Membrane Mutants: A Yeast Mutant with a Lesion in Phosphatidylserine Biosynthesis

Abstract

A single-gene nuclear choline-requiring mutant of S. cerevisiae was studied. Choline as a growth supplement to synthetic media was substituted by low concentrations of dimethylethanolamine, monomethylethanolamine or ethanolamine. DL-Serine supported growth, but only at high concentrations: on a molar basis it was .apprx. 100 times less effective than choline. When cultured in unsupplemented medium mutant cells ceased to grow. Growth-arrested cells contained less than 1/5 of the phosphatidylethanolamine present in wild-type cells and only traces of phosphatidylserine. The relative content of 2 phospholipid species was raised by growing mutant cells in the presence of choline of the other supplements but remained lower than in wild-type cells. Mutant cells depleted of phosphatidylethanolamine and phosphatidylserine had diminished ability to fuse with other cells in mating and their protoplasts showed increased resistance to hypotonic lysis. Respiration was not affected by the deficit of the 2 phospholipid species in the mutant. In cell-free preparations, the affinity of the phosphatidylserine synthesizing system for serine was 2 orders of magnitude lower in the mutant than in the wild-type. Impairment of phosphatidylserine synthesis accounted for growth requirement and the abnormal phospholipid composition of the mutant cells.