Yeast Sec14-like lipid transfer proteins Pdr16 and Pdr17 bind and transfer the ergosterol precursor lanosterol in addition to phosphatidylinositol

Yeast Sec14-like phosphatidylinositol transfer proteins (PITPs) contain a hydrophobic cavity capable of accepting a single molecule of phosphatidylinositol (PI) or another molecule in a mutually exclusive manner. We report here that two yeast Sec14 family PITPs, Pdr16p (Sfh3p) and Pdr17p (Sfh4p), possess high-affinity binding and transfer towards lanosterol. To our knowledge, this is the first identification of lanosterol transfer proteins. In addition, a pdr16Δpdr17Δ double mutant had a significantly increased level of cellular lanosterol compared with the corresponding wild-type. Based on the lipid profiles of wild-type and pdr16Δpdr17Δ cells grown in aerobic and anaerobic conditions, we suggest that PI-lanosterol transfer proteins are important predominantly for the optimal functioning of the post-lanosterol part of sterol biosynthesis.

Schizosaccharomyces pombe cardiolipin synthase is part of a mitochondrial fusion protein regulated by intron retention

Cardiolipin (CL) is a unique lipid component of mitochondria in all eukaryotes. It is important for the architecture of mitochondrial membranes and for mitochondrial dynamics. CL also creates a highly specific microenvironment of mitochondrial protein machineries. CL biosynthetic pathway is, however, only partially characterized in the fission yeast Schizosaccharomyces pombe. Here we show that CL synthase is an essential protein in S. pombe. It is encoded by the ORF SPAC22A12.08c as a C terminal part of a tandem fusion protein together with a mitochondrial hydrolase of unknown function. Expression of S. pombe CL synthase is able to complement deletion of the CRD1 gene of Saccharomyces cerevisiae and, vice versa, S. cerevisiae CRD1 gene complements deletion of S. pombe SPAC22A12.08c. The proper expression of CL synthase and its partner in the tandem protein, the mitochondrial hydrolase, is regulated at the level of alternate intron splicing. The first part of the SPAC22A12.08c fusion protein could be translated from both major SPAC22A12.08c derived mRNAs, with and without intron IV. Functional CL synthase, however, is produced only from the minor SPAC22A12.08c derived mRNA that has intron IV retained. Thus, intron retention is a novel mechanism for the differential expression of two proteins that evolved as a fusion protein and are under the control of the same promoter.

Yeast Pgc1p (YPL206c) controls the amount of phosphatidylglycerol via a phospholipase C-type degradation mechanism

The product of the open reading frame YPL206c, Pgc1p, of the yeast Saccharomyces cerevisiae displays homology to bacterial and mammalian glycerophosphodiester phosphodiesterases. Deletion of PGC1 causes an accumulation of the anionic phospholipid, phosphatidylglycerol (PG), especially under conditions of inositol limitation. This PG accumulation was not caused by increased production of phosphatidyl-glycerol phosphate or by decreased consumption of PG in the formation of cardiolipin, the end product of the pathway. PG accumulation in the pgc1Delta strain was caused rather by inactivation of the PG degradation pathway. Our data demonstrate an existence of a novel regulatory mechanism in the cardiolipin biosynthetic pathway in which Pgc1p is required for the removal of excess PG via a phospholipase C-type degradation mechanism.