Dimerization of the Muscle-specific Kinase Induces Tyrosine Phosphorylation of Acetylcholine Receptors and Their Aggregation on the Surface of Myotubes*
- From the Max-Planck-Institut für Entwicklungsbiologie, Abteilung Biochemie, Spemannstrasse 35, D-72076 Tübingen, Germany
Abstract
During development of the neuromuscular junction, neuronal splice variants of agrin initiate the aggregation of acetylcholine receptors on the myotube surface. The muscle-specific kinase is thought to be part of an agrin receptor complex, although the recombinant protein does not bind agrin with high affinity. To specify its function, we induced phosphorylation and activation of this kinase in the absence of agrin by incubating myotubes with antibodies directed against its N-terminal sequence. Antibody-induced dimerization of the muscle-specific kinase but not treatment with Fab fragments was sufficient to trigger two key events of early postsynaptic development: acetylcholine receptors accumulated into aggregates, and their β-subunits became phosphorylated on tyrosine residues. Heparin partially inhibited receptor aggregation induced by both agrin and anti-muscle-specific kinase antibodies. In contrast, it did not affect kinase or acetylcholine receptor phosphorylation. These data indicate that agrin induces postsynaptic differentiation by dimerizing the muscle-specific kinase. They also suggest that activation of the kinase domain can account for only part of agrin’s effects. Dimerization of this molecule appears to activate an additional signal, most likely by organizing a scaffold for other postsynaptic proteins.
Footnotes
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↵* This work was supported by the Max-Planck Society.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ Supported by the Graduiertenkolleg Neurobiologie Tübingen.
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↵§ To whom correspondence should be addressed: Max-Planck-Institut für Entwicklungsbiologie, Abteilung Biochemie, Spemannstr. 35, D-72076 Tübingen, Germany. Tel.: 07071-601415; Fax: 07071-601447; E-mail: werner.hoch{at}tuebingen.mpg.de.
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↵1 The abbreviations used are: AChR, nicotinic acetylcholine receptor; DMEM, Dulbecco’s modified Eagle’s medium; MuSK, muscle-specific kinase; MASC, MuSK-accessory specificity component; mAb, monoclonal antibody; pAb, polyclonal antibody; RATL, rapsyn-associated transmembrane linker; s-agrin, soluble agrin; PAGE, polyacrylamide gel electrophoresis; TGFβR I, transforming growth factor β receptor I.
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↵2 C. Hopf and W. Hoch, submitted for publication.
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↵3 C. Hopf and W. Hoch, unpublished results.
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- Received August 26, 1997.
- Revision received December 16, 1997.
- The American Society for Biochemistry and Molecular Biology, Inc.
