Some growing MTs (black) get released from the centrosome and sent to the edge of the cell.

The centrosome is a nucleating center for microtubules (MTs) and the hub of the MT aster. On page 731, Abal et al. report that the centrosome in fibroblasts also seeds a second population of short MTs that are released and transported to the periphery. Both anchored and free MTs are needed for cell migration.

Abal et al. distinguished the two MT populations using GFP-tagged EB1, a protein that associates with growing MT plus ends. Visualization of EB1 in fibroblasts revealed that one set of MTs moved away from the centrosome toward the cell membrane more rapidly than could be explained by polymerization alone. The speed of this faster group stemmed from active transport of short but growing MTs by the motor dynein. Inhibition of dynein greatly diminished the number of free MTs. Free MTs were also reduced by increasing the centrosomal content of ninein, which anchors astral MTs to the centrosome.

The transport of free MTs participates in cell migration, as ninein-overexpressing fibroblasts could no longer migrate, but membrane ruffling (regulated by astral MTs) was not affected. Although it is still unclear how MT transport to the leading edge improves cell motility, proteins that break down focal adhesions might piggy-back on the free MTs. Alternatively, the short MTs may be captured by actin fibers to stabilize leading edge lamellipodia. ▪