STRUCTURE AND FUNCTION OF CYTOPLASMIC MOTORS

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The project seeks to understand the functions of cytoplasmic motors in neurons in order to understand fast and slow axonal transport. New types of myosin motors attached to the surfaces of squid axonal organelles now appear to belong to at least two species of myosin. We have developed antibodies to various nervous system myosins to determine which myosins move organelles. Potential organelle myosins are now being sequenced and probes developed to explore their role in axonal transport. New studies of the organization of the actin substrates for these motors in the axon have shown that parallel actin filaments intertwine with the microtubule bundles, and that they have mixed polarities, suggesting that they might carry organelles to and from the microtubule bundles. Regarding slow axonal transport, negatively charged macomolecular assemblies injected into the squid giant axon move in the anterograde direction at rates up to 0.5 um/sec. Of particular interest is that neurofilament proteins as well as actin and microtubule fragments move anterogradely, and that these movements appear to be along some type of intracellular tract. Recent results suggest that these tracts are microtubule bundles and that the motors come from the soluble pools of conventional kinesin. Indeed, depolymerizing the axonal microtubules blocks slow neurofilament transport.