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J B Helms

Publications and source records attributed to J B Helms.

26 records · Page 2Linked to original sources
PubMed1993

Binding of coatomer to Golgi membranes requires ADP-ribosylation factor.

Coatomer, a complex of seven proteins, appears to be the precursor of the coat structure of non-clathrin-coated Golgi-derived vesicles. Another component of this vesicle coat is the cytosolic protein ADP-ribosylation factor (ARF). Like coatomer, ARF appears to reversibly associate with Golgi membranes. We now report that ARF is required for coatomer binding to Golgi membranes and that myristoylated, but not non-myristoylated, ARF is the required species. We utilize an antibody directed against the beta-subunit of coatomer (beta-COP) to follow coatomer binding. ARF and beta-COP bind stoichiometrically to Golgi membranes. ARF-dependent beta-COP binding requires a membrane-associated protein, is saturable, and is enhanced in the presence of stable GTP analogues like guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S). ARF and beta-COP bind sequentially to Golgi membranes, since beta-COP can be bound to reisolated membranes that had been previously incubated with ARF and GTP gamma S. We conclude that membrane-bound ARF confers to Golgi membranes all of the requirements for specific beta-COP binding.

ADP-Ribosylation Factors↗
PubMed1993

Two distinct populations of ARF bound to Golgi membranes.

ADP-ribosylation factor (ARF) is a small molecular weight GTP-binding protein (20 kD) and has been implicated in vesicular protein transport. The guanine nucleotide, bound to ARF protein is believed to modulate the activity of ARF but the mechanism of action remains elusive. We have previously reported that ARF binds to Golgi membranes after Brefeldin A-sensitive nucleotide exchange of ARF-bound GDP for GTP gamma S. Here we report that treatment with phosphatidylcholine liposomes effectively removed 40-60% of ARF bound to Golgi membranes with nonhydrolyzable GTP, presumably by competing for binding of activated ARF to lipid bilayers. This revealed the presence of two different pools of ARF on Golgi membranes. Whereas total ARF binding did not appear to be saturable, the liposome-resistant pool is saturable suggesting that this pool of ARF is stabilized by interaction with a Golgi membrane-component. We propose that activation of ARF by a guanine nucleotide-exchange protein results in association of myristoylated ARF GTP with the lipid bilayer of the Golgi apparatus. Once associated with the membrane, activated ARF can diffuse freely to associate stably with a target protein or possibly can be inactivated by a GTPase activating protein (GAP) activity.

ADP-Ribosylation Factors↗
PubMed1992

Inhibition by brefeldin A of a Golgi membrane enzyme that catalyses exchange of guanine nucleotide bound to ARF.

A wide variety of membrane transformations important in intracellular transport are inhibited by the fungal metabolite brefeldin A (refs 1-4), implying that the target for this drug is central to the formation and maintenance of subcellular compartments. Brefeldin A added to cells causes the rapid and reversible dissociation of a Golgi-associated peripheral membrane protein (M(r) 110,000) which was found to be identical to one of the subunits of the coat of Golgi-derived (non-clathrin) coated vesicles, beta-COP, implying that brefeldin A prevents transport by blocking the assembly of coats and thus the budding of enclosed vesicles. In addition to the coatomer (a cytosol-derived complex of seven polypeptide chains, one of which is beta-COP), the non-clathrin (COP) coat of Golgi-derived vesicles contains stoichiometric amounts of a small (M(r) approximately 20,000) GTP-binding protein, the ADP-ribosylation factor (ARF). Binding of ARF to Golgi membranes is necessary before coatomer/beta-COP can bind these membranes (ref. 12; and D. J. Palmer et al., manuscript submitted), so the primary effect of brefeldin A seems to be on the reaction responsible for ARF binding. Indeed, like beta-COP, ARF is dissociated from the Golgi complex by treatment with brefeldin A and brefeldin A prevents ARF from associating in vitro, but the mechanism of this action by brefeldin A has been unclear. Here we report the discovery of an enzyme in a Golgi-enriched fraction that catalyses guanine nucleotide (GDP-GTP) exchange on ARF-1 protein, and which is inhibited by brefeldin A. We suggest that activation of ARF proteins for membrane localization by compartmentalized exchange enzymes is in general the first committed step in membrane transformation pathways.

ADP-Ribosylation Factors↗
PubMed1992

Racial differences in patients with adenocarcinoma of the endometrium.

OBJECTIVE: To determine the differences between white and black women with regard to the presentation and behavior of adenocarcinoma of the endometrium. METHOD: Records of 273 (68%) white patients and 117 (32%) black patients with endometrial adenocarcinoma were reviewed in Bloemfontein, South Africa. Survival data was calculated according to the direct method where losses in follow-up were regarded as tumor deaths. RESULTS: Most patients (82%) were treated by pre-operative radium followed by total abdominal hysterectomy and bilateral salpingo-oophorectomy, with post-operative external irradiation where indicated. Pre-operatively, fewer black women had reached FIGO stage I, while a larger number had advanced to stages II-IV (P = 0.0024). In addition, the tumor differentiation was more often poor in the black group (P < 0.0001). Ten-year follow-up was achieved in 84% of the white patients and 51% of the black patients and the 10-year survival figures were 67% for white patients and 28% for blacks (P < 0.0001). CONCLUSION: Endometrial adenocarcinoma is a more aggressive disease in black women than it is in whites.

Adenocarcinoma↗
PubMed1991

Synthesis of phosphatidylinositol 4,5-bisphosphate in the endoplasmic reticulum of Chinese hamster ovary cells.

We have investigated the intracellular localization and synthesis of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) in Chinese hamster ovary (CHO) cells by analyzing membrane fractions that were obtained by sucrose density gradient centrifugation. After labeling the cells for 24 h with [3H]inositol, the bulk of [3H] PtdInsP2 was found in the plasma membrane fraction, yet this lipid was also distinctly present in the microsomal fraction (20% of total cellular [3H]PtdInsP2). To determine the origin of this microsomal PtdInsP2, gradient fractions from unlabeled CHO cells were incubated with [3H]inositol in the presence of an ATP-generating system. Under these conditions of labeling, [3H]PtdIns was exclusively present in the microsomal fractions and found to be partially converted to [3H] phosphatidylinositol 4-phosphate ([3H]PtdInsP) and phosphatidylinositol 4-phosphate ([3H]PtdInsP) and [3H]PtdInsP2. The ability of microsomes to synthesize PtdInsP and PtdInsP2 was confirmed by assaying the gradient fractions for PtdIns and PtdInsP kinase activity using endogenous substrate and [gamma-32P]ATP. In the presence of exogenous substrate and Triton X-100, PtdInsP kinase activity was particularly high in the plasma membrane fractions. When phosphoinositide synthesis was studied in permeabilized CHO cells under conditions of sustained membrane vesicle flow (Helms, J. B., Karrenbauer, A., Wirtz, K. W. A., Rothman, J. E., and Wieland, F. T. (1990) J. Biol. Chem. 265, 20027-20032), no lag-time could be detected between the synthesis of [3H]PtdIns and the formation of [3H]PtdInsP2. Moreover, when lipid transport pathways were blocked in these permeabilized cells either by omission of membrane-free cytosol, addition of GTP gamma S and brefeldin A, or temperature block, PtdInsP2 formation still occurred at normal levels. These results strongly suggest that PtdInsP2 can be formed at the site of PtdIns synthesis, i.e. the endoplasmic reticulum (ER). The relationship between PtdInsP2, generated in the ER, and PtdInsP2 present in the plasma membrane, remains to be established.

1-Phosphatidylinositol 4-Kinase↗
PubMed1990

Reconstitution of steps in the constitutive secretory pathway in permeabilized cells. Secretion of glycosylated tripeptide and truncated sphingomyelin.

The constitutive secretory pathway has been reconstituted in mechanically permeabilized Chinese hamster ovary cells using two secretory markers, an acyltripeptide (N-octanoyl-Asn-Tyr-Thr-NH2) that is glycosylated at Asn in the endoplasmic reticulum and a truncated ceramide that is converted to sphingomyelin. Secretion of these bulk phase markers is dependent on cytosolic proteins and ATP. Secretion of both the glycosylated tripeptide and truncated sphingomyelin was inhibited at 15 degrees C. These results are taken as evidence that the vesicle flow to the plasma membrane (rather than artificial lysis of endoplasmic reticulum or Golgi cisternae) is required for the release of markers to the medium. Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), a nonhydrolyzable analogue of GTP, inhibited secretion, resulting in an accumulation of both the glycosylated tripeptide and truncated sphingomyelin in the semi-intact cell. Inhibition of secretion by GTP gamma S was not observed in the presence of the aminoglycoside antibiotic neomycin.

Adenosine Triphosphate↗
PubMed1987

Purification of nonspecific lipid transfer protein (sterol carrier protein 2) from human liver and its deficiency in livers from patients with cerebro-hepato-renal (Zellweger) syndrome.

The nonspecific lipid transfer protein (i.e., sterol carrier protein 2) from human liver was purified to homogeneity using ammonium sulfate precipitation, CM-cellulose chromatography, molecular sieve chromatography and fast protein liquid chromatography. Its amino acid composition was determined and found to be very similar to that of the nonspecific lipid transfer protein from bovine and rat liver with, as main feature, the absence of arginine, histidine and tyrosine. By way of a specific enzyme immunoassay using affinity-purified antibodies, the levels of nonspecific lipid transfer protein were determined in human livers. Levels varied from approximately 150 ng nonspecific lipid transfer protein per mg 105,000 X g supernatant protein for juvenile and adult humans to 40 ng per mg supernatant protein for a young infant. Levels of nonspecific lipid transfer protein in livers of infants with cerebro-hepato-renal (Zellweger) syndrome were extremely low (i.e., 2 ng per mg supernatant protein). Immunoblotting revealed the presence of crossreactive proteins of molecular masses of 40,000 and 58,000. The 40 kDa and 58 kDa proteins occurred in control livers, whereas only the 40 kDa protein was present in Zellweger livers. As in rat the 58 kDa protein could be demonstrated in a peroxisomal preparation isolated from an adult liver. A possible link between the occurrence of nonspecific lipid transfer protein and the presence of peroxisomes is discussed.

Amino Acids↗