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Rad53 is an essential protein kinase governing DNA damage and replication stress checkpoints in budding yeast. It also appears to be involved in cellular morphogenesis processes. Mass spectrometry analyses revealed that Rad53 is phosphorylated at multiple SQ/TQ and at SP/TP residues, which are typical consensus sites for phosphatidylinositol 3-kinase-related kinases and CDKs, respectively. Here we show that Clb-CDK1 phosphorylates Rad53 at Ser(774) in metaphase. This phosphorylation event does not influence the DNA damage and replication checkpoint roles of Rad53, and it is independent of the spindle assembly checkpoint network. Moreover, the Ser-to-Asp mutation, mimicking a constitutive phosphorylation state at site 774, causes sensitivity to calcofluor, supporting a functional linkage between Rad53 and cellular morphogenesis.

Haspin is a protein kinase identified in mouse and human cells, and genes coding for haspin-like proteins are present in virtually all eukaryotic genomes sequenced so far. Two haspin homologues, called Alk1 and Alk2, are present in the yeast Saccharomyces cerevisiae. Both Alk1 and Alk2 exhibit a weak auto-kinase activity in vitro, are phosphoproteins in vivo and are hyperphosphorylated in response to DNA damage. The amount and modification of the two proteins is greatly regulated during the cell cycle. In fact, Alk1 and Alk2 levels peak in mitosis and late-S/G2, respectively, and phosphorylation of both proteins is maximal in mitosis. Control of protein stability plays a major role in Alk2 regulation. The half-life of Alk2 is particularly short in G1; mutagenesis and genetic analysis indicate that its degradation is controlled by the APC pathway. Overexpression of ALK2, but not of ALK1, causes a mitotic arrest, which is correlated to the kinase activity of the protein. This finding, together with its cell cycle regulation, suggests a role for Alk2 in the control of mitosis.