PRODUCTION AND REUTILIZATION OF AN EXTRACELLULAR PHOSPHATIDYLINOSITOLCATABOLITE, GLYCEROPHOSPHOINOSITOL, BY SACCHAROMYCES-CEREVISIAE

Phosphatidylinositol catabolism in Saccharomyces cerevisiae is known t o result in the formation of extracellular glycerophosphoinositol (Gro PIns). We now report that S. cerevisiae not only produces but also reu tilizes extracellular GroPIns and that these processes are regulated i n response to inositol availability. A wild-type strain uniformly prel abeled with [H-3] inositol displayed dramatically higher extracellular GroPIns levels when cultured in medium containing inositol than when cultured in medium lacking inositol. This difference in extracellular accumulation of GroPIns in response to inositol availability was shown to be a result of both regulated production and regulated reutilizati on. In a strain in which a negative regulator of phospholipid and inos itol biosynthesis had been deleted (an opi1 mutant), this pattern of e xtracellular GroPIns accumulation in response to inositol availability was altered. An inositol permease mutant (itr1 itr2), which is unable to transport free inositol, was able to incorporate label from exogen ous glycerophospho [H-3] inositol, indicating that the inositol label did not enter the cell solely via the transporters encoded by itr1 and itr2. Kinetic studies of a wild-type strain and an itr1 itr2 mutant s train revealed that at least two mechanisms exist for the utilization of exogenous GroPIns: an inositol transporter-dependent mechanism and an inositol transporter-independent mechanism. The inositol transporte r-independent pathway of exogenous GroPIns utilization displayed satur ation kinetics and was energy dependent. Labeling studies employing [C -14] glycerophospho [H-3] inositol indicated that, while GroPIns enter s the cell intact, the inositol moiety but not the glycerol moiety is incorporated into lipids.