Heterologous expression of the human cyclin-dependent kinase inhibitor p21Cip1 in the fission yeast, Schizosaccharomyces pombe reveals a role for PCNA in the chk1+ cell cycle checkpoint pathway.

Fission yeast cells expressing the human gene encoding the cyclin-dependent kinase inhibitor protein p21Cip1 were severely compromised for cell cycle progress. The degree of cell cycle inhibition was related to the level of p21Cip1 expression. Inhibited cells had a 2C DNA content and were judged by cytology and pulsed field gel electrophoresis to be in the G2 phase of the cell cycle. p21Cip1 accumulated in the nucleus and was associated with p34cdc2 and PCNA. Thus, p21Cip1 interacts with the same targets in fission yeast as in mammalian cells. Elimination of p34cdc2 binding by mutation within the cyclin-dependent kinase binding domain of p21Cip1 exaggerated the cell cycle delay phenotype. By contrast, elimination of PCNA binding by mutation within the PCNA-binding domain completely abolished the cell cycle inhibitory effects. Yeast cells expressing wild-type p21Cip1 and the mutant form that is unable to bind p34cdc2 showed enhanced sensitivity to UV. Cell cycle inhibition by p21Cip1 was largely abolished by deletion of the chk1+ gene that monitors radiation damage and was considerably enhanced in cells deleted for the rad3+ gene that monitors both DNA damage and the completion of DNA synthesis. Overexpression of PCNA also resulted in cell cycle arrest in G2 and this phenotype was also abolished by deletion of chk1+ and enhanced in cells deleted for rad3+. These results formally establish a link between PCNA and the products of the rad3+ and chk1+ checkpoint genes.