Dynactin Is Required for Coordinated Bidirectional Motility, but Not for Dynein Membrane Attachment

Abstract

Transport of cellular and neuronal vesicles, organelles, and other particles along microtubules requires the molecular motor protein dynein (Mallik and Gross, 2004). Critical to dynein function is dynactin, a multiprotein complex commonly thought to be required for dynein attachment to membrane compartments (Karki and Holzbaur, 1999). Recent work also has found that mutations in dynactin can cause the human motor neuron disease amyotrophic lateral sclerosis (Puls et al., 2003). Thus, it is essential to understand the in vivo function of dynactin. To test directly and rigorously the hypothesis that dynactin is required to attach dynein to membranes, we used both a Drosophila mutant and RNA interference to generate organisms and cells lacking the critical dynactin subunit, actin-related protein 1. Contrary to expectation, we found that apparently normal amounts of dynein associate with membrane compartments in the absence of a fully assembled dynactin complex. In addition, anterograde and retrograde organelle movement in dynactin deficient axons was completely disrupted, resulting in substantial changes in vesicle kinematic properties. Although effects on retrograde transport are predicted by the proposed function of dynactin as a regulator of dynein processivity, the additional effects we observed on anterograde transport also suggest potential roles for dynactin in mediating kinesin-driven transport and in coordinating the activity of opposing motors (King and Schroer, 2000).

Transient binding of dynein controls bidirectional long-range motility of early endosomes

“…These motors simultaneously bind to their cargo, which was shown in various cell systems, including fish and frog melanosomes (37), amoebas (38), lipid droplets in flies (14), intraflagellar transport in worms and algae (39,40), and vesicles in mammalian axons (41,42). The concurrent presence of kinesin and dynein is thought to be crucial for motor activity (3,13,43), but also recycles the motor back to MT plus ends (39,41). In fungi, kinesin-1 is thought to be involved in this process (26,30), making it likely that the observed anterograde motility of dynein is due to kinesin-1 activity.…”

The axonal transport of mitochondria

“…Biochemical and immunoelectron microscopy analyses have demonstrated that dynein and dynactin associate with mitochondria (Habermann et al, 2001;Pilling et al, 2006;Varadi et al, 2004), and that blocking the interaction between DIC and dynactin components in Xenopus extracts reduces the association of dynein heavy chains with cytoplasmic membranes (Steffen et al, 1997). Finally, genetic inhibition of dynactin components in Drosophila causes axon degeneration and results in defective axonal transport of mitochondria (Haghnia et al, 2007;Martin et al, 1999;Pilling et al, 2006). Although these observations support the notion that an interaction between dynein and mitochondria is mediated through DIC and dynactin, direct evidence is lacking.…”

Motor Neuron Disease Occurring in a Mutant Dynactin Mouse Model Is Characterized by Defects in Vesicular Trafficking

“…Although dynactin has been postulated to enhance the processivity and efficiency of the dynein motor (LaMonte et al, 2002;Schroer, 2004), previous studies demonstrate in vitro that the dynein-dynactin complexes exhibit bidirectional processive motility (Ross et al, 2006) and in vivo that dynactin is required for coordinated anterograde and retrograde organelle movement (Haghnia et al, 2007). The disruption of the dynactin complex by overexpression of dynamitin (LaMonte et al, 2002) results in motor neuron disease in mice, but the exact pathogenic mechanism remains unknown.…”