Trafficking of early endosomes requires the small GTPase RhoD. Zerial's group identified the first known downstream effector for RhoD, hDia2C, a relative of the actin-polymerizing protein formin. In vivo, RhoD recruited hDia2C onto endosomes, where the two proteins probably stabilize the association of endosomes with the actin network, as their overexpression caused the vesicles to align along actin filaments. In the overexpressing cells, endosomal motility was also slowed and long-range trafficking was inhibited, probably because the endosomes were unable to jump from actin to microtubules, where Rab5 stimulates long-range movements.
The results indicate a similarity between endosome motility and adhesion formation. Endosome-localized hDia2C recruited and activated c-Src kinase, which was required for the effects of RhoD and hDia2C on trafficking. Src is therefore another example of a signaling molecule required for organelle transport, although its relevant phosphorylation targets have not been identified. Actin remodeling during focal contact formation involves similar players, namely, RhoA, mDia1, and Src. Thus, although different molecules are used, the mechanisms underlying actin-dependent organelle trafficking and adhesion assembly appear to be conserved. ▪