βIII spectrin binds to the Arp1 subunit of dynactin

Cytoplasmic dynein is an intracellular motor responsible for endoplasmic reticulum-to-Golgi vesicle trafficking and retrograde axonal transport. The accessory protein dynactin has been proposed to mediate the association of dynein with vesicular cargo. Dynactin contains a 37-nm filament made up of the actin-related protein, Arp1, which may interact with a vesicle-associated spectrin network. Here, we demonstrate that Arp1 binds directly to the Golgi-associated beta III spectrin isoform. We identify two Arp1-binding sites in beta III spectrin, one of which overlaps with the actin-binding site conserved among spectrins. Although conventional actin binds weakly to beta III spectrin, Arp1 binds robustly in the presence of excess F-actin. Dynein, dynactin, and PHI spectrin co-purify on vesicles isolated from rat brain, and beta III spectrin co-immunoprecipitates with dynactin from rat brain cytosol. In interphase cells, beta III spectrin and dynactin both localize to cytoplasmic vesicles, co-localizing most significantly in the perinuclear region of the cell. In dividing cells, beta III spectrin and dynactin co-localize to the developing cleavage furrow and mitotic spindle, a novel localization for beta III spectrin. We hypothesize that the interaction between PHI spectrin and Arp1 recruits dynein and dynactin to intracellular membranes and provides a direct link between the microtubule motor complex and its membrane-bounded cargo.