Two Distinct Classes of Mitotic Cyclin Homologues, Cyc1 and Cyc2, Are Involved In Cell Cycle Regulation In the Ciliate Paramecium Tetraurelia

Abstract

The eukaryotic cell cycle is regulated by the sequential activation of different CDK/cyclin complexes. Two distinct classes of mitotic cyclin homologues, CYC1 and CYC2, have been identified and cloned for the first time in the ciliate Paramecium. Cyc1 is 324 amino acids long with a predicted molecular mass of 38 kDa, whereas Cyc2 is 336 amino acids long with a predicted molecular mass of 40 kDa. They display 42-51% sequence identity to other eukaryotic mitotic cyclins within the 'cyclin box' region. The conserved 'cyclin box' and 'destruction box' elements can be identified within each of the sequences. Genomic Southern blot analysis indicated that the CYC1 gene has two isoforms, with 92.3% and 85.9% identify at the amino acid level and at the nucleotide level, respectively. Both Cyc1 and Cyc2 proteins showed characteristic patterns of accumulation and destruction during the vegetative cell cycle, with Cyc1 peaking at the point of commitment to division (PCD), and Cyc2 reaching the maximal level late in the cell cycle. Immunoprecipitation experiments with antibodies specific to Cyc1 and Cyc2 indicated that Cyc1 and Cyc2 associate with distinct CDK homologues. Both immunoprecipitates exhibited histone H1 kinase activity that oscillated in the cell cycle in parallel with the respective amount of cyclins present. Histone H1 kinase activity associated with Cyc1 reached a peak at PCD while Cyc2 showed maximal activity when about 75% cells have completed cytokinesis. We propose that Cyc1 may be involved in commitment to division, in association with the CDK that binds to p13suc1, Cdk3, and that the Cyc2/Cdk2 complex may regulate cytokinesis. PCR-amplification revealed similar sequences in Tetrahymena, Sterkiella, Colpoda and Blepharisma, suggesting the conservation of the cyclin genes within ciliates. Although cell cycle regulation in ciliates differs in some respects from that of other eukaryotes, the cyclin motifs have clearly been conserved during evolution.

The H1 Phosphorylation State Regulates Expression of CDC2 and Other Genes in Response to Starvation in Tetrahymena thermophila

“…This mechanism clearly differs from the conserved, cyclin-dependent phosphorylation cascade that regulates cdc2 kinase activity in growing eukaryotic cells. The possibility that growing Tetrahymena cells also have a novel mechanism for regulating cdc2 kinase activity seems unlikely, inasmuch as a related ciliate, Paramecium tetraurelia, contains at least two genes encoding cyclins involved in cell cycle regulation and PCR analyses suggest that similar genes are present in Tetrahymena (69), and a number of cyclin-related sequences can be found in the recently released sequence of the Tetrahymena macronuclear genome (http://tigrblast.tigr.org/er-BLAST/index .cgi?projectϭttg).…”

Riding The Ciliate Cell Cycle—A Thirty‐Five‐Year Prospectivea

“…Immunoprecipitates obtained with antibodies to either PtCycl or PtCyc2 showed histone H1 kinase activity (Fig. 20; Zhang and Berger 1999b). The activity associated with PtCyc2 occurred at the end of the cell cycle during cytokinesis.…”

“… Unrooted tree showing relation of Paramecium cyclins (box with dashed line) to A‐type and B‐type mitotic cyclins of various organisms (from Zhang and Berger 1999b). …”

“…D. Histone kinase activity of the CDK2‐Cyc2 complex. The relative positions of initiation of macronuclear DNA synthesis (IDS), the point of commitment to division (PCD), and cytokinesis are indicated (data from Zhang and Berger 1999b).…”

Purification of GVBD‐Inducing Protein from the Ciliate Tetrahymenathermophila

“…In P. tetraurelia (Tang, Pelech, and Berger 1995), the gene of a homologue was cloned. Recently, cyclin genes were cloned in P. tetraurelia , and a cyclin‐like sequence was also found in T. thermophila by PCR‐amplification (Zhang, Adl, and Berger 1999). These studies suggest that complexes of cyclin and cyclin‐dependent kinases (CDKs) are also involved in the control of transitions between the phases of the cell cycle in ciliates.…”