Phylogenetic Analyses of Proteins Coordinating G2 Size Control in Fission Yeast
Regulation of G2 phase is based on inhibition of MPF (M-phase Promoting Factor) through phosphorylation by Wee1-like kinases. Removal of the inhibiting phosphate group requires Cdc25-like phosphatases. In fission yeast, size control is achieved by monitoring cell length via interactions of Pom1, Nif1, Cdr1 and Cdr2 proteins, regulating MPF via the Wee1 kinase. Here, a search for homologues of these key proteins was performed in the genomes of several model organisms to analyze the evolution of G2 size control. Both the known upstream pathways regulating Wee1 protein (Pom1 → Cdr2, and Nif1 → Cdr1) have been found to be characteristic only in fission yeasts. Mik1, a backup copy of Wee1 kinase probably appeared in the common ancestor of the fission yeasts. The duplication resulting in Wee1A and Wee1B isoforms probably happened in a common ancestor of higher animals, while the Myt1 protein (found only in animals) could be a variant between an ancient serine / threonine kinase and the Wee1 tyrosine kinase. Probably both the ancestors of plants and that of fungi may have lost the myt1 gene. In fission yeasts, Pyp3 is a backup phosphatase of Cdc25, also activating MPF in late G2. Interestingly, we found that the small Ibp1 phosphatase appeared to be a closer homologue of Cdc25, although its function is different. Moreover, Cdc25 homologues identified in plants were found to be more closely related to Ibp1 rather than to Cdc25 of fission yeast. In the Cdc25-like proteins, a novel conserved region was found with the consensus sequence LxxG(Y/F).