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 acetyl-CoA carboxylase acc1^ts/mtr7, the rate-limiting enzyme of fatty acid synthesis, suggested that the synthesis of C₂₆ VLCFAs (very-long-chain fatty acids) is important for maintaining the structure and function of the nuclear membrane. To characterize this C₂₆-dependent pathway in more detail, we have now examined cells that are blocked in pathways that require C₂₆. In yeast, ceramide synthesis and remodelling of GPI (glycosylphosphatidylinositol)-anchors are two pathways that incorporate C₂₆ 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 C₂₆-containing lipid may be affected in acc1^ts mutant cells. Lipid analysis of isolated nuclear membranes revealed the presence of a novel C₂₆-substituted PI (phosphatidylinositol). This C₂₆-PI accounts for approx. 1% of all the PI species, and is present in both the nuclear and the plasma membrane. Remarkably, this C₂₆-PI is the only C₂₆-containing glycerophospholipid that is detectable in wild-type yeast, and the C₂₆-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 (C₁₆ or C₁₈), the C₂₆-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.