Characterization of a novel cdk1-related kinase

The p13suc1/p9CKShs proteins bind tightly to the cyclin-dependent kinases cdk1 and cdk2. The distantly related protein, p15cdk-BP, binds cdk4/6, cdk5 and cdk8. We now show that immobilized p15cdk-BP binds both an HMG-I kinase and a 35-kDa protein that cross-reacts with anti-PSTAIRE antibodies (PSTAIRE is a totally conserved motif located in subdomain III of cdk). This 'cdkX' and the HMG-I kinase also bind to an immobilized inhibitor of cdks (HD). Several properties clearly distinguish cdkX, and its associated HMG-I kinase, from known anti-PSTAIRE cross-reactive cdks: (a) cdkX migrates, in SDS/PAGE, in a position intermediate between prophase phosphorylated cdk1 and metaphase dephosphorylated cdk1; (b) in contrast with cdk1, cdkX and associated HMG-I kinase activity do not decrease following successive depletions on p9CKShs1-sepharose; (c) cdkX and associated HMG-I kinase activity, but not cdk1, decrease following depletions on immobilized inhibitor; (d) cdkX is expressed during the early development of sea urchin embryos; in contrast with cdk1/cyclin B kinase, the p15cdk-BP-bound HMG-I kinase is active throughout the cell cycle; compared with cdk1 it is active later in development; (e) p15cdk-BP-bound HMG-I kinase is essentially insensitive to powerful inhibitors of cdk such as purvalanol, roscovitine, olomoucine, p21cip1 and p16INK4A; HD is only moderately inhibitory. Altogether these results suggest the existence of a new cdk1-related kinase, possibly involved in the regulation of early development. The presence of this kinase in all organisms investigated so far, from plants to mammals, calls for its definitive identification.