Dynein (pink) transports vimentin (green) along microtubules.

Move over, vesicles. Intermediate filaments (IFs) are also a major cargo of microtubule-based motor proteins, according to new results from Helfand et al. on page 795. The results explain how rapid changes in IF structure are produced by transport of nonfilamentous vimentin particles.

Dynamic changes in vimentin IF structure occur at the onset of cell spreading, requiring the transport of vimentin particles along microtubules. The motor protein kinesin is responsible for plus-end–directed movement of vimentin toward the periphery of spreading cells. But bidirectional transport occurs, meaning kinesin is not the only motor protein involved.

Now, dynein is identified as the motor responsible for vimentin transport back toward the cell center. Helfand et al. found a strong in vivo association between vimentin particles and both dynein and components of dynactin, a dynein-associated complex.

Disruption of dynein activity caused massive relocation of vimentin filaments to the cell periphery, due to continued transport by kinesin. Thus, transport and maintenance of vimentin filaments are balanced by its association with the two motors. Vimentin is one of the most abundant proteins in fibroblasts and, given its strong association with dynein, is probably a major cargo for the motor protein.

When dynein activity was inhibited, a disrupted Golgi structure was seen along with the altered vimentin IF organization. The Golgi changes occurred before any obvious disruption in microtubule organization, suggesting that IFs may also function in the organization and distribution of the Golgi and possibly of other membranous organelles. ▪