No abstract

Summary The microtubule motors kinesin and dynein function collectively to drive vesicular transport. High resolution tracking of vesicle motility in the cell indicates that transport is often bidirectional, characterized by frequent directional changes. However, the mechanisms coordinating the collective activities of oppositely-oriented motors bound to the same cargo are not well understood. To examine motor coordination, we purified neuronal transport vesicles and analyzed their motility using automated particle tracking with nanometer resolution. The motility of purified vesicles reconstituted in vitro closely models the movement of Lysotracker-positive vesicles in primary neurons, where processive bidirectional motility is interrupted with frequent directional switches, diffusional movement and pauses. Quantitative analysis indicates that vesicles co-purify with a low number of stably-bound motors: 1–5 dynein and 1–4 kinesin motors. These observations compare well to predictions from a stochastic tug-of-war model, where transport is driven by the force-dependent kinetics of teams of opposing motors in the absence of external regulation. Together, these observations indicate that vesicles move robustly with a small complement of tightly-bound motors, and suggest an efficient regulatory scheme for bidirectional motility where small changes in the number of engaged motors manifest in large changes in the motility of cargo.

To test the hypothesis that inhibition of axonal transport is sufficient to cause motor neuron degeneration such as that observed in amyotrophic lateral sclerosis (ALS), we engineered a targeted disruption of the dynein-dynactin complex in postnatal motor neurons of transgenic mice. Dynamitin overexpression was found to disassemble dynactin, a required activator of cytoplasmic dynein, resulting in an inhibition of retrograde axonal transport. Mice overexpressing dynamitin demonstrate a late-onset progressive motor neuron degenerative disease characterized by decreased strength and endurance, motor neuron degeneration and loss, and denervation of muscle. Previous transgenic mouse models of ALS have shown abnormalities in microtubule-based axonal transport. In this report, we describe a mouse model that confirms the critical role of disrupted axonal transport in the pathogenesis of motor neuron degenerative disease.

The mechanistic heart of the ubihydroquinone-cytochrome c oxidoreductase (cyt bc1 complex) is the catalytic oxidation of ubihydroquinone (QH2) at the Qo site. QH2 oxidation is initiated by ferri-cyt c, mediated by the cyt c1 and [2Fe-2S] cluster of the cytochrome bc1 complex. QH2 oxidation in turn drives transmembrane electronic charge separation through two b-type hemes to another ubiquinone (Q) at the Qi site. In earlier studies, residues F144 and G158 of the b-heme containing polypeptide of the Rhodobacter capsulatus cyt bc1 complex were shown to be influential in Qo site function. In the present study, F144 and G158 have each been singly substituted by neutral residues and the dissociation constants measured for both Q and QH2 at each of the strong and weak binding Qo site domains (Qos and Qow). Various substitutions at F144 or G158 were found to weaken the affinities for Q and QH2 at both the Qos and Qow domains variably from zero to beyond 10(3)-fold. This produced a family of Qo sites with Qos and Qow domain occupancies ranging from nearly full to nearly empty at the prevailing approximately 3 x 10(-2) M concentration of the membrane ubiquinone pool (Qpool). In each mutant, the affinity of the Qos domain remained typically 10-20-fold higher than that of the Qow domain, as is found for wild type, thereby indicating that the single mutations caused comparable extents of the weakening at each domain. Moreover, the substitutions were found to cause similar decreases of the affinities of both Q and QH2 in each domain, thereby maintaining the Q/QH2 redox midpoint potentials (Em7) of the Qo site at values similar to that of the wild type. Measurement of the yield and rate of QH2 oxidation generated by single turnover flashes in the family of mutants suggests that the Qos and Qow domains serve different roles for the catalytic process. The yield of the QH2 oxidation correlates linearly with Qos domain occupancy (QH2 or Q), suggesting that the Qos domain exchanges Q or QH2 with the Qpool at a rate which is much slower than the time scale of turnover.(ABSTRACT TRUNCATED AT 400 WORDS)

microtubule motility ͉ membrane trafficking ͉ molecular motors ͉ bidirectional motility ͉ Huntington's disease