Involvement of a specific ubiquitin ligase in the assembly of the dynein motor in the filamentous fungus Neurospora crassa
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Eukaryotic cells utilize multiple molecular motor proteins to accomplish intracellular transport. The two microtubule based motors, kinesin and cytoplasmic dynein, work in concert to move cargoes outward and inward, respectively, in the cell. In polarized cells, molecular motors are of even greater importance due to a need for long distance transport and maintenance of proper polarity. Although there exists numerous forms of kinesin for anterograde transport, only a single cytoplasmic form of dynein carries out the functions of retrograde transport. To accomplish its tasks, dynein makes use of multiple subunits and accessory proteins, including heavy chains, light chains, intermediate chains, light intermediate chains, and dynactin to form a motor complex of several megadaltons. The last two thirds of the dynein heavy chains contain the main motor unit required for force production, while the N-terminal tail region of the dynein heavy chain along, with other dynein components, aid in cargo binding. Force production and microtubule-based movement is accomplished by coordinating ATP hydrolysis in the motor heads with a microtubule-binding domain. The tail domain allows for both homodimerization of motor heads as well as binding of other dynein subunits. In this work, various genetic, cell biology, and biochemical methods were used to study cytoplasmic dynein in the filamentous fungus, Neurospora crassa. Numerous dynein heavy chain mutants were isolated previously from a genetic screen, with a subset located to the C-terminal region, which were the focal point of this work. To explore the mechanism by which these mutations affect dynein function, both intragenic and extragenic suppressors were identified. A novel extragenic suppressor of dynein mutations was discovered, a gene encoding a putative E3 ubiquitin ligase with homologs present in higher organisms, including humans. Mutation or deletion of the suppressor gene results in restoration of wild type-like growth and in vivo dynein localization for each of the C-terminal dynein heavy chain mutants, as well as certain other dynein heavy chain mutants. Results suggest that the activity of this protein affects interaction of dynein heavy chain with dynein intermediate chain and likely is a regulator of dynein motor assembly.
Table of Contents
Introduction -- Materials and methods -- Results -- Discussion