A cyclin-dependent protein kinase homologue associated with the basal body domains in the ciliate Tetrahymena thermophila

The tight coupling between cell cycle progression and morphogenetic development in the unicellular ciliates presents a unique model system for examination of the roles of Cdks in developmental processes. We here describe the isolation and characterization of the first cyclin-dependent kinase (Cdk) homologue, TtCdk1, from Tetrahymena thermophila. TtCdk1 corresponds to the larger of the two polypeptides recognized by anti-PSTAIRE antibody in a whole cell lysate, which differ from each other in their affinity for yeast p13(suc1) protein. In contrast to the constant protein expression levels of typical eukaryotic Cdks, the TtCdk1 protein level fluctuates periodically over the vegetative cell cycle, reaching a maximum at the end of the cell cycle, correlating with its histone H1 kinase activity. Its association with the membrane-skeletal domains that surround mature, but not nascent, basal bodies in the cell cortex suggests that TtCdk1 plays a role in the regulation of cortical morphogenesis in T. thermophila. A partial TtCDK1 knockout cell line constructed through somatic biolistic transformation resulted in a reduction of the regularity of the rows of basal bodies plus an additional effect on chromatin condensation in both macro- and micronuclei. Unlike the situations in higher eukaryotic cells, no apparent effect on basal body duplication was found upon disruption of the TtCDK1 gene.