Two populations of microtubules have different functions during cytokinesis.

Two distinct populations of spindle microtubules provide signals that initiate and complete cytokinesis, according to a proposition from Inoue et al. (page 49).

During interphase, microtubules radiate from centrosomes in all directions. But after chromosome segregation a structure called the central spindle forms. It provides the signals that place the actin-based cytokinetic furrow in the right place and then orchestrates its actions.

The authors take a close look at the central spindle in living fly spermatocytes and find that this structure, previously thought to be uniform, is composed of two populations of microtubules. These peripheral and interior microtubules were both geographically and biochemically distinct, with only the interior microtubules associated with the Orbit/Mast protein.

In mutants with reduced levels of Orbit protein, formation of the interior bundle of central spindle microtubules was defective. As in wild-type cells, peripheral microtubules still probed the actin-rich cortex leading to cleavage furrow initiation, but the furrows in mutants eventually snapped back. This failure in cleavage was accompanied by the mislocalization of several proteins thought to be involved in generating the cytokinesis signal.

Orbit helps to stabilize microtubule plus ends. This stabilization may not be favorable for the highly dynamic peripheral microtubules but may be essential for the maintenance of the more stable interior microtubules. As is clear from mutants with less Orbit, a full description of cytokinesis may have to account for the coordination not only of the actin and microtubule systems but also of two distinct microtubule populations. â–ª