Regulation of activity in vitro and in vivo of three phospholipases B from Saccharomyces cerevisiae.

The genome of the yeast, Saccharomyces cerevisiae, contains three highly similar genes coding for phospholipases B/lysophospholipases. These enzymes behave differently with respect to substrate preferences in vitro and relative contributions to phospholipid catabolism in vivo [Merkel, Fido, Mayr, Pruger, Raab, Zandonella, Kohlwein and Paltauf (1999) J. Biol. Chem. 274, 28121-28127]. It is shown in the present study that, in vitro, pH markedly affects the substrate preference of Plb1p and Plb2p, but not of Plb3p. At the pH optimum of 2.5-3.5, the order of substrate preference of Plb1p and Plb2p is PtdSer (phosphatidylserine)>PtdIns>PtdCho (phosphatidylcholine>PtdEtn (phosphatidylethanolamine). At pH values of 5 and above, the substrate preferences change to PtdCho=PtdEtn for Plb1p and PtdSer=PtdEtn for Plb2p. Accordingly, with cultured cells the ratio of PtdIns/PtdCho breakdown, as reflected in the ratio of GroPIns (glycerophosphoinositol)/GroPCho (glycerophosphocholine) released into the culture medium, is inversely related to the pH of the growth medium. This effect is ascribed to the pH response of Plb1p, because Plb2p does not contribute to the degradation of PtdIns and PtdCho in vivo. Bivalent and tervalent cations activate phospholipases B at pH 5.5, but are inhibitory at pH 2.5. Al3+ at a concentration of 20 mM increases Plb1p activity in vitro by 8-fold and leads to a 9-fold increase in GroPCho release by whole cells. In vivo, cycloheximide strongly inhibits the breakdown of PtdIns, and to a lesser extent PtdCho. However, Al3+-stimulated GroPCho release is almost completely inhibited by cycloheximide. Deletion of PLB3 leads to increased sensitivity to toxic Al3+. Addition of SDS or melittin to cultured cells leads to a significant increase in phospholipid degradation, which is insensitive to inhibition by cycloheximide. Deletion mutants defective in the PLB1 gene are significantly more resistant to SDS than are wild-type cells.