Mutations Causing DOK7 Congenital Myasthenia Ablate Functional Motifs in Dok-7

• Published in: The Journal of biological chemistry Vol. 283; no. 9; pp. 5518 - 5524
• Main Authors: Hamuro, Johko, Higuchi, Osamu, Okada, Kumiko, Ueno, Makiko, Iemura, Shun-ichiro, Natsume, Tohru, Spearman, Hayley, Beeson, David, Yamanashi, Yuji
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 29-02-2008; American Society for Biochemistry and Molecular Biology
• Subjects: Alleles; Amino Acid Motifs - genetics; Animals; Cell Line; Enzyme Activation - genetics; Frameshift Mutation; Humans; Mice; Muscle Fibers, Skeletal - metabolism; Muscle Fibers, Skeletal - pathology; Muscle Proteins - genetics; Muscle Proteins - metabolism; Myasthenia Gravis - genetics; Myasthenia Gravis - metabolism; Myasthenia Gravis - pathology; Neuromuscular Junction - genetics; Neuromuscular Junction - metabolism; Neuromuscular Junction - pathology; Nuclear Localization Signals - genetics; Nuclear Localization Signals - metabolism; Protein Structure, Tertiary - genetics; Receptor Protein-Tyrosine Kinases - genetics; Receptor Protein-Tyrosine Kinases - metabolism; Receptors, Cholinergic - genetics; Receptors, Cholinergic - metabolism; Signal Transduction - genetics; Syndrome
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M708607200

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.