OLIGOMERS OF THE CDC7 DBF4 PROTEIN-KINASE EXIST IN THE YEAST-CELL/

Cdc7/Dbf4 protein kinase is required for the initiation of DNA replica tion in Saccharomyces cerevisiae. Cdc7/Dbf4 protein kinase is not a cy clin-dependent kinase (CDK), but is regulated in a similar fashion in that the Cdc7 kinase subunit is inactive in the absence of the regulat ory subunit Dbf4. In contrast to what is known about CDKs, Cdc7/Dbf4 p rotein kinase is shown to be an oligomer in the cell in this report. G enetic data that support this claim include interallelic complementati on between several cdc7ts alleles and the cdc7T281A allele and also th e results of experiments using the two-hybrid system with Cdc7 in both DNA-binding and transactivation domain plasmids. A molecular interact ion between two different Cdc7 molecules was shown by using a HA-tagge d Cdc7 protein that differs in size from the wild-type Cdc7 protein: a n anti-HA antibody immunoprecipitates both proteins in appproximately equal stoichiometry. Analysis of the native molecular weight of Cdc7/D bf4 protein kinase is consistent with oligomerization of the Cdc7 prot ein in that complexes of about 180 and 300 kDa were found. Oligomers o f Cdc7 protein may exist for the purpose of allosteric regulation or t o allow phosphorylation of multiple substrate protein molecules.