The cyclin B-dependent protein kinase Cdk1 is a master regulator of mitosis and phosphorylates numerous proteins on the minimal consensus motif Ser/Thr-Pro (S/T-P). At least in several proteins, however, not well-defined motifs lacking a Pro in the +1 position, referred herein to as non-S/T-P motifs, have been shown to be phosphorylated by Cdk1. Here we show that non-S/T-P motifs in fact form consensus sequences for Cdk1 and probably play roles in mitotic regulation of physiologically important proteins. First, we show, by in vitro kinase assays, that previously identified non-S/T-P motifs all harbour one or more C-terminal Arg/Lys residues essential for their phosphorylation by Cdk1. Second, using Arg/Lys-scanning oriented peptide libraries, we demonstrate that Cdk1 phosphorylates a minimal sequence S/T-X-X-R/K and more favorable sequences (P)-X-S/T-X-[R/K] 2-5 as its non-S/T-P consensus motifs. Third, on the basis of these results, we find that highly conserved linkers (typically, T-G-E-K-P) of C2H2 zinc finger proteins and a nuclear localization signal-containing sequence (matching P-X-S-X-[R/K] 5) of the cytokinesis regulator Ect2 are inhibitorily phosphorylated by Cdk1, well accounting for the known mitotic regulation and function of the respective proteins. We suggest that non-S/T-P Cdk1 consensus motifs identified here may function to regulate many other proteins during mitosis.
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