Cdc37 is a molecular chaperone that functions with Hsp90 to promote protein kinase folding. Analysis of 65 Saccharomyces cerevisiae protein kinases (∼50% of the kinome) in a cdc37 mutant strain showed that 51 had decreased abundance compared with levels in the wild-type strain. Several lipid kinases also accumulated in reduced amounts in the cdc37 mutant strain. Results from our pulse-labeling studies showed that Cdc37 protects nascent kinase chains from rapid degradation shortly after synthesis. This degradation phenotype was suppressed when cdc37 mutant cells were grown at reduced temperatures, although this did not lead to a full restoration of kinase activity. We propose that Cdc37 functions at distinct steps in kinase biogenesis that involves protecting nascent chains from rapid degradation followed by its folding function in association with Hsp90. Our studies demonstrate that Cdc37 has a general role in kinome biogenesis.

The kelch repeat-containing proteins Krh1p and Krh2p are negative regulators of the Gpa2p signaling pathway that directly interact with the G protein ␣-subunit Gpa2p in the yeast Saccharomyces cerevisiae. A screen was carried out to identify Gpa2p variants that are defective in their ability to bind Krh1p but retain the ability to bind another Gpa2p-interacting protein, Ime2p. This screen identified amino acids Gln-419 and Asn-425 as being important for the interaction between Gpa2p and Krh1p. Gpa2p variants with changes at these positions are defective for Krh1p binding in vivo. Cells containing these forms of Gpa2p display decreased heat shock resistance and increased expression of a gene required for pseudohyphal growth. These findings indicate that the substitutions at positions 419 and 425 confer a degree of constitutive activity to the Gpa2p ␣-subunit. Residues Gln-419 and Asn-425 are located in the ␤6-␣5 loop and ␣5 helix of Gpa2p, which is the region that couples receptor binding to guanine nucleotide exchange. The results suggest that binding of Gpa2p to Krh1p does not resemble the binding of G␣ subunits to either G␤ subunits or effectors, but it instead represents a novel type of functional interaction.