Pyp3 Involvement in Mitotic Control and Cell Growth in Fission Yeast

Abstract

In fission yeast, Schizosaccharomyces pombe, tyrosine dephosphorylation of p34cdc2 is required for mitotic initiation and this reversible tyrosine phosphorylation plays a crucial role in the regulation of the cell cycle. The kinases wee1+, mik1+ and the phosphatases cdc25+, pyp3+ play opposite roles in the regulation of p34cdc2 activity. In cells lacking wee1, tyrosine phosphorylation of p34cdc2 is carried out by Mik1; in cells lacking cdc25+, the activity of the Pyp3 phosphatase is responsible for tyrosine dephosphorylation of p34cdc2 at the G2/M phase transition. Pyp3 was first identified as a high copy number plasmid suppressor of a temperature sensitive cdc25-22 mutation. Loss of pyp3+ function causes only a minor delay in the onset of mitosis in wild type cells, but is lethal in wee1- cdc25- double mutants suggesting an essential function in this genetic background. However, it is not clear whether Pyp3 plays a direct minor role in dephosphorylating Cdc2 when Cdc25 is fully functional. In order to examine the function and regulation of Pyp3, the yeast two-hybrid system was used to isolate S. pombe cDNA clones encoding proteins that interact with Pyp3. One of the positive interacting proteins is tif1. eIF4E (Tif1) is the mRNA cap-binding protein and essential for recruitment of ribosomes to mRNA. Our results show that Pyp3 physically interacts with eIF4E in vivo and affects both the phosphorylation state and cellular distribution of eIF4E. In addition, over-expression of eIF4E causes delay of cell cycle G2/M transition and an elongated cell size phenotype at division in S. pombe. Importantly, over-expression of the Pyp3 tyrosine phosphatase counteracts the cell cycle inhibition induced by over-expression of wild type eIF4E, and this depends on the conserved eIF4E binding site on Pyp3. In addition, Pyp3 also interacts with the 14-3-3 proteins in vivo. Collectively, these data provide novel insights into Pyp3 function and eIF4E regulation, and suggest Pyp3 might have additional important roles in the coordination of cell growth with cell cycle control in fission yeast.