Filopodia sprout from a cell after a cargo-free MyoX (green) dimerizes.

The motor protein MyoX is more than a cellular U-Haul, as Tokuo et al. now show. The protein also helps a cell crawl by muscling actin filaments into position at the front edge of the membrane.

As a cell slithers, it sends out skinny extensions called filopodia that help guide its movements. Previous work has shown that the cargo-hauling protein MyoX spurs formation of these structures. The molecule's head grips and slides along actin filaments, while its tail holds cargo. MyoX travels to the tips of filopodia, and researchers assumed that the cargos it takes there stimulate the extensions to sprout and grow.

That explanation was only half right, as Tokuo et al. found when they tested tailless MyoX molecules that can't ferry anything. Dimers of the trimmed molecules still triggered filopodia, but the extensions were stumpy and short lived. Bundles of actin filaments normally line up along the leading edge of a crawling cell. This orderly arrangement vanished when MyoX was eliminated using RNAi.

The results indicate that MyoX has two jobs during filopodium formation. First, the motor portion bunches up actin filaments at the base of the incipient filopodium, prompting it to bulge out. Then MyoX can slide along the actin fibers into the protrusion, where it deposits its cargos that cause further elongation. The team now wants to determine how MyoX gets to the cell's leading edge and how it wrenches the actin filaments into place.


Tokuo, H., et al.
J. Cell Biol.