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R Sealock

Publications and source records attributed to R Sealock.

At least 19 recordsLinked to original sources
PubMed2001

FIGQY phosphorylation defines discrete populations of L1 cell adhesion molecules at sites of cell-cell contact and in migrating neurons.

Phosphorylation of neurofascin, a member of the L1 family of cell adhesion molecules (L1 CAMs), at the conserved FIGQY-tyrosine abolishes the ankyrin-neurofascin interaction. This study provides the first evidence, in Drosophila melanogaster and vertebrates, for the physiological occurrence of FIGQY phosphorylation in L1 family members. FIGQY tyrosine phosphorylation is localized at specialized cell junctions, including paranodes of sciatic nerve, neuromuscular junctions of adult rats and Drosophila embryos, epidermal muscle attachment sites of Drosophila, and adherens junctions of developing epithelial cells of rat and Drosophila. In addition, FIGQY-phosphorylated L1 CAMs are abundantly expressed in regions of neuronal migration and axon extension, including the embryonic cortex, the neonatal cerebellum and the rostral migratory stream, a region of continued neurogenesis and migration throughout adulthood in the rat. Based on our results, physiological FIGQY-tyrosine phosphorylation of the L1 family likely regulates adhesion molecule-ankyrin interactions establishing ankyrin-free and ankyrin-containing microdomains and participates in an ankyrin-independent intracellular signaling pathway at specialized sites of intercellular contact in epithelial and nervous tissue.

Adherens Junctions↗
PubMed2000

Absence of alpha-syntrophin leads to structurally aberrant neuromuscular synapses deficient in utrophin.

The syntrophins are a family of structurally related proteins that contain multiple protein interaction motifs. Syntrophins associate directly with dystrophin, the product of the Duchenne muscular dystrophy locus, and its homologues. We have generated alpha-syntrophin null mice by targeted gene disruption to test the function of this association. The alpha-Syn(-/)- mice show no evidence of myopathy, despite reduced levels of alpha-dystrobrevin-2. Neuronal nitric oxide synthase, a component of the dystrophin protein complex, is absent from the sarcolemma of the alpha-Syn(-/)- mice, even where other syntrophin isoforms are present. alpha-Syn(-/)- neuromuscular junctions have undetectable levels of postsynaptic utrophin and reduced levels of acetylcholine receptor and acetylcholinesterase. The mutant junctions have shallow nerve gutters, abnormal distributions of acetylcholine receptors, and postjunctional folds that are generally less organized and have fewer openings to the synaptic cleft than controls. Thus, alpha-syntrophin has an important role in synapse formation and in the organization of utrophin, acetylcholine receptor, and acetylcholinesterase at the neuromuscular synapse.

Acetylcholinesterase↗
PubMed2000

Syntrophin isoforms at the neuromuscular junction: developmental time course and differential localization.

The syntrophins are a family of cytoplasmic adapter proteins that associate with dystrophin family proteins and have putative signaling and structural roles at the neuromuscular junction. We have localized the syntrophin family members within the rodent junction from birth to adulthood. Alpha-syntrophin is the only isoform on the postsynaptic membrane at birth. In the adult, it occurs on the crests of the junctional folds, with utrophin, and in the troughs, with dystrophin. Surprisingly, neuronal nitric oxide synthase (nNOS) does not accompany alpha-syntrophin onto the crests. Beta2-syntrophin, a junction-specific form, is not present at birth and occurs mainly in the troughs in the adult. Beta1-syntrophin is a sarcolemmal form at birth, not concentrated at the junction, and disappears entirely from most fibers by 6 weeks. In positive fibers, junctional beta1-syntrophin occurs exclusively in the troughs. These results suggest that the syntrophin isoforms have distinct functions at the junction and show that the known protein-protein associations of the syntrophins and nNOS in skeletal muscle are not sufficient to explain their localizations.

Animals↗
PubMed1998

Differential membrane localization and intermolecular associations of alpha-dystrobrevin isoforms in skeletal muscle.

alpha-Dystrobrevin is both a dystrophin homologue and a component of the dystrophin protein complex. Alternative splicing yields five forms, of which two predominate in skeletal muscle: full-length alpha-dystrobrevin-1 (84 kD), and COOH-terminal truncated alpha-dystrobrevin-2 (65 kD). Using isoform-specific antibodies, we find that alpha-dystrobrevin-2 is localized on the sarcolemma and at the neuromuscular synapse, where, like dystrophin, it is most concentrated in the depths of the postjunctional folds. alpha-Dystrobrevin-2 preferentially copurifies with dystrophin from muscle extracts. In contrast, alpha-dystrobrevin-1 is more highly restricted to the synapse, like the dystrophin homologue utrophin, and preferentially copurifies with utrophin. In yeast two-hybrid experiments and coimmunoprecipitation of in vitro-translated proteins, alpha-dystrobrevin-2 binds dystrophin, whereas alpha-dystrobrevin-1 binds both dystrophin and utrophin. alpha-Dystrobrevin-2 was lost from the nonsynaptic sarcolemma of dystrophin-deficient mdx mice, but was retained on the perisynaptic sarcolemma even in mice lacking both utrophin and dystrophin. In contrast, alpha-dystrobrevin-1 remained synaptically localized in mdx and utrophin-negative muscle, but was absent in double mutants. Thus, the distinct distributions of alpha-dystrobrevin-1 and -2 can be partly explained by specific associations with utrophin and dystrophin, but other factors are also involved. These results show that alternative splicing confers distinct properties of association on the alpha-dystrobrevins.

Alternative Splicing↗
PubMed1998

Yotiao, a novel protein of neuromuscular junction and brain that interacts with specific splice variants of NMDA receptor subunit NR1.

The molecular machinery underlying neurotransmitter receptor immobilization at postsynaptic sites is poorly understood. The NMDA receptor subunit NR1 can form clusters in heterologous cells via a mechanism dependent on the alternatively spliced C1 exon cassette in its intracellular C-terminal tail, suggesting a functional interaction between NR1 and the cytoskeleton. The yeast two-hybrid screen was used here to identify yotiao, a novel coiled coil protein that interacts with NR1 in a C1 exon-dependent manner. Yotiao mRNA (11 kb) is present modestly in brain and abundantly in skeletal muscle and pancreas. On Western blots, yotiao appears as an approximately 230 kDa band that is present in cerebral cortex, hippocampus, and cerebellum. Biochemical studies reveal that yotiao fractionates with cytoskeleton-associated proteins and with the postsynaptic density. With regard to immunohistochemistry, two anti-yotiao antibodies display a somatodendritic staining pattern similar to each other and to the staining pattern of NR1. Yotiao was colocalized by double-label immunocytochemistry with NR1 in rat brain and could be coimmunoprecipitated with NR1 from heterologous cells. Thus yotiao is an NR1-binding protein potentially involved in cytoskeletal attachment of NMDA receptors. Consistent with a general involvement in postsynaptic structure, yotiao was also found to be specifically concentrated at the neuromuscular junction in skeletal muscle.

A Kinase Anchor Proteins↗
PubMed1998

Interaction of muscle and brain sodium channels with multiple members of the syntrophin family of dystrophin-associated proteins.

Syntrophins are cytoplasmic peripheral membrane proteins of the dystrophin-associated protein complex (DAPC). Three syntrophin isoforms, alpha1, beta1, and beta2, are encoded by distinct genes. Each contains two pleckstrin homology (PH) domains, a syntrophin-unique (SU) domain, and a PDZ domain. The name PDZ comes from the first three proteins found to contain repeats of this domain (PSD-95, Drosophila discs large protein, and the zona occludens protein 1). PDZ domains in other proteins bind to the C termini of ion channels and neurotransmitter receptors containing the consensus sequence (S/T)XV-COOH and mediate the clustering or synaptic localization of these proteins. Two voltage-gated sodium channels (NaChs), SkM1 and SkM2, of skeletal and cardiac muscle, respectively, have this consensus sequence. Because NaChs are sarcolemmal components like syntrophins, we have investigated possible interactions between these proteins. NaChs copurify with syntrophin and dystrophin from extracts of skeletal and cardiac muscle. Peptides corresponding to the C-terminal 10 amino acids of SkM1 and SkM2 are sufficient to bind detergent-solubilized muscle syntrophins, to inhibit the binding of native NaChs to syntrophin PDZ domain fusion proteins, and to bind specifically to PDZ domains from alpha1-, beta1-, and beta2-syntrophin. These peptides also inhibit binding of the syntrophin PDZ domain to the PDZ domain of neuronal nitric oxide synthase, an interaction that is not mediated by C-terminal sequences. Brain NaChs, which lack the (S/T)XV consensus sequence, also copurify with syntrophin and dystrophin, an interaction that does not appear to be mediated by the PDZ domain of syntrophin. Collectively, our data suggest that syntrophins link NaChs to the actin cytoskeleton and the extracellular matrix via dystrophin and the DAPC.

Amino Acid Sequence↗
PubMed1994

beta 2-Syntrophin: localization at the neuromuscular junction in skeletal muscle.

The syntrophins are a multigene family of proteins which bind C-terminal domains of dystrophin, utrophin and homologs thereof. We report here that antibodies specific for one isoform, beta 2-syntrophin, labeled only the neuromuscular junction (NMJ) in rat skeletal muscle. Anti-alpha 1-syntrophin antibodies gave strong labeling of the sarcolemma and NMJ in normal rat and mouse muscle, and similar but much weaker labeling in dystrophin-minus mdx muscle. beta 2-Syntrophin therefore appears to be specific to the NMJ in normal muscle, as is utrophin, and may be involved in acetylcholine receptor clustering. alpha 1-Syntrophin appears to be associated mainly with dystrophin, as expected, but a small portion must be associated with another protein, possibly homologs of the electric tissue 87K protein.

Amino Acid Sequence↗
PubMed1994

Association of utrophin and multiple dystrophin short forms with the mammalian M(r) 58,000 dystrophin-associated protein (syntrophin).

Electric tissue syntrophin, originally described as an M(r) 58,000 postsynaptic protein having homologs in mammalian muscle, was previously shown to associate with dystrophin in Triton extracts of Torpedo postsynaptic membranes. It also associates with the Torpedo M(r) 87,000 postsynaptic protein (87K), the core of which is a superdomain homologous to the cysteine-rich (CR) and COOH-terminal (CT) domains of human dystrophin. Using immunoaffinity purifications from various rat tissues and immunoblotting, we find that syntrophin associates with dystrophin, utrophin (the chromosome 6-encoded dystrophin homolog formerly known as dystrophin-related protein), multiple proteins which are cross-reactive with 87K, and two subfamilies of 71K-like proteins (CRCT-containing proteins encoded by the dystrophin gene under the control of an alternative promoter in intron 62). One 71K subfamily retains the dystrophin COOH-terminal sequence; the other has an alternative COOH-terminal sequence caused by deletion of the penultimate exon by alternative splicing. The relative masses of the members of the subfamilies suggest they arise by alternative splicing at other previously described sites within CT. These results establish that syntrophin is a general ligand for the CRCT domain in mammalian dystrophin and its homologs. They also reveal a greater diversity in 71K proteins than has previously been apparent.

Amino Acid Sequence↗
PubMed1994

Association of acetylcholine receptors with peripheral membrane proteins: evidence from antibody-induced coaggregation.

Acetylcholine receptors (AChR) are associated with several peripheral membrane proteins that are concentrated on the cytoplasmic face of the plasma membrane at the neuromuscular junction, and at aggregates of AChR that form in vitro. We tested the linkage among these proteins by inducing microaggregation of AChR, then determining if a given peripheral membrane protein accumulated with the receptors in microaggregates. In most experiments, we used isolated membrane fragments that are rich in AChR and accessible to antibodies against intracellular antigens. We showed that the 43 kD receptor-associated protein always aggregated with AChR, whether microaggregation was driven by antibodies to the 43 kD protein, or to the receptor itself. Antibodies to the 58 kD receptor-associated protein also always aggregated the 58 kD protein with the receptor. Our results are consistent with a model for AChR-rich membrane in which the 43 kD and 58 kD proteins are both closely associated with the AChR. When we induced microaggregation in intact muscle cells with anti-AChR antibodies, our results were less definitive. The 43 kD receptor-associated protein microaggregated with AChR, but the 58 kD protein was not especially enriched at AChR microaggregates. We discuss the advantages of using isolated AChR-rich membrane fragments to study the association of AChR with peripheral membrane proteins.

Animals↗
PubMed1992

The 30 S lobster skeletal muscle Ca2+ release channel (ryanodine receptor) has functional properties distinct from the mammalian channel proteins.

The 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (Chaps)-solubilized ryanodine receptor (RyR) of lobster skeletal muscle has been isolated by rate density centrifugation as a 30 S protein complex. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of the purified 30 S receptor revealed a single high molecular weight protein band with a mobility intermediate between those of the mammalian skeletal and cardiac M(r) 565,000 RyR polypeptides. Immunoblot analysis showed no or only minimal cross-reactivity with the rabbit skeletal and canine cardiac RyR polypeptides. By immunofluorescence the lobster RyR was localized to the junctions of the A-I bands. Following planar lipid bilayer reconstitution of the purified 30 S lobster RyR, single channel K+ and Ca2+ currents were observed which were modified by ryanodine and optimally activated by millimolar concentrations of cis (cytoplasmic) Ca2+. Vesicle-45Ca2+ flux measurements also indicated an optimal activation of the lobster Ca2+ channel by millimolar Ca2+, whereas 45Ca2+ efflux from mammalian skeletal and cardiac muscle sarcoplasmic reticulum (SR) vesicles is optimally activated by micromolar Ca2+. Further, mammalian muscle SR Ca2+ release activity is modulated by Mg2+ and ATP, whereas neither ligand appreciably affected 45Ca2+ efflux from lobster SR vesicles. These results suggested that lobster and mammalian muscle express immunologically and functionally distinct SR Ca2+ release channel protein complexes.

Animals↗
PubMed1992

Association of the Mr 58,000 postsynaptic protein of electric tissue with Torpedo dystrophin and the Mr 87,000 postsynaptic protein.

Dystrophin was purified by immunoaffinity chromatography from detergent-solubilized Torpedo electric organ postsynaptic membranes using monoclonal antibodies. A major doublet of proteins at Mr 58,000 and minor proteins at Mr 87,000, Mr 45,000, and Mr 30,000 reproducibly copurified with dystrophin. The Mr 58,000 and Mr 87,000 proteins were identical to previously described peripheral membrane proteins (Mr 58,000 protein and 87,000 protein) whose muscle homologs are associated with the sarcolemma (Froehner, S. C., Murnane, A. A., Tobler, M., Peng, H. B., and Sealock, R. (1987) J. Cell Biol. 104, 1633-1646; Carr, C., Fischbach, G. D., and Cohen, J. B. (1989) J. Cell Biol. 109, 1753-1764). The copurification of dystrophin and Mr 58,000 protein was shown to be specific, since dystrophin was also captured with a monoclonal antibody against the Mr 58,000 protein but not by several control antibodies. The Mr 87,000 protein was a major component (along with the Mr 58,000 protein) in material purified on anti-58,000 columns, suggesting that the Mr 58,000 protein forms a distinct complex with the Mr 87,000 protein, as well as with dystrophin. Immunofluorescence staining of skeletal and cardiac muscle from the dystrophin-minus mdx mouse with the anti-58,000 antibody was confined to the sarcolemma as in normal muscle but was much reduced in intensity, even though immunoblotting demonstrated that the contents of Mr 58,000 protein in normal and mdx muscle were comparable. Thus, the Mr 58,000 protein appears to associate inefficiently with the sarcolemmal membrane in the absence of dystrophin. This deficiency may contribute to the membrane abnormalities that lead to muscle necrosis in dystrophic muscle.

Animals↗
PubMed1992

Trichohyalin: purification from porcine tongue epithelium and characterization of the native protein.

Trichohyalin, a protein of Mr between 190 and 220 kDa in different species, was first demonstrated in large granules of the inner root sheath and medulla of hair follicles and may provide a matrix for keratin filaments. We have purified trichohyalin in milligram quantities from a citric acid-insoluble fraction derived from pig tongue epithelium. Trichohyalin was extracted under conditions of low ionic strength from the citric acid-insoluble fraction, separated by gel-filtration chromatography in buffer containing 1 M NaBr, and concentrated by ion-exchange chromatography in buffer containing 4 M urea. The purified material, which is soluble in buffers containing 1 M NaBr, was considered to be trichohyalin because of its characteristic molecular weight and amino acid composition and its localization to hair follicle inner root sheath and medulla by indirect immunofluorescence using antibodies against the purified protein. Immunofluorescence showed that trichohyalin is a major protein of filiform papillae of the tongue. Unlike trichohyalin from other animals examined, the porcine protein is a doublet on SDS polyacrylamide gels of 195 and 210 kDa; both bands are recognized by different antibodies, their two-dimensional peptide maps are nearly identical, and they have nearly identical isoelectric points of about 6.6. Trichohyalin has a Stokes radius of 124 A on gel filtration and a Svedberg constant of 6, consistent with an extended structure. The protein probably associates reversibly in solution, and the native protein we have isolated may be dimeric, because crosslinking of the iodinated purified protein with disuccinimidyl suberate demonstrated the presence of a dimer, which could be dissociated in the presence of high concentrations of urea. Rotary shadowing electron microscopy of the native protein showed a filamentous structure averaging 85 nm in length with a single globular-appearing end-domain. The purification of native trichohyalin provides a basis for future functional studies.

Amino Acids↗
PubMed1992

Amphibian ryanodine receptor isoforms are related to those of mammalian skeletal or cardiac muscle.

The ryanodine receptor (RyR)-Ca2+ release channels of frog skeletal muscle have been purified as 30S protein complexes comprised of two high molecular weight polypeptides. The upper and lower bands of the frog doublet comigrated on sodium dodecyl sulfate polyacylamide gels with the mammalian skeletal and cardiac RyR polypeptides, respectively. Immunoblot analysis showed that a polyclonal antiserum to the rat skeletal RyR preferentially cross-reacted with the upper band, whereas monoclonal antibodies to the canine cardiac RyR preferentially cross-reacted with the lower band of the frog receptor doublet. Immunoprecipitation studies indicated the presence of two homooligomer 30S RyR complexes comprised of either the lower or upper polypeptide band of the frog doublet, and immunocytochemical staining revealed their colocalization in frog gastrocnemius muscle. After planar lipid bilayer reconstitution of the 30S frog RyR, single-channel currents were observed that exhibited a Na+ and Ca2+ conductance and pharmacological characteristics similar to those of the mammalian skeletal and cardiac Ca2+ release channels. These results suggest that amphibian skeletal muscle expresses two distinct RyR isoforms that share epitopes in common with the mammalian skeletal or cardiac RyR.

Animals↗
PubMed1991

Localization of paxillin, a focal adhesion protein, to smooth muscle dense plaques, and the myotendinous and neuromuscular junctions of skeletal muscle.

In this report we have demonstrated that paxillin, a cytoskeletal protein which is present in focal adhesions, localizes in vivo to regions of cell-extracellular matrix interaction which are believed to be analogous to focal adhesions. Specifically, it is enriched in the dense plaques of chicken gizzard smooth muscle tissue and in the myotendinous junctions formed in Xenopus laevis tadpole tail skeletal muscle. In addition, paxillin was identified at the rat diaphragm neuromuscular junction. The distribution of paxillin is thus comparable to that of other focal adhesion proteins, for example, talin and vinculin, in these structures.

Animals↗
PubMed1991

Localization of dystrophin relative to acetylcholine receptor domains in electric tissue and adult and cultured skeletal muscle.

Two high-affinity mAbs were prepared against Torpedo dystrophin, an electric organ protein that is closely similar to human dystrophin, the gene product of the Duchenne muscular dystrophy locus. The antibodies were used to localize dystrophin relative to acetylcholine receptors (AChR) in electric organ and in skeletal muscle, and to show identity between Torpedo dystrophin and the previously described 270/300-kD Torpedo postsynaptic protein. Dystrophin was found in both AChR-rich and AChR-poor regions of the innervated face of the electroplaque. Immunogold experiments showed that AChR and dystrophin were closely intermingled in the AChR domains. In contrast, dystrophin appeared to be absent from many or all AChR-rich domains of the rat neuromuscular junction and of AChR clusters in cultured muscle (Xenopus laevis). It was present, however, in the immediately surrounding membrane (deep regions of the junctional folds, membrane domains interdigitating with and surrounding AChR domains within clusters). These results suggest that dystrophin may have a role in organization of AChR in electric tissue. Dystrophin is not, however, an obligatory component of AChR domains in muscle and, at the neuromuscular junction, its roles may be more related to organization of the junctional folds.

Animals↗