Identification and biophysical characterization of a very-long-chain-fatty-acid-substituted phosphatidylinositol in yeast subcellular membranes

Morphological analysis of a conditional yeast mutant in acetylCoA carboxylase acc1(ts)/mtr7, the rate-limiting enzyme of fatty acid synthesis, suggested that the synthesis of C-26 VLCFAs (very-long-chain fatty acids) is important for maintaining the structure and function of the nuclear membrane. To characterize this C-26-dependent pathway in more detail, we have now examined cells that are blocked in pathways that require C-26. In yeast, ceramide synthesis and remodelling of GPI (glycosylphosphatidylinositol)-anchors are two pathways that incorporate C-26 into lipids. Conditional mutants blocked in either ceramide synthesis or the synthesis of GPI anchors do not display the characteristic alterations of the nuclear envelope observed in acc1(ts), indicating that the synthesis of another C-26-containing lipid may be affected in acc1(ts) mutant cells. Lipid analysis of isolated nuclear membranes revealed the presence of a novel C-26-substituted PI (phosphatidylinositol). This C-26-PI accounts for approx. 1 % of all the PI species, and is present in both the nuclear and the plasma membrane. Remarkably, this C-26-PI is the only C-26-containing glycerophospholipid that is delectable in wild-type yeast, and the C-26-substitution is highly specific for the sn - 1 position of the glycerol backbone. To characterize the biophysical properties of this lipid, it was chemically synthesized. In contrast to PIs with normal long-chain fatty acids (C-16 or C-18), the C-26-PI greatly reduced the bilayer to hexagonal phase transition of liposomes composed of 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine (DEPE). The biophysical properties of this lipid are thus consistent with a possible role in stabilizing highly curved membrane domains.