beta III spectrin binds to the Arp1 subunit of dynactin

Cytoplasmic dynein is an intracellular motor responsible for endoplasmic re ticulum-to-Golgi vesicle trafficking and retrograde axonal transport. The a ccessory protein dynactin has been proposed to mediate the association of d ynein with vesicular cargo. Dynactin contains a 37-nm filament made up of t he actin-related protein, Arp1, which may interact with a vesicle-associate d spectrin network. Here, we demonstrate that Arp1 binds directly to the Go lgi-associated beta III spectrin isoform. We identify two Arp1-binding site s in beta III spectrin, one of which overlaps with the actin-binding site c onserved 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 c ytosol. In interphase cells, beta III spectrin and dynactin both localize t o cytoplasmic vesicles, co-localizing most significantly in the perinuclear region of the cell. In dividing cells, beta III spectrin and dynactin co-l ocalize to the developing cleavage furrow and mitotic spindle, a novel loca lization for beta III spectrin. We hypothesize that the interaction between PHI spectrin and Arp1 recruits dynein and dynactin to intracellular membra nes and provides a direct link between the microtubule motor complex and it s membrane-bounded cargo.