Functional characterization of the fission yeast phosphatidylserine synthase gene, pps1, reveals novel cellular functions for phosphatidylserine

We describe the characterization of the pps1 gene encoding a predicted phosphatidylserine (PS) synthase in the fission yeast, Schizosaccharomyces pombe. pps1 Δ mutants grow slowly in rich medium and are inviable in synthetic minimal medium. They do not produce detectable PS in vivo and possess negligible in vitro PS synthase activity, indicating that pps1 encodes the major PS synthase activity in S. pombe. Supplementation of media with ethanolamine partially suppresses the growth defect of pps1 Δ cells, reflecting the likely importance of PS as a precursor for phosphatidylethanolamine in S. pombe. pps1 Δ mutants exhibit morphology, cytokinesis, cytoskeletal, and cell wall defective phenotypes. Overexpression of pps1 also leads to cell morphology and cytokinesis defects, implicating PS as a dosage‐dependent regulator of these processes. During log phase, GFP‐Pps 1p fusion proteins are concentrated at the cell and nuclear peripheries and presumptive ER membranes, while in stationary phase cells, they are redistributed to unusual cytoplasmic structures. Moreover, stationary phase pps1 Δ cultures retain poor viability relative to wild type S. pombe cells, even in medium containing ethanolamine, demonstrating a critical role for PS in stationary phase survival. Our findings demonstrate novel cellular functions for PS and the usefulness of S. pombe as a model organism for elucidating its molecular functions.