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P J Novick

Publications and source records attributed to P J Novick.

26 records · Page 2Linked to original sources
PubMed1989

Mutational analysis of SEC4 suggests a cyclical mechanism for the regulation of vesicular traffic.

Mutant alleles of SEC4, an essential gene required for the final stage of secretion in yeast, have been generated by in vitro mutagenesis. Deletion of the two cysteine residues at the C terminus of the protein results in a soluble non-functional protein, indicating that those two residues are required for normal localization of Sec4p to secretory vesicles and the plasma membrane. A mutant allele of SEC4 generated to mimic an activated, transforming allele of H-ras, as predicted, does not bind GTP. The presence of this allele in cells containing wild-type SEC4 causes a secretory defect and the accumulation of secretory vesicles. The results of genetic studies indicate that this allele behaves as a dominant loss of function mutant and as such prevents wild-type protein from functioning properly. We propose a model in which Sec4p cycles between an active and an inactive state in order to mediate the fusion of vesicles to the plasma membrane.

Alleles↗
PubMed1989

Fractionation of yeast organelles.

In summary, organelles of the secretory pathway can be effectively separated from one another using differential centrifugation followed by sucrose density gradient fractionation of wild-type or vesicle-accumulating mutant yeast cells. Up to 10-fold enrichment of the plasma membrane fraction is obtained, and resolution of the peak fractions of several organelles allows one to localize specific proteins to particular components of the pathway. Additionally, a highly purified population of constitutive secretory vesicles can be isolated from the 100,000 g membrane fraction of sec 6-4 cells on a Sephacryl S-1000 column. The success of this procedure is due to the homogeneous size of the vesicles and the high concentration of vesicles accumulated in the sec 6-4 cells. From other laboratories, methods have been described for the isolation of other organelles including the vacuole (Wiemken, 1975), plasma membrane (Tschopp and Schekman, 1983), and nuclei (Mann and Mecke, 1980), as well as an alternative procedure for the purification of secretory vesicles from yeast (Holcomb et al., 1987). For the localization of proteins to particular organelles the ability to lyse cells osmotically is an important improvement over the glass bead lysis procedure. The shear forces generated during glass bead lysis could potentially remove proteins from the surface of organelles that otherwise would be membrane-attached, causing them to appear soluble. Similarly, because the conditions required for stabilizing the association of a protein with a membrane can be quite variable depending on the lysis buffer, confirmation of localization using alternative schemes is prudent. With the advent of such techniques as confocal immunofluorescent microscopy and immunoelectron microscopy, effective methods for confirming localizations are becoming available.

Cell Fractionation↗
PubMed1989

The Sec15 protein responds to the function of the GTP binding protein, Sec4, to control vesicular traffic in yeast.

SEC15 function is required at a late stage of the yeast secretory pathway. Duplication of the gene encoding the ras-like, GTP-binding protein, Sec4, can suppress the partial loss of function resulting from the sec15-l mutation, but cannot suppress disruption of sec15. Analysis of the SEC15 gene predicts a hydrophilic protein product of 105 kD. Anti-Sec15 antibody recognizes a protein of 116-kD apparent molecular mass which is associated with a microsomal fraction of yeast in a strongly pH dependent fashion. Overproduction of Sec15 protein interferes with the secretory pathway, resulting in the formation of a cluster of secretory vesicles, and a patch of Sec15 protein revealed by immunofluorescence. The sec4-8 and sec2-4l mutations, but not mutations in other SEC genes, prevent formation of the Sec15 protein patch. We propose that Sec15 protein responds to the function of the Sec4 protein to control vesicular traffic.

Amino Acid Sequence↗
PubMed1989

Mutations in the SAC1 gene suppress defects in yeast Golgi and yeast actin function.

The budding mode of Saccharomyces cerevisiae cell growth demands that a high degree of secretory polarity be established and directed toward the emerging bud. We report here our demonstration that mutations in SAC1, a gene identified by virtue of its allele-specific genetic interactions with yeast actin defects, were also capable of suppressing sec14 lethalities associated with yeast Golgi defects. Moreover, these sac1 suppressor properties also extended to sec6 and sec9 secretory vesicle defects. The genetic data are consistent with the notion that SAC1p modulates both secretory pathway and actin cytoskeleton function. On this basis, we suggest that SAC1p may represent one aspect of the mechanism whereby secretory and cytoskeletal activities are coordinated, so that proper spatial regulation of secretion might be achieved.

Actins↗
PubMed1988

A GTP-binding protein required for secretion rapidly associates with secretory vesicles and the plasma membrane in yeast.

SEC4, one of the 10 genes involved in the final stage of the yeast secretory pathway, encodes a ras-like, GTP-binding protein. In wild-type cells, Sec4 protein is located on the cytoplasmic face of both the plasma membrane and the secretory vesicles in transit to the cell surface. In all post-Golgi blocked sec mutants, Sec4p is predominantly associated with the secretory vesicles that accumulate as a result of the secretory block. Sec4p is synthesized as a soluble protein that rapidly (t1/2 less than or equal to 1 min) and tightly associates with secretory vesicles and the plasma membrane by virtue of a conformational change of a covalent modification. These data suggest that Sec4p may function as a "G" protein on the vesicle surface to transduce an intracellular signal needed to regulate transport between the Golgi apparatus and the plasma membrane.

Cell Membrane↗
PubMed1987

A ras-like protein is required for a post-Golgi event in yeast secretion.

Secretion is blocked at the post-Golgi stage within 5 min of a shift of sec4-8 cells from 25 degrees C to 37 degrees C. Analysis of SEC4 predicts a protein product of 23.5 kd molecular weight that shares 32% homology with ras proteins and is essential for growth. The regions of best homology are those involved in the binding and hydrolysis of GTP. Duplication of SEC4 suppresses post-Golgi-blocked mutations in three sec genes. These mutations are lethal when combined with sec4-8 at 25 degrees C. Mutations that block elsewhere on the pathway are not suppressed by the SEC4 duplication and are not lethal when combined with sec4-8. We propose that the SEC4 product is a GTP-binding protein that plays an essential role in controlling a late stage of the secretory pathway.

Amino Acid Sequence↗
PubMed1987

Purification and characterization of constitutive secretory vesicles from yeast.

We have developed a purification procedure for the isolation of constitutive post-Golgi secretory vesicles from Saccharomyces cerevisiae. Although the post-Golgi stage of the secretion pathway is normally very rapid, we have used a temperature-sensitive secretory mutant, sec 6-4, to greatly expand the population of secretory vesicles. Following invertase as a marker, intact vesicles are enriched 36-fold from the crude lysate. The final preparation contains few contaminants as assessed by morphologic and biochemical examination. Three proteins (110, 40-45, and 18 kD) co-purify with the vesicle marker enzyme invertase. Metabolic labeling experiments indicate that these vesicle-associated proteins are synthesized during the period of vesicle accumulation. They are not apparent in the corresponding fractions from wild-type cells. Analysis of these proteins indicates that the 110-kD protein is a major glycoprotein residing in the vesicle lumen, while the 40-45- and 18-kD proteins are not glycosylated and are firmly associated with the vesicle membrane, each with at least one domain exposed on the cytoplasmic surface.

Cell Fractionation↗