Fig. 1. MCK-Musk and MCK-MMT transgenes are expressed and active in skeletal muscle. (A) Expression of Musk and MMT are controlled by the 5' regulatory region of the muscle creatine kinase (MCK) gene. MCK-Musk and MCK-MMT transgenes each contain a FLAG epitope-tag in the extracellular domain of Musk (Herbst and Burden, 2000). The MCK-Musk transgene encodes a fusion protein between Musk and GFP. MMT, a chimera between the extracellular and transmembrane regions of Musk and the intracellular region of TrkA, includes a substitution of 13 amino acids from the juxtamembrane (JM) of Musk with the comparable region in TrkA (Herbst and Burden, 2000). (B) Transgene expression was measured by immunoprecipitation followed by western blotting. MCK-Musk and MCK-MMT are expressed strongly in skeletal muscle; heart expression is detected in some, but not all lines (arrows). TrkA is expressed strongly in brain. (C) Transgene expression, determined by western blotting of skeletal muscle lysates, differs among the different transgenic (Tg) lines: MCK-MMT, line 5; expresses more MMT in skeletal muscle than MCK-MMT (line 29), and MCK-Musk (line 23) expresses more Musk in skeletal muscle than MCK-Musk, line 47. (D) Musk as well as MMT are tyrosine phosphorylated. Musk and MMT were immunoprecipitated from skeletal muscle with antibodies to Musk and antibodies to TrkA, respectively, and western blots were probed with antibodies to phosphotyrosine (pY) and FLAG.

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