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At least 955 records · Page 53Linked to original sources
PubMed1984

Ganglioside biosynthesis in Golgi apparatus: new perspectives on its mechanism.

The synthesis of gangliosides GM1 and GM2 in intact rat liver Golgi vesicles is stimulated by phosphatidylglycerol as much as or even more than by detergents (Triton X-100 and octyglucoside, respectively). The antibiotic tunicamycin, known as an inhibitor of the N-glycosylation of proteins, strongly inhibits the synthesis of the above gangliosides, in the presence as well as in the absence of the phospholipid. Both phosphatidylglycerol dependence and tunicamycin inhibition disappear when the Golgi vesicles are solubilized by addition of detergents or disrupted by ultrasonication or pretreated with pronase. Transport studies with UDP-[3H]Gal show that tunicamycin blocks the penetration of the sugar nucleotide into the Golgi vesicles in a concentration-dependent manner up to 80%. The results show that tunicamycin inhibits ganglioside biosynthesis by blocking the transport of the nucleotide sugar and not by inhibiting the transferase directly. Studies on glycoprotein-galactosyltransferase with ovalbumin as exogenous acceptor showed that phosphatidylglycerol does not destroy the integrity of the Golgi vesicles. So this phospholipid is an excellent tool for studying ganglioside biosynthesis at optimal transferase activities without solubilizing the Golgi membranes.

Animals↗
PubMed1989

A novel GTP-binding protein, Sar1p, is involved in transport from the endoplasmic reticulum to the Golgi apparatus.

SAR1, a gene that has been isolated as a multicopy suppressor of the yeast ER-Golgi transport mutant sec12, encodes a novel GTP-binding protein. Its nucleotide sequence predicts a 21-kD polypeptide that contains amino acid sequences highly homologous to GTP-binding domains of many ras-related proteins. Gene disruption experiments show that SAR1 is essential for cell growth. To test its function further, SAR1 has been placed under control of the GAL1 promoter and introduced into a haploid cell that had its chromosomal SAR1 copy disrupted. This mutant grows normally in galactose medium but arrests growth 12-15 h after transfer to glucose medium. At the same time, mutant cells accumulate ER precursor forms of a secretory pheromone, alpha-mating factor, and a vacuolar enzyme, carboxypeptidase Y. We propose that Sec12p and Sarlp collaborate in directing ER-Golgi protein transport.

Amino Acid Sequence↗
PubMed1976

Concanavalin A binding to membranes of the Golgi apparatus and resultant modification of galactosyltransferase activity.

125I-Concanavalin A was found to bind to purified Golgi membranes in a manner which was time-dependent, proportional to the amount of Golgi protein, saturable, and inhibited by alpha-methyl-D-glucoside and unlabeled concanavalin A. Approximately 2 nmol of 125I-concanavalin A were bound per mg of membrane protein with a mean Ka of 0.2 x 10(7) M-1. After binding of concanavalin A to the membranes, all unbound lectin was removed by repeated washing prior to assay of galactosyltransferase activity to assure that bulk concanavalin A would not bind to the sugar acceptor during the enzyme assay. Therefore, the effects of the lectin were not due to altered acceptor concentrations in the assay. As a result of concanavalin A binding the activity of the galactosyltransferase of these membranes was enhanced by 70 to 100%. The enhancement was abolished by treatment of the concanavalin A-bound Golgi with alpha-methylglucoside. Kinetically this stimulation was reflected as an altered Km for N-acetylglucosamine and UDP-galactose. The data are suggestive that the stimulation of the enzyme results indirectly from perturbation of the membranes on binding of the lectin.

Animals↗
PubMed1987

Nascent lipoproteins from recirculating and nonrecirculating liver perfusions and from the hepatic Golgi apparatus of African green monkeys.

Perfusate apoB-100-containing lipoproteins from the isolated, perfused livers of African green monkeys consist of significant amounts of d greater than 1.006 g/ml particles in addition to very low density lipoproteins (VLDL). Distinguishing characteristics of these perfusate lipoproteins are the relative abundance of surface lipids and deficiency of core lipids. The present studies were performed to determine the likelihood that the d greater than 1.006 g/ml perfusate lipoproteins are secretion products instead of products of post-secretory modification (e.g., lipolysis) of secreted VLDL. [14C]Leucine from the perfusate became incorporated into the apoB of each of the perfusate lipoprotein classes to a similar extent in both recirculating and nonrecirculating perfusions. When endogenously radiolabeled perfusate VLDL from one liver was recirculated through a second liver, only about 15% of the radiolabeled protein appeared in the d greater than 1.006 g/ml fraction. The particle morphology and the cholesterol and apoB distribution between VLDL and d greater than 1.006 g/ml fractions were similar in recirculating and nonrecirculating perfusions. A Golgi apparatus-rich fraction was isolated from the homogenates of fresh liver samples and the isolated Golgi VLDL and d greater than 1.006 g/ml lipoproteins exhibited morphologic evidence of extra surface material analogous to that seen in perfusate. Taken together, these data support the possibility that significant amounts of d greater than 1.006 g/ml lipoproteins, many with surface-rich properties, are nascent, secretory products of the primate liver. The low level of lecithin:cholesterol acyltransferase (LCAT) in this perfusion system appears to permit detection of these secretion products and it is significant to note that the perfusate lipoprotein profile, which is unlike that of normal plasma, is similar to that of LCAT-deficient patients.

Animals↗
PubMed2001

Physical and functional association of glycolipid N-acetyl-galactosaminyl and galactosyl transferases in the Golgi apparatus.

Glycolipid glycosyltransferases catalyze the stepwise transfer of monosaccharides from sugar nucleotides to proper glycolipid acceptors. They are Golgi resident proteins that colocalize functionally in the organelle, but their intimate relationships are not known. Here, we show that the sequentially acting UDP-GalNAc:lactosylceramide/GM3/GD3 beta-1,4-N-acetyl-galactosaminyltransferase and the UDP-Gal:GA2/GM2/GD2 beta-1,3-galactosyltransferase associate physically in the distal Golgi. Immunoprecipitation of the respective epitope-tagged versions expressed in transfected CHO-K1 cells resulted in their mutual coimmunoprecipitation. The immunocomplexes efficiently catalyze the two transfer steps leading to the synthesis of GM1 from exogenous GM3 in the presence of UDP-GalNAc and UDP-Gal. The N-terminal domains (cytosolic tail, transmembrane domain, and few amino acids of the stem region) of both enzymes are involved in the interaction because (i) they reproduce the coimmunoprecipitation behavior of the full-length enzymes, (ii) they compete with the full-length counterpart in both coimmunoprecipitation and GM1 synthesis experiments, and (iii) fused to the cyan and yellow fluorescent proteins, they localize these proteins to the Golgi membranes in an association close enough as to allow fluorescence resonance energy transfer between them. We suggest that these associations may improve the efficiency of glycolipid synthesis by channeling the intermediates from the position of product to the position of acceptor along the transfer steps.

Animals↗
PubMed2005

Mon2, a relative of large Arf exchange factors, recruits Dop1 to the Golgi apparatus.

The protein Mon2 is distantly related to the guanine nucleotide exchange factors (GEFs) that activate Arf1 on Golgi membranes. However, unlike these "large" Arf GEFs, Mon2 lacks the Sec7 domain that catalyzes nucleotide exchange on Arf1. Here we report that yeast Mon2 shares extensive homology with the noncatalytic parts of both the BIG and Golgi brefeldin A resistance factor subfamilies of Arf GEFs and is located to the trans-Golgi. Moreover, we find that Mon2 forms a complex with Dop1, a large cytoplasmic protein conserved in evolution from humans to protozoa. Deletion of Mon2 results in mislocalization of Dop1 from the Golgi and defects in cycling between endosomes and the Golgi. However, unlike Mon2, Dop1 is essential for yeast viability. A conditional allele of Dop1 shows that loss of Dop1 activity not only affects endosome to Golgi transport but also causes a severe perturbation of the organization of the endoplasmic reticulum. Thus, it appears that Dop1 plays a widespread role in membrane organization, and Mon2 acts as a scaffold to recruit the Golgi-localized pool of Dop1.

Alleles↗