Characterization and function in vivo of two novel phospholipases B/lysophospholipases from Saccharomyces cerevisiae.

The yeast genome contains two genes, designated as PLB2 and PLB3, that are 67% and 62% identical, respectively, to PLB1, which codes for a phospholipase B/lysophospholipase in yeast (Lee, S. K., Patton, J. L., Fido, M., Hines, L. K., Kohlwein, S. D., Paltauf, F., Henry, S. A., and Levin, D. E. (1994) J. Biol. Chem. 269, 19725-19730). Deletion and overexpression studies and in vivo and in vitro activity measurements suggest that both genes indeed code for phospholipases B/lysophospholipases. In cell free extracts of a plb1 plb2 plb3 triple mutant, no phospholipase B activity was detectable. Upon overexpression of PLB2 in a plb1 plb3 mutant background, phospholipase B activity was detectable in the plasma membrane, periplasmic space extracts and the culture supernatant. Similar to Plb1p, Plb2p appears to accept all major phospholipid classes, with a preference for acidic phospholipids including phosphatidylinositol 3',4'-bisphosphate and phosphatidic acid. Consistent with a function as an extracellular lysophospholipase, PLB2 overexpression conferred resistance to lyso-phosphatidylcholine. Deletion of Plb2p function had no effect on glycerophosphoinositol or glycerophosphocholine release in vivo, in contrast to a deletion of Plb3p function, which resulted in a 50% reduction of phosphatidylinositol breakdown and glycerophosphoinositol release from the cells. In vitro, Plb3p hydrolyzes only phosphatidylinositol and phosphatidylserine and, to a lesser extent, their lyso-analogs. Plb3p activity in a plb1 plb2 mutant background was observed in periplasmic space extracts. Both Plb3p and Plb2p display transacylase activity in vitro, in the presence or absence, respectively, of detergent.

The Saccharomyces cerevisiae PLB1 gene encodes a protein required for lysophospholipase and phospholipase B activity.

Several enzymes with lysophospholipase/phospholipase B activity have been described from the budding yeast Saccharomyces cerevisiae. In vitro, these enzymes are capable of hydrolyzing all phospholipids that can be extracted from yeast cells. Two forms of the enzyme have been isolated from plasma membranes and a third from culture supernatants and the periplasmic space, but their biological roles have not been determined. These highly glycosylated enzymes were reported to have very similar catalytic properties but differed with respect to apparent molecular weight. We isolated a gene from S. cerevisiae, encoding a protein predicted to share 45% amino acid sequence identity with phospholipase B from Penicillium notatum. This yeast gene, designated PLB1, was mapped to the left arm of chromosome VIII. No residual lysophospholipase/phospholipase B activity was detected upon assay of extracts or culture supernatants of a plb1 delta mutant. Thus, either the PLB1 gene encodes all of the previously detected isoforms of phospholipase B or its gene product is required for their expression or activation. Deletion of PLB1 did not result in any apparent phenotypic defect, suggesting either that we failed to identify the growth conditions that would betray such a defect or that Plb1p is functionally redundant with another protein, whose activity has gone undetected. A plb1 delta mutant released wild-type levels of the soluble phosphatidylinositol metabolite glycerophosphoinositol into the growth medium but released greatly reduced levels of the corresponding phosphatidylcholine and phosphatidylethanolamine metabolites. These results indicate that PLB1 is principally responsible for the production of the deacylation products of phosphatidylcholine and phosphatidylethanolamine but not phosphatidylinositol.