Glycerophospholipid Variation in Choline and Inositol Auxotrophs of Neurospora Crassa. Internal Compensation Among Zwitterionic and Anionic Species

The glycerophospholipids of cultures of Neurospora crassa were extracted, deacylated, and analyzed. In addition to a wild-type strain, several auxotrophic mutant strains were examined: chol-1 (defective S-adenosylmethionine: phosphatidylethanolamine methyltransferase), chol-2 (defective S-adenosyl methionine:phosphatidylmonomethylethanolamine (dimethylethanolamine) methyltransferase), and inos (defective myoinositol-1-phosphate phosphatase). In addition, a double mutant strain, chol-1;chol-2, was constructed. Cultures of the mutant strains grown with concentrations of supplement(s) just adequate to support growth had bizarre phospholipid compositions. By appropriate choice of mutant and supplement(s), it was possible to vary the relative level of every phospholipid of the organism, with the exception of cardiolipin. The maximum ranges encountered for the zwitterionic species, expressed as per cent of total phospholipid phosphorus, were lecithin (0.9 to 53.1%), phosphatidyldimethylethanolamine (0.0 to 55.5%), phosphatidylmonomethylethanolamine (0.0 to 53.9%), and phosphatidylethanolamine (9.8 to 43.3%). For the anionic species, the ranges were phosphatidylserine (1.7 to 10.4%) and phosphatidylinositol (3.6 to 25.1%). Despite the wide variation of the relative proportions of the individual phospholipid species, five quantities remained constant: the cardiolipin content, the total phospholipid content, the total content of the zwitterionic species, the total content of the anionic species, and the ratio of the zwitterionic to anionic totals. The data suggest the existence of an internal compensation mechanism, the net effect of which is maintenance of a fairly constant contribution by the phospholipid components to the over-all membrane charge.