STRUCTURE AND FUNCTION OF CYTOPLASMIC MOTORS

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The goal of this project is to understand the distribution and functions of cytoplasmic motors in the axon of neurons. This information is intended to lead to an understanding, at the molecular level, of fast and slow axonal transport as well as the cytoplasmic organization in the axon. This is the central project in the Section, and has been very productive this year, providing a plethora of new findings. A novel subclass of myosin II-like motors on the surfaces of squid axonal organelles has been discovered and this myosin as well as its calmodulin- like light chain can now be purified in quantity from brain, and is being characterized. Immunolabeling shows that this myosin adheres strongly to the surfaces of axonal organelles. The organization of the actin substrates for these motors in the axon has showed that actin filaments intertwine with the microtubule bundles. An assay that may be useful for approaching the difficult problem of defining the mechanism of slow transport has also been introduced. Macro-molecular assemblies injected into the squid giant axon are move in the anterograde direction at rates up to 0.5 Mm/sec. Of particular interest is that short actin filaments also move anterogradely and that all movements appear to be along some type of intracellular tract. This in vitro assay should make it possible to define the motors for such movements. The bacterial flagellar motor in E. coli has been studied as another example of a motor system than can switch direction of translocation. A new cytoplasmic component of the flagellar motor thought to be involved in directional switching has now been isolated and purified, and we are making progress in devising a structural model of how the proteins in this component of the motor are organized.