Multi-Kinase Phosphorylation of the APC/C Activator Cdh1 Revealed by Mass Spectrometry

by activating a large ubiquitin ligase called the anaphase-promoting complex, or cyclosome (APC/C). At the end of G1, APC/CCdh1 is inhibited by cyclin-dependent kinase (CDK) phosphorylation of Cdh1. The specific Cdh1 phosphorylation sites used to regulate APC/CCdh1 activity have not been directly identified. Here, we used a mass spectrometric approach to identify the in vivo phosphorylation sites on yeast Cdh1. Surprisingly, in addition to several expected CDK phosphorylation sites, we discovered numerous non-CDK phosphorylation sites. In total, at least 19 serine and threonine residues on Cdh1 are phosphorylated in vivo. Seventeen of these sites are located in the N-terminal half of Cdh1, outside the highly conserved WD40 repeats. The pattern of phosphorylation was the same when Cdh1 was purified from yeast cultures arrested in S, early M and late M. Mutation of CDK consensus sequences eliminated detectable phosphorylation at many of the non-CDK sites. In contrast, mutation of non-CDK sites had no signif...