Secretion and cellular distribution of the phospholipase B homolog, Plb1, are regulated in response to nutrient limitation and KCl stress in fission yeast

Like related phospholipase B (PLB) enzymes in other fungi, the primary translation product of the Schizosaccharomyces pombe plb1 gene has a putative N‐terminal signal peptide and multiple potential N ‐glycosylation sites, which are predicted to target the protein for processing through the secretory pathway. We have investigated the secretion, subcellular localization, and glycosylation of Plb1 by analysis of S. pombe cells expressing the protein as a fusion to green fluorescent protein (Plb1‐GFP). In S. pombe cells cultured in nutrient‐rich medium (YES) containing glucose as the primary carbon source, all detectable Plb1 protein was highly glycosylated but secreted at negligible levels. By comparison, cells cultured in YES containing glycerol as the primary carbon source (YES‐G) or in synthetic minimal medium containing glucose as the primary carbon source (EMM) secreted substantial quantities of Plb1 into the culture medium, indicating that Plb1 secretion is induced by nutrient limitation. Intracellularly, Plb1 was localized to vacuoles in S. pombe cells under all tested culturing conditions. In cells grown in YES‐G, but not YES or EMM, Plb1 was also enriched in the periplasmic space, while in cells cultured in YES‐G or EMM, but not YES, it was localized to the cell division site in dividing cells. KCl stress resulted in downregulation of Plb1 secretion and increased intracellular concentration of the protein in cells cultured in EMM medium, suggesting a role for intracellular Plb1 in cellular adaptation to KCl stress. Consistent with this notion, a nonsecretable mutant form of Plb1 was capable of rescuing the KCl‐sensitive growth defect of the S. pombe plb1 Δ mutant.