Production and reutilization of an extracellular phosphatidylinositol catabolite, glycerophosphoinositol, by Saccharomyces cerevisiae

Abstract

Phosphatidylinositol catabolism in Saccharomyces cerevisiae is known to result in the formation of extracellular glycerophosphoinositol (GroPIns). We now report that S. cerevisiae not only produces but also reutilizes extracellular GroPIns and that these processes are regulated in response to inositol availability. A wild-type strain uniformly prelabeled with [ 3 H]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 reutilization. In a strain in which a negative regulator of phospholipid and inositol biosynthesis had been deleted (an opi1 mutant), this pattern of extracellular 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 exogenous glycerophospho[ 3 H]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 strain revealed that at least two mechanisms exist for the utilization of exogenous GroPIns: an inositol transporter-dependent mechanism and an inositol transporterindependent mechanism. The inositol transporter-independent pathway of exogenous GroPIns utilization displayed saturation kinetics and was energy dependent. Labeling studies employing [ C]glycerophospho[ H] inositol indicated that, while GroPIns enters the cell intact, the inositol moiety but not the glycerol moiety is incorporated into lipids.The extracellular production of glycerophosphoinositol (GroPIns) represents a major pathway of phosphatidylinositol (PI) metabolism in growing cultures of Saccharomyces cerevisiae (1, 2). GroPIns accumulates in the growth medium at levels equivalent to about 25% of the amount of cellular PI, in contrast to much lower extracellular levels of the deacylated forms of the major yeast phospholipids phosphatidylcholine and phosphatidylethanolamine (1). In pulse-chase turnover experiments, Angus and Lester (1) have shown that GroPIns accounts for approximately 50% of the phosphorus and inositol lost from PI during growth. Furthermore, the release of GroPIns is regulated by glucose in the medium (2). Thus, the extracellular accumulation of GroPIns was shown to be specific, regulated, and a major route of PI turnover in a growing yeast culture. Recently, Hawkins et al. (9) confirmed that the addition of glucose to stationary-phase cultures results in the extracellular production of not only GroPIns but also low levels of GroPIns 4-phosphate and GroPIns 4,5-bisphosphate.GroPIns, GroPIns 4-phosphate, and GroPIns 4,5-bisphosphate are produced from PI, PI 4-phosphate, and PI 4,5-bisphosphate, respectively, by a phospholipase A and a lysophospholipase acting sequentially or by a phospholipas...

Glycerophosphocholine Metabolism in Higher Plant Cells. Evidence of a New Glyceryl-Phosphodiester Phosphodiesterase

“…In other eukaryotes, GPX-PDE activities have been described in yeast (Paltauf et al, 1985;Patton et al, 1995) and animal cells (Zablocki et al, 1991;Bauernschmitt and Kinne, 1993) and were often found to be extracellular. All of these activities hydrolyze glycerophosphodiester into Gro-3-P and the corresponding alcohol (choline, ethanolamine, inositol, or glycerol) and can function in alkaline conditions.…”

“…In yeast (Saccharomyces cerevisiae), glycerophosphodiesters are secreted in the extracellular medium and hydrolyzed at the outer surface of the cell (Patton et al, 1995;Dowd et al, 2001). In animal cells, glycerophosphodiesters, mainly glycerophosphocholine (GroPCho) are synthesized from phospholipids.…”

Reorganization of Actin Cytoskeleton by the Phosphoinositide Metabolite Glycerophosphoinositol 4-Phosphate

“…The identity of such a transporter is unknown, but a reasonable hypothesis is that it might be the mammalian ortholog of the GroPIns transmembrane carrier already identified and studied in yeast (Patton-Vogt and Henry, 1998). In yeast, the glycerophosphoinositols are released (Hawkins et al, 1993) and can be taken back up as a source of the essential nutrient inositol when this component is lacking in the growth medium (Patton et al, 1995). In higher eukaryotes, the glycerophosphoinositols are unlikely to have nutritional roles, but the molecules presiding over their transport and metabolism might be conserved.…”

Glycerophosphoinositol, a Novel Phosphate Source Whose Transport Is Regulated by Multiple Factors in Saccharomyces cerevisiae

“…When inositol is limiting, GroPIns is transported into the cell, where it is catabolized, and its inositol portion is used for the synthesis of PI ( Fig. 1) (11). Transport of GroPIns is mediated through the permease encoded by the GIT1 gene (12).…”

“…Turbidity was monitored by measurement at A 600 nm on a Biomate 3 Thermo Spectronic spectrophotometer. Synthetic complete medium was prepared as described previously (11). "High P i ," "Low P i ", and "no P i " media, (see Table II and Figs.…”