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D Branton

Publications and source records attributed to D Branton.

At least 37 records · Page 2Linked to original sources
PubMed1987

Drosophila spectrin. II. Conserved features of the alpha-subunit are revealed by analysis of cDNA clones and fusion proteins.

Drosophila alpha-spectrin cDNA sequences were isolated from a lambda gt11 expression library. These cDNA clones encode fusion proteins that include portions of the Drosophila alpha-spectrin polypeptide as shown by a number of structural and functional criteria. The fusion proteins elicited antibodies that reacted strongly with Drosophila and vertebrate alpha-spectrins and a comparison of cyanogen bromide peptide maps demonstrated a clear structural correspondence between one fusion protein and purified Drosophila alpha-spectrin. Alpha-spectrin fusion protein also displayed calcium-dependent calmodulin-binding activity in blot overlay experiments and one fusion protein bound specifically to both Drosophila and bovine brain beta-spectrin subunits on protein blots. A region of the Drosophila cDNA cross-hybridized at lowered stringency with an avian alpha-spectrin cDNA. Together these data show that the composition, structure, and binding properties of the spectrin family of proteins have been remarkably well conserved between arthropods and vertebrates. Drosophila cDNA hybridized to an mRNA of greater than or equal to 9 kb on blots of total Drosophila poly A+ RNA; and hybridized in situ to a single site in polytene region 62B, 1-7. This result and Southern blot analysis of genomic DNA indicate that the sequences are likely to be single copy in the Drosophila genome.

Animals↗
PubMed1986

The molecular basis of erythrocyte shape.

Recent discoveries about the molecular organization and physical properties of the mammalian erythrocyte membrane and its associated structural proteins can now be used to explain, and may eventually be used to predict, the shape of the erythrocyte. Such explanations are possible because the relatively few structural proteins of the erythrocyte are regularly distributed over the entire cytoplasmic surface of the cell membrane and because the well-understood topological associations of these proteins seem to be stable in comparison with the time required for the cell to change shape. These simplifications make the erythrocyte the first nonmuscle cell for which it will be possible to extend our knowledge of molecular interactions to the next hierarchical level of organization that deals with shape and shape transformations.

Energy Metabolism↗
PubMed1986

Clathrin-coated vesicles contain two protein kinase activities. Phosphorylation of clathrin beta-light chain by casein kinase II.

Incubation of clathrin-coated vesicles with Mg2+-[gamma-32P]ATP results in the autophosphorylation of a 50-kDa polypeptide (pp50) (Pauloin, A., Bernier, I., and Jollès, P. (1982) Nature 298, 574-576). We describe here a second protein kinase that is associated with calf brain and liver coated vesicles. This kinase, which phosphorylates casein and phosvitin but not histone and protamine using either ATP or GTP, co-fractionates with coated vesicles as assayed by gel filtration, electrophoresis, and sedimentation. The enzyme can be extracted with 0.5 M Tris-HCl or 1 M NaCl, and can be separated from the pp50 kinase as well as the other major coat proteins. We identified this enzyme as casein kinase II based on physical and catalytic properties and by comparative studies with casein kinase II isolated from brain cytosol. It has a Stokes radius of 4.5 nm, a catalytic moiety of approximately 45 kDa, and labels a polypeptide of 26 kDa when the pure enzyme is assayed for autophosphorylation. Its activity is inhibited by heparin and not affected by cAMP, phospholipids, or calmodulin. This protein kinase preferentially phosphorylates clathrin beta-light chain. The phosphorylation is markedly stimulated by polylysine and inhibited by heparin. Isolated beta-light chain as well as beta-light chain in triskelions or in intact coated vesicles is phosphorylated. All of the phosphate (0.86 mol of Pi/mol of clathrin beta-light chain) is incorporated into phosphoserine.

Adenosine Triphosphate↗
PubMed1986

Protein kinase C of human erythrocytes phosphorylates bands 4.1 and 4.9.

Addition of 10 nM 12-O-tetradecanoylphorbol 13-acetate (TPA) to intact human erythrocytes results in rapid phosphorylation of two cytoskeletal components, bands 4.1 and 4.9. The synthetic diacylglycerol, 1-oleoyl-2-acetylglycerol, shows a similar effect, while the biologically inactive phorbol ester, 4 alpha-phorbol didecanoate, fails to enhance phosphorylation. That TPA and 1-oleoyl-2-acetylglycerol stimulate this phosphorylation suggests that protein kinase C is being activated. In the presence of TPA, bands 4.1 and 4.9 incorporate 1.5 mol Pi/mol protein and 1.2 mol Pi/mol protein, respectively. The pattern and extent of phosphorylation shows that it is not due to cAMP-dependent protein kinases, which also phosphorylate bands 4.1 and 4.9. Ca2+-phospholipid-dependent protein kinase activity is demonstrable in the soluble fraction of erythrocytes, and has been partially purified (2200-fold) from the hemolysate by affinity chromatography (Uchida and Filburn, 1984. J. Biol. Chem. 259, 12311-12314). The affinity purified erythrocyte kinase has a 42 A Stokes' radius and phosphorylates purified bands 4.1 and 4.9 in vitro in a Ca2+- and phospholipid-dependent manner. These results show that human erythrocytes contain protein kinase C, and that band 4.1 and 4.9 are the major endogenous substrates for this kinase.

Blood Proteins↗
PubMed1986

Purification of erythrocyte band 4.1 and other cytoskeletal components using hydroxyapatite-Ultrogel.

An improved method for purifying erythrocyte band 4.1, the protein which mediates the interaction between spectrin and actin, has been developed. The new procedure, using adsorption chromatography on hydroxylapatite crystals immobilized within a crosslinked agarose gel (HA-Ultrogel), is simple and reproducibly provides a high yield of band 4.1 which is essentially free of protein kinase. Other components eluted from the hydroxylapatite matrix include band 4.9, ankyrin, and a 35,000-Da polypeptide that appears to be glyceraldehyde-3-phosphate dehydrogenase that remains bound to the erythrocyte membrane in 150 mM NaCl.

Actins↗
PubMed1985

Visualization of the protein associations in the erythrocyte membrane skeleton.

We have obtained clear images of the erythrocyte membrane skeleton from negatively stained preparations that originate directly from the intact cell but in which the spectrin meshwork is artificially spread to allow close inspection. Our procedure requires less than 2 min at 5 degrees C in phosphate buffers. We find 200-nm-long spectrin tetramers crosslinked by junctional complexes. Each junction contains a regular 37-nm rod, probably an actin oligomer of approximately 13 monomers. Densities appear at variable places in the meshwork but distinct globules occur with great frequency 78 nm from the spectrin tetramer's junctional insertion end, very close to the known binding site for ankyrin. Most frequently, five or six spectrin tetramers insert into each junction, producing a meshwork that displays remarkably regular long range order.

Actins↗
PubMed1985

Partial purification and characterization of an actin-bundling protein, band 4.9, from human erythrocytes.

Band 4.9 (a 48,000-mol-wt polypeptide) has been partially purified from human erythrocyte membranes. In solution, band 4.9 polypeptides exist as trimers with an apparent molecular weight of 145,000 and a Stokes radius of 50 A. Electron microscopy shows that the protein is a three-lobed structure with a radius slightly greater than 50 A. When gel-filtered rabbit muscle actin is polymerized in the presence of band 4.9, actin bundles are generated that are similar in appearance to those induced by "vinculin" or fimbrin. The bundles appear brittle and when they are centrifuged small pieces of filaments break off and remain in the supernatant. At low band 4.9 to actin molar ratios (1:30), band 4.9 lowers the apparent steady-state low-shear falling ball viscosity by sequestering filaments into thin bundles; at higher ratios, the bundles become thicker and obstruct the ball's movement leading to an apparent increase in steady-state viscosity. Band 4.9 increases the length of the lag phase and decreases the rate of elongation during actin polymerization as measured by high-shear Ostwald viscometry or by the increase in the fluorescence of pyrene-labeled actin. Band 4.9 does not alter the critical actin monomer concentration. We hypothesize that band 4.9, together with actin, erythrocyte tropomyosin, and spectrin, forms structures in erythroid precursor cells analogous to those formed by fimbrin, actin, tropomyosin, and TW 260/240 in epithelial brush borders. During erythroid development and enucleation, the actin filaments may depolymerize up to the membrane, leaving a membrane skeleton with short stubs of actin bundled by band 4.9 and cross-linked by spectrin.

Actins↗
PubMed1985

Solubilization of proteins from bovine brain coated vesicles by protein perturbants and Triton X-100.

To identify integral and peripheral membrane proteins, highly purified coated vesicles from bovine brain were exposed to solutions of various pH, ionic strength, and concentrations of the nonionic detergent Triton X-100. At pH 10.0 or above most major proteins were liberated, but four minor polypeptides sedimented with the vesicles. From quantitative analysis of phospholipids in the pellet and extract, we determined that at a pH of up to 12 all phospholipids could be recovered in the pellet. Electron microscopic examination of coated vesicles at pH 12.0 showed all vesicles devoid of coat structures. Treatment with high ionic strength solutions (0-1.0 M KCl) at pH 6.5-8.5 also liberated all major proteins, except tubulin, which remained sedimentable. The addition of Triton X-100 to coated vesicles or to stripped vesicles from which 90% of the clathrin had been removed resulted in the release of four distinct polypeptides of approximate Mr 38,000, 29,000, 24,000 and 10,000. The 38,000-D polypeptide (pK approximately 5.0), which represents approximately 50% of the protein liberated by Triton X-100, appears to be a glycoprotein on the basis of its reaction with periodic acid-Schiff reagent. Extraction of 90% of the clathrin followed by extraction of 90% of the phospholipids with Triton X-100 produced a protein residue that remained sedimentable and consisted of structures that appeared to be shrunken stripped vesicles. Together our data indicate that most of the major polypeptides of brain coated vesicles behave as peripheral membrane proteins and at least four polypeptides behave as integral membrane proteins. By use of a monoclonal antibody, we have identified one of these polypeptides (38,000 mol wt) as a marker for a subpopulation of calf brain coated vesicles.

Animals↗
PubMed1984

Modulation of actin polymerization by the spectrin-band 4.1 complex.

The effect of human erythrocyte spectrin dimer and band 4.1 on the polymerization of actin was studied by two independent methods: by following the increase in fluorescence of actin covalently conjugated to N-pyrenyl-iodoacetamide (pyrenylactin) and by following the increase in light scattered by actin polymers. Both techniques indicated that the complex of spectrin dimer and band 4.1, but neither spectrin nor band 4.1 alone, stimulates the rate of nucleation (decreases the lag phase of polymerization) and stabilizes oligomers of F-actin. While the band 4.1-spectrin complex, but not spectrin alone, had no immediate effect on the rate of polymerization after the lag phase, it eventually decreases the rate of actin filament growth when the molecular ratio of actin-spectrin-band 4.1 approaches the physiological range. The complex has no detectable effect on the critical actin concentration and does not significantly alter the apparent order of the nucleation reaction.

Actins↗
PubMed1984

The association of clathrin fragments with coated vesicle membranes.

The association between clathrin triskelions and the clathrin-stripped membranes of coated vesicles has been investigated using a filter assay to separate bound from unbound clathrin. The filter assay is more sensitive and less cumbersome than a sedimentation assay used previously (1). While confirming the high affinity interaction between clathrin and the vesicle membrane, our results yield Scatchard plots that are curvilinear and consistent with a positively cooperative interaction between clathrin and the vesicle membranes. Controlled digestion with trypsin removes the distal portions of the triskelion legs leaving the proximal 31 nm portions that form the hub of the triskelions. These hubs are trimers of large 112,000- and 124,000-dalton fragments of clathrin heavy chains. They competitively inhibit the binding of 125I-labeled intact triskelions to stripped vesicles with a KI identical to the KD for the association of 125I-labeled intact triskelions to stripped vesicles. Furthermore, these large fragment trimers bind to stripped vesicles with approximately the same high affinity as do intact triskelions and also show evidence of a positively cooperative interaction. It is concluded that clathrin binds to coated vesicles by an interaction that is mediated by the proximal 112,000-dalton fragment of the clathrin heavy chains.

Animals↗
PubMed1984

Identification of coated vesicles in Saccharomyces cerevisiae.

Clathrin-coated vesicles were found in yeast, Saccharomyces cerevisiae, and enriched from spheroplasts by a rapid procedure utilizing gel filtration on Sephacryl S-1000. The coated vesicles (62-nm diam) were visualized by negative stain electron microscopy and clathrin triskelions were observed by rotary shadowing. The contour length of a triskelion leg was 490 nm. Coated vesicle fractions contain a prominent band with molecular weight of approximately 185,000 when analyzed by SDS PAGE. The presence of coated vesicles in yeast cells suggests that this organism will be useful for studying the function of clathrin-coated vesicles.

Chromatography↗
PubMed1983

Fusion of coated vesicles with lysosomes: measurement with a fluorescence assay.

A fluorescence assay was developed to measure the rate of fusion of highly purified clathrin-coated vesicles isolated from bovine brain with purified lysosomes isolated from bovine kidney. Coated vesicles and stripped vesicles, prepared by removal of clathrin from coated vesicles with dilute alkaline buffer, were labeled with the nonfluorescent dye 6-carboxydiacetylfluorescein. Fusion of the vesicles with lysosomes resulted in mixing of the vesicle contents and exposure of 6-carboxydiacetylfluorescein to lysosomal esterases, which hydrolyze the probe's acetate groups to give the fluorescent 6-carboxyfluorescein. Fusion was therefore measured by recording the increase in fluorescence obtained upon mixing the vesicles with lysosomes. The results of the experiments indicated that the clathrin coat of coated vesicles inhibited the fusion of the vesicle membrane with that of the lysosome. In addition, fusion appears to require free Ca2+ and does not require vesicle-surface protein.

Animals↗
PubMed1983

Clathrin-coated vesicles contain an ATP-dependent proton pump.

Clathrin-coated vesicles isolated from calf brain contain an ATP-dependent proton pump. Proton movement was monitored by measuring [14C]methylamine distribution. Addition of Mg2+ and ATP to coated vesicles equilibrated with [14C]methylamine resulted in the generation of a 4- to 5-fold concentration gradient, corresponding to a delta pH of 0.6-0.7 units between the medium and the acidic inside of the coated vesicles. ATP-dependent [14C]methylamine uptake was abolished by the proton ionophore carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP) and partially inhibited by the carboxyl reagent N,N'-dicyclohexylcarbodiimide but was unaffected by the Na+, K+-ATPase inhibitors strophanthidin (100 microM) and vanadate (10 microM) and the mitochondrial ATPase inhibitors oligomycin (10 microgram/ml) and aurovertin (1 microgram/ml). GTP, but not the nonhydrolyzable analog 5'-adenylyl imidodiphosphate, could support [14C]methylamine uptake. Dissipation of the membrane potential with K+ and valinomycin resulted in stimulation of [14C]methylamine uptake, whereas both FCCP and valinomycin stimulated the strophanthidin-resistant ATPase activity. These results are consistent with the existence of an electrogenic, ATP-dependent proton pump in clathrin-coated vesicles. This proton pump may play a role in the acidification events that are essential in receptor-mediated endocytosis.

Adenosine Triphosphatases↗
PubMed1982

A stable, high capacity, F-actin affinity column.

A high capacity F-actin affinity matrix is constructed by binding fluorescyl-actin to rabbit anti-fluorescein IgG that is covalently bound to Sepharose 4B. When stabilized with phalloidin, the actin remains associated with the Sepharose beads during repeated washes, activates the ATPase activity of myosin subfragment 1, and specifically binds 125I-heavy meromyosin and 125I-tropomyosin. The associations between the F-actin affinity matrix and the iodinated F-actin binding proteins are monitored both by affinity chromatography and by a rapid, low speed sedimentation assay. Anti-fluorescein IgG-Sepharose should be generally useful as a matrix for the immobilization of proteins containing accessible, covalently bound fluorescein groups.

Actins↗
PubMed1982

Mapping functional sites on biological macromolecules.

Electron microscopy of macromolecules dried from glycerol and rotary shadowed from a low angle can reveal the structure of individual molecules, or groups of molecules, with remarkable clarity. We used this technique to examine the interaction of the red blood cell cytoskeletal proteins spectrin, a 500,000 dalton protein which is long (750 A) and flexible;actin, a 43,000 dalton protein capable of polymerizing into double helical filaments; and band 4.1, an 82,000 dalton globular protein. By examining binary and ternary complexes of these molecules, the binding sites for actin, band 4.1 and a fourth protein ankyrin, which links the cytoskeleton to the membrane, have been mapped along the length of the spectrin molecule. These findings, which have enabled us to construct a model of the red cell cytoskeleton, show that low angle shadowing is a powerful but simple method for investigating associations among macromolecules.

Actins↗