Mitotic regulation of protein phosphatases by the fission yeast sds22 protein.

Cell cycle progression requires the activity of protein kinases and phosphatases at critical points in the cell cycle in all eukaryotes. We have previously reported that the dis2(+) and sds2(+) genes of fission yeast encode redundant catalytic subunits of a type 1-like protein phosphatase. The sds22(+) gene was shown to be essential for cell viability and to interact genetically with dis2(+) and sds21(+). Here we show by immunoprecipitation that the sds22 protein physically interacts with the dis2 and sds21 proteins, and that sds22-associated phosphatase activity has altered substrate specificity, The loss of sds22 function by a temperature sensitive mutation leads to cell cycle arrest at mid-mitosis, at which point cdc2-dependent histone Hl kinase activity is high while sds22-dependent H1 phosphatase activity is low. To examine the unusual properties of sds22 protein structure, we analyzed a collection of sds22 deletion and point mutants by a variety of functional criteria. We propose that sds22 is a regulatory subunit of the dis2/sds21 phosphatase catalytic subunits and that sds22-bound phosphatase carries a key phosphatase activity essential for the progression from metaphase to anaphase. Mutational analysis indicates that dis2/sds21 interacts with the central repetitive domain of sds22, while the C-terminal and central regions of sds22 may be involved in subcellular targeting and the N-terminus is important for stability.