Golgi phospholipid composition is a critical determinant of Golgi secretory competence in yeast
Golgi secretory function in Saccharomyces cerevisiae is strictly dependent upon the activity of the SEC14 gene product, the Sec14p. Yeast strains bearing temperature-sensitive mutations in the SEC14 gene exhibit defects in protein trafficking from the Golgi and otherwise wild type cells which lack any functional copy of the sec14 gene fail to grow. The product of this essential gene, the Sec14p, has been shown to be the phosphatidylinositol (PI)/phosphatidylcholine (PC) transfer protein of yeast and serves to transport PI and PC monomers between natural or artificial lipid bilayers, in vitro. The stimulation of yeast Golgi function by the Sec14p has further been shown to correlate with the physical co-localization between Golgi membranes and Sec14p. These data have resulted in the assignment of the Golgi membranes as an in vivo target for phospholipid transfer protein function, the first such assignment possible. In this report, I demonstrate that the normal requirement for the Sec14p may be bypassed by loss of function mutations in any of the genes encoding the structural enzymes of the CDP-choline pathway for PC biosynthesis. Analysis of the bulk membrane phospholipids from these bypass suppressor strains revealed increases in the fraction of total phospholipid that is represented by PI in both steady state and pulse labeling experiments. The alterations in phospholipid composition were observed to uniformly raise the PI/PC ratio of bulk membranes above the level of wild type membranes. These results suggest that the Sec14p functions to control the phospholipid composition of yeast Golgi membranes and that the net result of this activity is to increase the PI/PC ratio of the Golgi membranes. Consistent with this hypothesis, the phospholipid composition of enriched yeast Golgi membranes confirmed that wild type Golgi membranes exhibited high PI/PC ratios with respect to that of bulk membranes and that under conditions of Sec14p dysfunction, the PI/PC content of Golgi membranes decreased to that of bulk membranes.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign