Plasma membrane expansion terminates in Saccharomyces cerevisiae secretion-defective mutants while phospholipid synthesis continues

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Abstract
Phospholipid synthesis activity and plasma membrane growth were studied in S. cerevisiae temperature-sensitive, secretion-defective mutants. The mutants, sec1 through sec23, do not grow at 37.degree. C and exhibit lower rates of phospholipid synthesis than does the wild-type strain X2180. None of the mutants exhibits a decline in lipid synthesis rapid enough to explain secretion failure. Plasma membrane growth was assessed indirectly by examining the osmotic sensitivity of spheroplasts derived from cultures transferred from 24.degree. to 37.degree. C. Spheroplasts from the normal-growing strain X2180 exhibited a small rapid increase in osmotic sensitivity and stabilized at a more sensitive state. Spheroplasts from the sec mutants exposed to the same temperature shift exhibited progressively increasing osmotic sensitivity. Cycloheximide treatment prevented progressive increasing osmotic fragility. Possibly, plasma membrane expansion is restricted in the sec, mutants. during incubation at 37.degree. C, the accumulation of intracellular materials within the no-longer expanding plasma membrane exerts osmotic stress on the membrane, increasing with time. The gene products defective in the sec mutants appear to be required for extracellular protein secretion and plasma membrane growth in yeast cells.