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Cdc37 is a molecular chaperone whose clients are predominantly protein kinases, many of which are important in cell-cycle progression. Temperature-sensitive mutants of cdc37 in Schizosaccharomyces pombe are lethal at the restrictive temperature, arresting cell division within a single cell cycle. These mutant cells elongate during incubation at the restrictive temperature, consistent with a cell-cycle defect. The cell-cycle arrest arises from defective function of the mutant Cdc37 proteins rather than a reduction in Cdc37 protein levels. Around 80% of the arrested, elongated cells contain a single nucleus and replicated (2C) DNA content, indicating that these mutants arrest the cell cycle in G2 or mitosis (M). Cytological observations show that the majority of cells arrest in G2. In fission yeast, a G2 cell-cycle arrest can arise by inactivation of the cyclin-dependent kinase (Cdk) Cdc2 that regulates entry into mitosis. Studies of the cdc37 temperature-sensitive mutants show a genetic interaction with some cdc2 alleles and overexpression of cdc2 rescues the lethality of some cdc37 alleles at the restrictive temperature, suggesting that Cdc2 is a likely client for the Cdc37 molecular chaperone. In cdc37 temperature-sensitive mutants at the restrictive temperature, the level of Cdc2 protein remains constant but Cdc2 protein kinase activity is greatly reduced. Inactivation of Cdc2 appears to result from the inability to form complexes with its mitotic cyclin partner Cdc13. Further evidence for Cdc2 being a client of Cdc37 in S. pombe comes from the identification of genetic and biochemical interactions between these proteins.

Cdc37 is a molecular chaperone that interacts with a range of clients and co-chaperones, forming various high molecular mass complexes. Cdc37 sequence homology among species is low. High homology between yeast and metazoan proteins is restricted to the extreme N-terminal region, which is known to bind clients that are predominantly protein kinases. We show that despite the low homology, both Saccharomyces cerevisiae and human Cdc37 are able to substitute for the Schizosaccharomyces pombe protein in a strain deleted for the endogenous cdc37 gene. Expression of a construct consisting of only the N-terminal domain of S. pombe Cdc37, lacking the postulated heat-shock protein (Hsp) 90-binding and homodimerization domains, can also sustain cellular viability, indicating that Cdc37 dimerization and interactions with the cochaperone Hsp90 may not be essential for Cdc37 function in S. pombe. Biochemical investigations showed that a small proportion of total cellular Cdc37 occurs in a high molecular mass complex that also contains Hsp90. These data indicate that the N-terminal domain of Cdc37 carries out essential functions independently of the Hsp90-binding domain and dimerization of the chaperone itself.