Characterization of a novel cdk1-related kinase

The pl3(sucl1)/p9(CKShS) proteins bind tightly to the cyclin-dependent kinases cdkl and cdk2. The distantly related protein, pl5(cdk-Bp) binds cdk4/6, cdk5 and cdk8. We now show that immobilized pl5(cdk-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 In 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 cdkl and metaphase dephosphorylated cdkl; (b) in contrast with cdkl, cdkX and associated HMG-I kinase activity do not decrease following successive depletions on p9(CKShS1)-sepharose; (c) cdkX and associated HMG-I kinase activity, but not cdkl, decrease following depletions on immobilized inhibitor; (d) cdkX is expressed during the early development of sea urchin embryos; in contrast with cdkl/cyclin B kinase, the pl5(cdk-bp)-bound HMG-I kinase is active throughout the cell cycle; compared with cdkl it is active later in development; (e) pl5(cdk-BP)-bound HMG-T kinase is essentially insensitive to powerful inhibitors of cdk such as purvalanol, roscovitine, olomoucine, p21(ciP1) and pl6(IMK4A); HD is only moderately inhibitory. Altogether these results suggest the existence of a new cdkl-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.