PHOSPHOLIPASE-D ACTIVITY IS REQUIRED FOR SUPPRESSION OF YEAST PHOSPHATIDYLINOSITOL TRANSFER PROTEIN DEFECTS

Yeast phosphatidylinositol transfer protein (Sec14p) function is essen tial for production of Golgi-derived secretory vesicles, and this requ irement is bypassed by mutations in at least seven genes. Analyses of such 'bypass Sec14p' mutants suggest that Sec14p acts to maintain an e ssential Golgi membrane diacylglycerol (DAG) pool that somehow acts to promote Golgi secretory function. SPO14 encodes the sole yeast phosph atidylinositol-4,5-bisphosphate-activated phospholipase D (PLD), PLD f unction, while essential for meiosis, is dispensable for vegetative gr owth. Herein, we report specific physiological circumstances under whi ch an unanticipated requirement for PLD activity in yeast vegetative G olgi secretory function is revealed, This PLD involvement is essential in 'bypass Sec14p' mutants where normally Sec14p-independent Golgi se cretory reactions are occurring in a Sec14p-independent manner. PLD ca talytic activity is necessary but not sufficient for 'bypass Sec14p', and yeast operating under 'bypass Sec14p' conditions are ethanol-sensi tive. These data suggest that PLD supports 'bypass Sec14p' by generati ng a phosphatidic acid pool that is somehow utilized in supporting yea st Golgi secretory function.