Phospholipid biosynthesis increases in RHD3-defective mutants

RHD3, a member of the ER-shaping dynamin-like GTPases, is required in the transition from a cisternal to a tubular ER architecture during cell growth. The aberrant ER morphology in rhd3 mutants may be correlated with alterations of the ER lipid bilayer. We analyzed the lipid fraction of rhd3 mutants at qualitative and quantitative levels. We observed an increase of the amount of phospholipids but also of proteins in the mutants, indicating an overall increase of ER membranes. This increase may indicate that phospholipid biosynthesis is deregulated in rhd3 mutants. It was shown that overexpression of PIS1 and PIS2 (involved in phosphatidylinositol biosynthesis) induces the synthesis of phosphatidylinositol (PI) but also of phosphatidic acid and that overexpression of PIS1 also induces the synthesis of phosphatidylethanolamine and diacylglycerol. (1) We wondered whether PIS1 or PIS2 could be linked to the increase of the amount of phospholipids in rhd3 mutants. To answer, we measured the phospholipid composition in the double mutants rhd3-7/pis1 and rhd3-7/pis2. The phospholipid increase in the rhd3 mutant was compensated in rhd3-7/pis1 but not rhd3-7/pis2. Our results suggest a possible deregulation of PIS1 in the rhd3 mutant.