Distinct domains of MuSK mediate its abilities to induce and to associate with postsynaptic specializations.
Agrin released from motor nerve terminals activates a muscle-specific receptor tyrosine kinase (MuSK) in muscle cells to trigger formation of the skeletal neuromuscular junction. A key step in synaptogenesis is the aggregation of acetylcholine receptors (AChRs) in the postsynaptic membrane, a process that requires the AChR-associated protein, rapsyn. Here, we mapped domains on MuSK necessary for its interactions with agrin and rapsyn. Myotubes from MuSK(-/)- mutant mice form no AChR clusters in response to agrin, but agrin-responsiveness is restored by the introduction of rat MuSK or a Torpedo orthologue. Thus, MuSK(-/)- myotubes provide an assay system for the structure-function analysis of MuSK. Using this system, we found that sequences in or near the first of four extracellular immunoglobulin-like domains in MuSK are required for agrin responsiveness, whereas sequences in or near the fourth immunoglobulin-like domain are required for interaction with rapsyn. Analysis of the cytoplasmic domain revealed that a recognition site for the phosphotyrosine binding domain-containing proteins is essential for MuSK activity, whereas consensus binding sites for the PSD-95/Dlg/ZO-1-like domain-containing proteins and phosphatidylinositol-3-kinase are dispensable. Together, our results indicate that the ectodomain of MuSK mediates both agrin- dependent activation of a complex signal transduction pathway and agrin-independent association of the kinase with other postsynaptic components. These interactions allow MuSK not only to induce a multimolecular AChR-containing complex, but also to localize that complex to a primary scaffold in the postsynaptic membrane.
Zhou
H
H
Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Glass
D J
DJ
Yancopoulos
G D
GD
Sanes
J R
JR
D000255:Adenosine Triphosphate / Q000378:metabolism; D018171:Agrin / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D002459:Cell Fusion; D002460:Cell Line; D004789:Enzyme Activation; D008024:Ligands; D051379:Mice; D009124:Muscle Proteins / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000166:cytology / Q000201:enzymology; D009154:Mutation; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D019000:Phosphotyrosine / Q000378:metabolism; D051381:Rats; D020794:Receptor Protein-Tyrosine Kinases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D020812:Receptor, trkC; D011950:Receptors, Cholinergic / Q000378:metabolism*; D017475:Receptors, Nerve Growth Factor / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D013570:Synaptic Membranes / Q000201:enzymology / Q000378:metabolism*; D014101:Torpedo
D018171:Agrin; D008024:Ligands; D009124:Muscle Proteins; D011950:Receptors, Cholinergic; D017475:Receptors, Nerve Growth Factor; C051609:peripheral membrane protein 43K; D019000:Phosphotyrosine; D000255:Adenosine Triphosphate; D019869:Phosphatidylinositol 3-Kinases; C095714:MUSK protein, human; D020794:Receptor Protein-Tyrosine Kinases; D020812:Receptor, trkC