Muscle-specific microtubule-associated protein 4 is expressed early in myogenesis and is not sufficient to induce microtubule reorganization

The expression of a muscle-specific variant of microtubule-associated protein 4 (mMAP4) has been analyzed during myogenesis of C(2)C(12) cells using an isoform-specific antibody. MMAP4 localizes to microtubules (MTs) and is expressed prior to a very early morphogenetic event, the formation of mononucleate spindle-shaped cells. MMAP4 protein appears at about the same time as titin and coincident with Golgi reorganization, but antedates myosin expression. Misexpression of EGFP-mMAP4 in non-muscle and proliferating C(2)C(12) cells does not induce dramatic changes in MT organization or stability, nor in Golgi organization. Expression of full-length mMAP4 or of a truncated form lacking the MT-binding domain does not disrupt myotube formation or myofibrillogenesis. While previous antisense studies indicated that mMAP4 is necessary for normal myotube formation [Mangan and Olmsted, 1996: Development 122:771-781], these data indicate mMAP4 is not sufficient to induce the reorganization of MTs or the Golgi into patterns typical of muscle cells. Thus, with respect to MT organizing properties, this tissue-specific variant differs from related neuronal MAPs, MAP2, and tau, which induce neural-like changes in MT organization.