Dual role for microtubules in regulating cortical contractility during cytokinesis

Microtubules stimulate contractile-ring formation in the equatorial cortex and simultaneously suppress contractility in the polar cortex; how they accomplish these differing activities is incompletely understood. We measured the behavior of GFP-actin in mammalian cells treated with nocodazole under conditions that either completely eliminate microtubules or selectively disassemble astral microtubules. Selective disassembly of astral microtubules resulted in functional contractile rings that were wider than controls and had altered dynamic activity, as measured by FRAP. Complete microtubule disassembly or selective loss of astral microtubules resulted in wave-like contractile behavior of actin in the non-equatorial cortex, and mislocalization of myosin II and Rho. FRAP experiments showed that both contractility and actin polymerization contributed to the wave-like behavior of actin. Wave-like contractile behavior in anaphase cells was Rho-dependent. We conclude that dynamic astral microtubules function to suppress Rho activation in the nonequatorial cortex, limiting the contractile activity of the polar cortex.