Analysis of All Protein Phosphatase Genes in Aspergillus nidulans Identifies a New Mitotic Regulator, Fcp1

Abstract

Reversible protein phosphorylation is an important regulatory mechanism of cell cycle control in which protein phosphatases counteract the activities of protein kinases. In Aspergillus nidulans , 28 protein phosphatase catalytic subunit genes were identified. Systematic deletion analysis identified four essential phosphatases and four required for normal growth. Conditional alleles of these were generated using the alcA promoter. The deleted phosphatase strain collection and regulatable versions of the essential and near-essential phosphatases provide an important resource for further analysis of the role of reversible protein phosphorylation to the biology of A. nidulans . We further demonstrate that nimT and bimG have essential functions required for mitotic progression since their deletions led to classical G ₂ - and M-phase arrest. Although not as obvious, cells with An pphA and An nem1 deleted also have mitotic abnormalities. One of the essential phosphatases, the RNA polymerase II C-terminal domain phosphatase An fcp1 , was further examined for potential functions in mitosis because a temperature-sensitive An fcp1 allele was isolated in a genetic screen showing synthetic interaction with the cdk1F mutation, a hyperactive mitotic kinase. The An fcp1 ^ts cdk1F double mutant had severe mitotic defects, including inability of nuclei to complete mitosis in a normal fashion. The severity of the An fcp1 ^ts cdk1F mitotic phenotypes were far greater than either single mutant, confirming the synthetic nature of their genetic interaction. The mitotic defects of the An fcp1 ^ts cdk1F double mutant suggests a previously unrealized function for AnFCP1 in regulating mitotic progression, perhaps counteracting Cdk1-mediated phosphorylation.