Actin cables (green) surround the spindle (red) as it assembles.

Cables of actin stretch from pole to pole of the mitotic spindle, and, with myosin, help control spindle length and shape, according to Woolner et al.

Microtubules and their motors are indispensible for mitotic spindle formation, but whether actin filaments and actin-based myosin motors are also part of the spindle apparatus is a controversial issue. Mitotic spindles interact with cortical actin, which is thought to anchor them. One protein that might be mediating this interaction is Myo10, an unconventional myosin that can bind both actin and microtubules. Because Myo10 is important for meiotic spindle formation, the authors sought to define its function in mitotic spindles.

They now find that knockdown of Myo10 leads to longer spindles, and fragmented star-shaped spindle poles. In the absence of Myo10, spindle rotation movements were slow and smooth rather than quick and jerky. The normally spasmodic spindle movement suggests that Myo10 anchors the spindle by forming brief, transient links with cortical actin.

Rescue experiments indicated that Myo10 has both actin-dependent and -independent functions during mitosis. Spindle pole fragmentation was rescued by Myo10 that lacked the actin-binding head domain. However, the long-spindle defect required the actin-binding region, or could be suppressed by disrupting actin, suggesting that actin helps elongate the spindle, whereas Myo10 shortens it.

As Myo10's actin-binding domain was required for normal spindle length, the authors sought actin in the spindle. Live cell imaging showed dynamic actin cables within mitotic spindles, oriented longitudinally and rotating along with the spindle as a whole, “as if the cables helped determine the direction of spindle motion,” says Woolner. The mechanism for coordination of actin and Myo10 is still unclear, but the authors envision Myo10 “walking along the actin, holding the poles at a fixed point.”

Woolner, S., et al.
J. Cell Biol.