Wee1 kinase differentially regulates maize CDKA2;1a and CDKB1;1

In eukaryotes, cyclin-dependent kinase/cyclin (CDK/Cyc) complexes regulate cell cycle progression by phosphorylating hundreds of substrates. Full CDK activation requires the phosphorylation of a conserved threonine residue and association with a cyclin that helps discriminate between different substrates. Kinase activity of the CDK/cyclin complex is also negatively regulated by phosphorylation of two conserved residues in the CDK (T14 and Y15 in human CDK1). This regulatory mechanism is particularly important in vertebrates for ensuring successful mitosis and is mediated by Wee1 kinase, an enzyme also conserved in plants, in which Wee1 function has been poorly studied. To investigate the conservation of Wee1 function in maize, we have studied the effect of Wee1 on maize CDKA2;1a and CDKB1;1 in vitro, two of the main CDKs that control cell cycle progression in plants. Unlike results reported for A. thaliana, we found that maize Wee1 phosphorylated CDKB1;1, but interestingly, inhibited kinase activity of CDKA2;1a and CDKB1;1 independently of its kinase activity; this inhibition was substrate-dependent. Previously, we reported that maize CDKs also participate in substrate recognition, and therefore, we suggest that Wee1 associates with CDKs and modifies their recognition of substrates. Additionally, CDKB1;1 phosphorylation by Wee1 inhibited autophosphorylation at the threonine residue necessary for its activation and also differentially affected substrate phosphorylation. Finally, both CDKA2;1a and CDKB1;1 acted on Wee1, differentially modifying its phosphorylation. Together, and differing from previous reports, our results show that in vitro, maize Wee1 modulates the kinase activity of CDKs through several mechanisms and does not only act as an inhibitor.