FUNCTIONAL REDUNDANCY OF CDP-ETHANOLAMINE AND CDP-CHOLINE PATHWAY ENZYMES IN PHOSPHOLIPID BIOSYNTHESIS - ETHANOLAMINE-DEPENDENT EFFECTS ON STEADY-STATE MEMBRANE PHOSPHOLIPID-COMPOSITION IN SACCHAROMYCES-CEREVISIAE

It has been established that yeast membrane phospholipid content is re sponsive to the inositol and choline content of the growth medium. Alt erations in the levels of transcription of phospholipid biosynthetic e nzymes contribute significantly to this response. We now describe cond itions under which ethanolamine can exert significant influence on yea st membrane phospholipid composition. We demonstrate that mutations wh ich block a defined subset of the reactions required for the biosynthe sis of phosphatidylcholine (PC) via the CDP-choline pathway cause etha nolamine-dependent effects on the steady-state levels of bulk PC in ye ast membranes. Such an ethanolamine-dependent reduction in bulk membra ne PC content was observed for both choline kinase (cki) and choline p hosphotransferase (cpt1) mutants, but it was not observed for mutants defective in cholinephosphate cytidylyltransferase, the enzyme that ca talyzes the penultimate reaction of the CDP-choline pathway for PC bio synthesis. Moreover, the ethanolamine effect observed for cki and cpt1 mutants was independent of the choline content of the growth medium. Finally, we found that haploid yeast strains defective in the activity of both the choline and ethanolamine phosphotransferases experienced an ethanolamine-insensitive reduction in steady-state PC content, an e ffect which was not observed in strains defective in either one of the se activities alone. The collective data indicate that specific enzyme s of the CDP-ethanolamine pathway for phosphatidylethanolamine biosynt hesis, while able to contribute to PC synthesis when yeast cells are g rown under conditions of ethanolamine deprivation, do not do so when y east cells are presented with this phospholipid headgroup precursor.