Cyclin-dependent kinase-like proteins in pea nuclei: Their presence and role in cell proliferation

Although many putative cdk (cyclin-dependent kinase) homologue genes have been identified in higher plants, their function and involvement in cell proliferation are still unclear. In this work we investigated the presence and distribution of cdk-like proteins in root tip meristem nuclei at different germination times (before, during, and after the onset of cell proliferation) and in nuclei of differentiated leaves. Nuclear cdk-like proteins were found in the root meristem throughout seed germination with a higher amount in actively proliferating cells, but were not detected in differentiated leaf. Characterization of the detected pea cdk-like proteins by immunoblotting led to the identification of two specific principal proteins of 33.2 and 34 kDa with the cdk conserved motif PSTAIRE. The p33.2 protein was also recognized by the anti-human p33(cdk2) antibody, suggesting that the p33.2 and p34 proteins could be pea homologues of human p33(cdk2) and p34(cdk1), involved in the G(1)-S and G(2)-M transitions, respectively. Additional analysis of pea cdk protein localization has shown partial localization of these proteins at DNA replication sites during the G(1) to S transition. These microscopical and biochemical data support the hypothesis that, in pea nuclei as in mammals, many PSTAIRE-cdks are present with different functions related to cell proliferation, one of which is probably involved in the control of the G(1)-S transition.