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J H Walker

Publications and source records attributed to J H Walker.

At least 91 records · Page 5Linked to original sources
PubMed1985

Characterization of calelectrin, a Ca2+-binding protein isolated from the electric organ of Torpedo marmorata.

We report a fast (less than 1 day) and efficient (2-3 mg protein/100 g tissue) isolation method for calelectrin, a protein of Mr 34,000 in the electric organ of Torpedo marmorata that binds to membranes in the presence of Ca2+. Purified protein was used to investigate the nature of its interaction with membranes and with Ca2+. Calelectrin binds to liposomes composed of total extractable lipids from the electric organ in a Ca2+-dependent and -specific manner with half-maximal binding between 3 and 7 microM free Ca2+. This binding is totally inhibited by 1 mM mercaptoethanol. It is also shown that calelectrin directly binds Ca2+ in solution by two techniques: at 1 and 10 microM Ca2+ it binds 45Ca2+ as measured by gel permeation chromatography, and it contains saturable Tb3+-binding sites that are Ca2+-displaceable. An investigation of the protein's endogenous fluorescence shows that although it contains both tryptophan and tyrosine, there is no change in the apparent quantum yield as a function of Ca2+. Ca2+-dependent hydrophobic affinity chromatography of the total soluble proteins from Torpedo electric organ shows that Torpedo calelectrin, like calmodulin and mammalian calelectrins, is specifically retained in the presence of Ca2+ and eluted by EGTA. Calelectrin also contains high-affinity sites for hydrophobic fluorescence probes such as N-phenyl-1-naphthylamine, 2-CP-toluidinylnaphthalene-6-sulfonic acid, and 1-anilinonaphthalene-8-sulfonic acid, which again unlike calmodulin, show no changes as a function of Ca2+. We conclude that calelectrin is a Ca2+-binding protein whose binding to the lipid moieties of membranes is regulated by physiological change in the Ca2+ concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Electron microscopic localization of calelectrin, a Mr 36 000 calcium-regulated protein, at the cholinergic electromotor synapse of Torpedo.

Calelectrin is a calcium-binding protein of Mr 36 000 which has previously been shown to be associated with membranes of the cholinergic synapse in a calcium-dependent manner. We report here that calelectrin was solubilized from the electric organ of Torpedo marmorata in the absence of calcium together with proteins of Mr 54 000 and Mr 15 000. In cholinergic nerve endings isolated from the electric organ only calelectrin was solubilized in a calcium-dependent manner. A specific antiserum to calelectrin was used to localize the antigen by immunofluorescence microscopy on sections of electric organ and showed that calelectrin is distributed throughout the postsynaptic cell. Calelectrin was also detected in axons and in the cell bodies of the cholinergic neurones where it was concentrated in discrete patches throughout the cells. Electric organ tissue was processed to localize calelectrin with the electron microscope using an immunoperoxidase method. The most intense staining was observed on the cytoplasmic face of the acetylcholine receptor-containing postsynaptic membrane and also associated with the intracellular filaments of the electrocyte. The intensity of staining associated with these structures could be greatly reduced by preincubating the tissue with calcium chelators. In nerve terminals calelectrin was associated with synaptic vesicles in a polarized fashion. Calelectrin was also found on the cytoplasmic face of the synaptosomal plasma membrane and associated with neurofilaments. No extracellular staining was ever observed. Our results strongly support our original hypothesis that calelectrin is a calcium-regulated component of intracellular structure associated both with membranes and filaments.

Animals↗
PubMed1985

Cytoskeletal proteins at the cholinergic synapse: distribution of desmin, actin, fodrin, neurofilaments, and tubulin in Torpedo electric organ.

Treatment of the electric organ of Torpedo marmorata with Triton X-100 in the presence of 2 mM MgCl2 generated a cytoskeletal fraction in which a 54 kDa polypeptide is a major constituent. This 54 kDa polypeptide accounted for about 8% of the cellular protein when total electric organ tissue was analyzed by two-dimensional gel electrophoresis. Immunoblotting experiments showed that this protein reacts with monoclonal antibodies to desmin, the major intermediate filament protein of avian and mammalian muscle tissue. Negative stain analysis revealed that filaments of about 10 nm diameter are the major structural elements of the electric organ cytoskeleton. In the presence of Ca2+ there was a rapid degradation of the desmin-like protein and intermediate filaments due to a Ca2+-activated protease. Some of the resulting fragments retained antigenic activity against the desmin antibodies. Immunoblotting of membrane fractions enriched in acetylcholine receptor revealed desmin in addition to some actin. A further cytoskeletal component was identified from biochemical and immunological properties as a homologue of the mammalian neurofilament L-polypeptide. Thus Torpedo expresses proteins homologous to the mammalian desmin and neurofilament L-protein which can be detected using immunological approaches. Immunofluorescence microscopy was used to map the location of various cytoskeletal proteins of the cholinergic synapse on paraffin sections and on en face preparations of membranes. Desmin staining was restricted to electrocytes and in en face preparations was seen associated with both the ventral receptor-containing membrane and with the non-innervated dorsal membrane. Antibodies to neurofilament L-protein stained only the axons and not the electrocytes. Staining for fodrin, a non-erythrocyte spectrin, resulted in submembraneous decoration of both the axons and the electrocytes. Axonal staining for neurofilaments and microtubules did not extend into the ends of the nerve terminal arborizations.

Actins↗
PubMed1985

A method of assessment during vocational training: report of a pilot study.

Problems arising from the present rate of failure of vocational trainees in the MRCGP examination are outlined; and the role of formative assessment during training in reducing this rate is considered. A study is described in which trainees in a number of centres were assessed by a method designed to measure specified cognitive abilities and areas of knowledge. The method, based on written papers, provides each candidate with a profile of performance and generates comparative standards. Reliability of marking, the distribution of candidate-scores within and between areas of assessment and techniques for monitoring the effectiveness of questions are reported. Use of the method by College as an educational service to trainees is considered, along with its potential as a Part I MRCGP examination.

Education, Medical, Graduate↗
PubMed1984

Cellular distribution of three mammalian Ca2+-binding proteins related to Torpedo calelectrin.

Addition of Ca2+ to post-microsomal fractions of bovine adrenal or liver produced a sedimentable complex of membrane vesicles and cytoplasmic proteins. Proteins with apparent mol. wts. 70 000, 36 000 and 32 500 were solubilized from this complex by Ca2+ chelation. The 36 000 mol. wt. protein (p36) was immunoprecipitated by an antiserum specific for pp36, a major substrate for Rous sarcoma virus src-gene tyrosine kinase. This protein was present in many mesenchymal cells and associated with membrane cytoskeleton of bovine fibroblasts in a Ca2+-dependent manner. The 70 000 and 32 500 mol. wt. proteins were widely distributed in established cell lines, but were not clearly associated with cell organelles in tissue sections, nor retained in cytoskeleton preparations. On immunoblots p36 reacted strongly with antibodies produced against the electric fish protein Torpedo calelectrin and the similar Ca2+-binding properties and subunit mol. wts. of these proteins suggests that they might be functionally related. Since Torpedo calelectrin, p70, p36 and p32.5 were bound by lipid vesicles or microsomal membranes at micromolar free Ca2+ concentrations, regulated association with intrinsic membrane components may be involved in the functions of these widespread proteins.

Adrenal Medulla↗
PubMed1984

Isolation of mammalian calelectrins: a new class of ubiquitous Ca2+-regulated proteins.

In a new approach to isolating proteins which participate in the Ca2+-dependent regulation of membrane traffic in animal cells, two new Ca2+-binding proteins (Mr 67 000 and 32 500) have been identified in and purified from bovine liver, brain, and adrenal medulla. These proteins specifically and reversibly bind to chromaffin granule membranes at low Ca2+ concentrations (half-maximal binding at 5.5 microM Ca2+) and greatly potentiate the Ca2+-induced aggregation of these membranes at higher concentrations (above 10 microM). In the presence of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetate, the isolated proteins have Stokes radii of 3.40 nm (Mr 67 000) and 2.53 nm (Mr 32 500) as estimated by gel filtration and therefore occur as monomers. They are slightly acidic proteins with pI's of 5.85 and 5.60. In bovine tissues, both proteins and a third protein of Mr 35 000 cross-react immunologically with each other and with Torpedo calelectrin (Mr 34 000) and are therefore identified as mammalian calelectrins. In all tissues of Torpedo marmorata tested, only a single molecular mass form of calelectrin exists, whereas multiple forms of calelectrin exist in mammalian tissues, indicating gene duplication during evolution. We suggest that the evolutionary conservation and diversification, the high tissue concentrations, and the Ca2+-specific interactions of the calelectrins make them candidates for Ca2+-dependent regulators of membrane events in animal cells.

Adrenal Medulla↗
PubMed1984

The 43-K protein, v1, associated with acetylcholine receptor containing membrane fragments is an actin-binding protein.

Acetylcholine receptor enriched membrane fragments were obtained from the electric organs of Torpedo marmorata. The purified membrane fragments contained several proteins in addition to the acetylcholine receptor subunits. One of these was shown to be actin by means of immune blotting with a monoclonal antibody. Brief treatment of the membranes with pH 11.0 buffer removed actin and the other non-receptor proteins including the receptor-associated 43 000 mol. wt. polypeptide. This polypeptide was shown to bind actin after transferring the proteins from one- and two-dimensional polyacrylamide gels to nitrocellulose paper and incubating the nitrocellulose blots with actin. Specifically bound actin was demonstrated using the monoclonal antibodies to actin. No calcium or calmodulin dependency of binding was observed. The findings suggest that the 43 000 mol. wt. polypeptide is a link between the membrane-bound acetylcholine receptor and the cytoskeleton.

Actins↗
PubMed1984

Calmodulin binding proteins of the cholinergic electromotor synapse: synaptosomes, synaptic vesicles, receptor-enriched membranes, and cytoskeleton.

Calmodulin binding proteins (CBPs) have been identified using a gel overlay technique for fractions isolated from Torpedo electromotor nerve endings. Different fractions possessed characteristic patterns of CBPs. Synaptosomes showed five major CBPs--Mr 220,000, 160,000, 125,000, 55,000, and 51,000. Polypeptides of Mr 55,000 and 51,000 were found in the cytoplasm and the others are membrane-associated. The Triton X-100-insoluble cytoskeleton of synaptosomes was isolated in the presence or absence of calcium. The major CBPs had Mr of 19,000, 18,000, and 16,000. In the presence of calcium, no other CBPs were seen. In the absence of calcium, an Mr 160,000 polypeptide was present in the Triton cytoskeleton. Synaptic vesicles showed CBPs of Mr 160,000, 25,000, and 20,000. Membrane fragments enriched in acetylcholine receptors contained two major CBPs, Mr 160,000 and 125,000, together with a less prominent protein at Mr 26,000. A protein of Mr similar to that of fodrin was present in synaptosomes and acetylcholine receptor membrane fragments, but only in small amounts relative to the other polypeptides observed. The heavy and light chains of clathrin-coated vesicles from pig brain did not bind calmodulin, although strong labelling of an Mr 47,000 polypeptide was found. Results showed that calelectrin does not bind calmodulin. The possible identity of the calmodulin binding proteins is discussed.

Animals↗
PubMed1983

Calelectrin, a calcium-dependent membrane-binding protein associated with secretory granules in Torpedo cholinergic electromotor nerve endings and rat adrenal medulla.

Calelectrin, a calcium-dependent membrane-binding protein of subunit molecular weight 32,000 has been isolated from the electric organ of Torpedo, and shown to occur in cholinergic neurones and in bovine adrenal medulla. In this study a monospecific antiserum against the Torpedo protein has been used to study the localization of calelectrin in the rat adrenal gland. The cortex was not stained, whereas in the medulla the cytoplasm of the chromaffin cells was stained in a particulate manner. An identical staining pattern was obtained with an antiserum against the chromaffin granule enzyme dopamine beta-hydroxylase, although the two antisera did not cross-react with the same antigen. The purified protein aggregates bovine chromaffin granule membranes and cholinergic synaptic vesicles and also self aggregates in a calcium-dependent manner. Negative staining results demonstrate that calcium induces a transformation of the purified protein from circular structures 30-80 nm in diameter into a highly aggregated structure. Calelectrin may have a structural or regulatory role in the intracellular organization of secretory cells.

Adrenal Medulla↗
PubMed1983

Identification of a proteoglycan antigen characteristic of cholinergic synaptic vesicles.

An antiserum to cholinergic synaptic vesicles isolated from the electric organ of Torpedo marmorata was purified by adsorption with fractions containing unwanted antigens. The adsorbed antiserum responds to the proteoglycan core material of the cholinergic synaptic vesicles. The major antigen migrates in an anomalous fashion on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), forming a broad band with an apparent molecular weight of approximately 120,000 - 300,000. The distribution of this antigen after sucrose density gradient centrifugation of synaptic vesicles is the same as that of vesicular ATP. The antigen comigrates with a substance that can be stained with Alcian-Blue after SDS-PAGE of highly purified synaptic vesicles. This substance is related to the low-molecular-weight, Alcian-Blue-positive glycosaminoglycan vesiculin, which is formed from the high-molecular-weight proteoglycan by prolonged dialysis against water or by protease treatment. No antibodies were detected against vesiculin itself, indicating that the antigenic determinants are restricted to the proteoglycan.

Animals↗
PubMed1983

Calelectrin in human blood cells.

Calelectrin is a new calcium-binding protein isolated from the cholinergic nerve terminals of the electric organ of Torpedo marmorata, which is widely distributed in nervous tissues and selectively binds to membranes, self-aggregates, and promotes calcium-induced membrane aggregation as a function of calcium concentration. We now show by immunofluorescence and immune blotting procedures that this protein is also present in human blood cells. Immunofluorescence demonstrates calelectrin in all human leucocytes, including mononuclear cells, but not in platelets or in erythrocytes. The immunofluorescence indicates an exclusively cytoplasmic location of calelectrin with a diffuse distribution and no primary association with the cytoskeleton or the cell membranes. SDS-polyacrylamide gel electrophoresis with immune blotting of fractionated blood cells (thrombocytes, mononuclear cells, granulocytes and erythrocytes) reveals the presence of a single protein crossreactive with calelectrin from Torpedo marmorata in the granulocyte and mononuclear cell fractions only. Human calelectrin has a molecular weight similar to Torpedo calelectrin (approximately 34-35 kD) and also binds to membranes in a Ca(2+)-dependent manner. Our results have several implications: (1) Calelectrin is conserved during evolution between the fish Torpedo marmorata and humans; (2) its expression in neural and mesenchymal cells points to an important functional role of the protein; (3) its absence from platelets excludes the hypothesis that it is a necessary participant in exocytosis per se and suggests some other function in Ca(2+)-triggered processes.

Annexins↗
PubMed1982

Calelectrin self-aggregates and promotes membrane aggregation in the presence of calcium.

Calelectrin is a protein that can be purified to homogeneity from the cholinergically innervated electric organ of Torpedo marmorata where it is present in large amounts. It has been shown to bind to the membranes of the electric organ in a Ca2+-dependent and specific manner. Using the purified protein we now report that it is specifically self-aggregated by Ca2+ in micromolar concentrations but not by Mg2+ at much higher concentrations. Sr2+ is also completely inactive, while Ba2+ and the trivalent lanthanides Tb3+, Eu3 +, and La3+ can substitute for Ca2+. Calelectrin also greatly enhances the Ca2+-induced aggregation of isolated synaptic vesicle membranes from the cholinergic nerve terminals of T. marmorata and of chromaffin granule membranes from the bovine adrenal medulla. The potentiation of membrane aggregation is mainly due to the appearance of a fast aggregatory phase in the presence of calelectrin . It is saturable with respect to calelectrin and can be demonstrated at very low calelectrin concentrations, suggesting a specific calelectrin membrane-binding component. This component seems to be of lipid nature since the aggregation of total extracted lipids from Torpedo electric organ and from chromaffin granules could also be enhanced by calelectrin . The Ca2+-induced self-association of calelectrin and its aggregation enhancing effect may be of great importance to the structural organization of neural and secretory cells and the mechanism of exocytosis.

Animals↗