Mutations causing DOK7 congenital myasthenia ablate functional motifs in Dok-7.
Dok-7 is a cytoplasmic activator of muscle-specific receptor-tyrosine kinase (MuSK). Both Dok-7 and MuSK are required for neuromuscular synaptogenesis. Mutations in DOK7 underlie a congenital myasthenic syndrome (CMS) associated with small and simplified neuromuscular synapses likely due to impaired Dok-7/MuSK signaling. The overwhelming majority of patients with DOK7 CMS have at least one allele with a frameshift mutation that causes a truncation in the COOH-terminal region of Dok-7 and affects MuSK activation. Dok-7 has pleckstrin homology (PH) and phosphotyrosine binding (PTB) domains in the NH2-terminal moiety, both of which are indispensable for MuSK activation in myotubes, but little is known about additional functional elements. Here, we identify a chromosome region maintenance 1-dependent nuclear export signal (NES) in the COOH-terminal moiety and demonstrate that the NES-mediated cytoplasmic location of Dok-7 is essential for regulating the interaction with MuSK in myotubes. The NH2-terminal PH domain is responsible for the nuclear import of Dok-7. We also show that the Src homology 2 target motifs in the COOH-terminal moiety of Dok-7 are active and crucial for MuSK activation in myotubes. In addition, CMS-associated missense mutations found in the PH or PTB domain inactivate Dok-7. Together, these findings demonstrate that, in addition to the NH2-terminal PH and PTB domains, the COOH-terminal NES and Src homology 2 target motifs play key roles in Dok-7/MuSK signaling for neuromuscular synaptogenesis. Ablation or disruption of these functional elements in Dok-7 probably underlies the neuromuscular junction synaptopathy observed in DOK7 CMS.
Hamuro
Johko
J
Department of Cell Regulation, Medical Research Institute, Tokyo Medical and Dental University, Bunkyo-ku, Yushima, Tokyo 113-8510, Japan.
Higuchi
Osamu
O
Okada
Kumiko
K
Ueno
Makiko
M
Iemura
Shun-ichiro
S
Natsume
Tohru
T
Spearman
Hayley
H
Beeson
David
D
Yamanashi
Yuji
Y
D000483:Alleles; D020816:Amino Acid Motifs / Q000235:genetics; D000818:Animals; D002460:Cell Line; D004789:Enzyme Activation / Q000235:genetics; D016368:Frameshift Mutation; D006801:Humans; D051379:Mice; D018485:Muscle Fibers, Skeletal / Q000378:metabolism* / Q000473:pathology; D009124:Muscle Proteins / Q000235:genetics / Q000378:metabolism*; D009157:Myasthenia Gravis / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D009469:Neuromuscular Junction / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D019913:Nuclear Localization Signals / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000235:genetics; D020794:Receptor Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism*; D011950:Receptors, Cholinergic / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000235:genetics; D013577:Syndrome
C511692:DOK7 protein, human; D009124:Muscle Proteins; D019913:Nuclear Localization Signals; D011950:Receptors, Cholinergic; C095714:MUSK protein, human; C511695:MuSK protein, mouse; D020794:Receptor Protein-Tyrosine Kinases