An essential role for a phospholipid transfer protein in yeast Golgi function

Abstract

Progression of proteins through the secretory pathway of eukaryotic cells involves a continuous rearrangement of macromolecular structures made up of proteins and phospholipids. The protein SEC14p is essential for transport of proteins from the yeast Golgi complex. Independent characterization of the SEC14 gene and the PIT1 gene, which encodes a phosphatidylinositol/phosphatidylcholine transfer protein in yeast, indicated that these two genes are identical. Phospholipid transfer proteins are a class of cytosolic proteins that are ubiquitous among eukaryotic cells and are distinguished by their ability to catalyse the exchange of phospholipids between membranes in vitro. We show here that the SEC14 and PIT1 genes are indeed identical and that the growth phenotype of a sec14-1ts mutant extends to the inability of its transfer protein to effect phospholipid transfer in vitro. These results therefore establish for the first time an in vivo function for a phospholipid transfer protein, namely a role in the compartment-specific stimulation of protein secretion.

Roles of Phosphoinositides and of Spo14p (phospholipase D)-generated Phosphatidic Acid during Yeast Sporulation

“…Consistent with this suggestion, increased expression of Shf2p and Shf4p, yeast PITPs that display PtdIns-but not PtdCho-transfer activity in vitro, can rescue the growth and secretory defects of sec14 mutants and requires the function of Spo14p to do so (Li et al, 2000). However, Sec14p itself exhibits both PtdCho and PtdIns transfer activity in vitro (Bankaitis et al, 1990), and the results of genetic analyses have not supported a model in which the essential vegetative function of Sec14p is to provide for activation of Spo14p (Sreenivas et al, 1998;Xie et al, 1998), because unlike sec14⌬ mutants, spo14⌬ mutants are viable (Honigberg et al, 1992;Rose et al, 1995). Moreover, homozygous sfh2⌬ and sfh4⌬ diploids sporulate at wild-type frequencies (Rabitsch et al, 2001).…”

“…Sec14p possesses both PtdCho and PtdIns transfer activity (Bankaitis et al, 1990). Thus, the reduction in Spo14p activity observed in sec14-1 cells could be due to the loss of Sec14p-dependent transport of the substrate, PtdCho, to the cellular compartment(s) where Spo14p functions, to loss of Sec14p-dependent transport of PtdIns and the consequent failure to synthesize the PLD activator PtdIns(4,5)P 2 , or both.…”

“…Exit of secretory proteins from the Golgi requires the function of Sec14p, a PtdIns/PtdCho transfer protein (Bankaitis et al, , 1990, and Pik1p, a PtdIns 4-kinase, which phosphorylates PtdIns to synthesize PtdIns(4)P (Hama et al, 1999;Walch-Solimena and Novick, 1999;Audhya et al, 2000). Consequently, SEC14 and PIK1 are essential genes in vegetatively growing cells.…”

Membrane stress is coupled to a rapid translational control of gene expression in chlorpromazine-treated cells

“…In this respect, it is noteworthy that CPZ increases the steady state level of PI4-P (phosphate at the D-4 position of the inositol ring) in human platelets and in mice Wbroblasts (Frolich et al 1992;Raucher and Sheetz 2001). PIs, but also PA and phosphatidylethanolamine (PE), which serve as a lipid platform for the recruitment of coat proteins and promote membrane curvature to initiate the formation of vesicles (Bankaitis et al 1990;Ktistakis et al 1996;Rothman and Wieland 1996), are potential CPZ binding sites. Thus, it is likely that CPZ, by interacting with negatively charged lipids, interferes with membrane transport and produces a synergistic growth defect when combined with mutants that block membrane traYcking between the late-Golgi and endosomal compartments.…”

Purification and cloning of phosphatidylinositol transfer proteins from Dictyostelium discoideum: homologues of both mammalian PITPs and Saccharomyces cerevisiae Sec14p are found in the same cell

“…The bestcharacterized protein, encoded by SEC14 of Saccharomyces cere isiae, is essential and is required for protein secretion from the Golgi complex [20]. Analysis of bypass mutants has led to the suggestion that Sec14p is required for maintenance of a critical pool of diacylglycerol [21,22].…”