draetta and Beach 1988;Dunphy et al 1988;Hadwiger et al 1989b). Both Sucl/Cksl antibodies and Sucl-Sepharose beads have been used to adsorb Cdc2/Cdc28 protein kinase activity.…”

“…These cyclins are presumed activators of the Cdc28/Cdc2 protein kinase in the G2-to M-phase transition (Swenson et al 1986;Beach 1987, 1988;Booher et al 1989;Lehner and O'Fartell 1989;Minshull et al 1989;Murray and Kirschner 1989b;Murray et al 1989). Recently, a second class of cyclins, the G~-specific cyclins, involved in activation of Cdc28/Cdc2 kinase in the G~-to S-phase transition, have been identified in the budding yeast S. cerevisiae (Cross 1988;Nash et al 1988;Hadwiger et al 1989b;Richardson et al 1989;Wittenberg et al 1990).…”

Proliferating cell nuclear antigen and p21 are components of multiple cell cycle kinase complexes.

“…Each CDK requires the association of a cyclin that serves as a positive regulatory subunit for the kinase activity. In both budding and fission yeasts, a single catalytic subunit, p34cDc /cDc28, is required for both the G1/S and G2/M transitions through association with either Gl cyclins (CLN-type) or mitotic cyclins (B-type) (Booher and Beach, 1987;Hadwiger et al, 1989;Wittenberg et al, 1990;Bueno et al, 1991;Ghiara et al, 1991;Surana et al, 1991). In vertebrates, whereas p34CDC2 plays an essential role in mitosis (Riabowol et al, 1989; reviewed by Draetta, 1990), the function of p34CDC2 in Gl and S phases appears to be mediated not by a single kinase but by a family of CDC2-related proteins.…”