Axonal isoforms of myosin-I

We have examined spinal motor neurons in Sprague-Dawley rats to further characterize a mechanoenzyme, myosin-Iγ (myr4), which is found in high concentration during axon tract formation in neonates. We raised an antibody to myr4 and made riboprobes for in situ hybridization. Myr4 mRNA was abundant in spinal cord motor neurons (particularly during axon regrowth). Nerves undergoing Wallerian degeneration (from a crush 7 days earlier) showed anti-myr4 labeling of the axolemma and SER-after microtubules, neurofilaments, and F-actin had already been degraded-which is consistent with a described lipid-binding domain in the tail region of myosin-Is. Newly synthesized myr4 was carried in axons by the slow component (SC) of axonal transport at 1-8 mm/day, whereas, none was carried by the fast component (FC). We conclude that SC delivers myr4 to the cytoplasmic surfaces of stationary axonal membranes (SER and axolemma). This positioning would anchor the tail domain of myr4 and leave the catalytic head domain free to interact with F-actin.