Department of Zoology, University of British Columbia, 6270 University Boulevard, Vancouver, British Columbia V6T 1Z4, Canada
ABSTRACT. Cyclin-dependent kinases (CDKs) and associated proteins play essential roles in eukaryotic cell cycle progression. We report the identification and isolation of the gene encoding a novel class of cyclin-dependent protein kinase, Cdk2, and the characterization of its role in the Paramecium vegetative cell cycle control. It is 301 amino acids long, 7 amino acids short than Cdk1, the CDK that is associated with macronuclear DNA synthesis. All the catalytic domains typical of protein kinases can be located within the sequence and putative regulatory phosphorylation sites equivalent to Thr14, Tyr15 and Thr161 in human CDK1 are also conserved. In the "PSTAIRE’ region, 16/16 amino acids are perfectly conserved. Cdk2 shares only 48% homology to Cdk1 at the amino acid level, suggesting the evolutionary separation of Cdk1 and Cdk2 is ancient and implying that they have different roles in cell cycle regulation. Like Cdk1, Cdk2 does not bind to yeast p13suc1, even though it has better conservation of p13suc1 binding sites than Cdk1 does. The Cdk2 protein level is relatively constant throughout the vegetative cell cycle, but drops in starving cells. Cdk2 exhibits kinase activity towards bovine histone H1 in vitro with the maximal activity at cytokinesis, suggesting Cdk2 may be involved in the regulation of cytokinesis. Our results further support the view that an analogue of the p34cdc2 cell cycle regulatory system like that of yeast and higher eukaryotic cells operates in Paramecium and that a family of p34cdc2-related kinases may control different aspects of the Paramecium cell cycle.
Supplementary key words. cell cycle regulation, histone H1 kinase, p13suc1.