Spindles are defective without cortical myosin pulling on them.


Mutants, RNAi, and extracts were unanimous: myosin II is not needed in mitosis before cytokinesis. But now Jody Rosenblatt, Karen McGee (University College London, England), and colleagues report that myosin II does, indeed, function in mitosis to pull apart and position centrosomes.

Rosenblatt did not set out to contradict the myosin II dogma. She was working on her primary interest of cell extrusion, checking the effect of blocking myosin, when “one day I was staining for tubulin and noticed spindle defects.” One colleague told her, “you really need to focus,” but she continued to see the unexpected phenotype. “Basically it was really crazy but it was happening,” she says. “It took me a long time to convince myself.”

That convincing involved getting the same phenotype—defective spindles and displaced or misaligned chromosomes—after any one of four treatments: myosin RNAi, an immobilizing cross-linking of the cortex with extracellular lectins, and applying either one of two different myosin-inhibiting chemicals. Some cells were not affected because their centrosomes had migrated sufficiently far apart along the nuclear envelope by the time of nuclear envelope breakdown. (This lack of an absolute requirement may have obscured myosin effects in earlier experiments.) But up to half needed some later maneuvering by myosin II.

Latex beads on the surface of cultured cells moved in the direction of centrosome movement. Rosenblatt suggests that myosin II activity in the cortex near centrosomes may be dampened by microtubules contacting a cortical protein. That would leave active only the myosin-based contraction on the opposite side of the cell. Centrosomes would be pulled apart toward that other side of the cell until microtubule–cortex contacts were equalized. ▪


Rosenblatt, J., et al. 2004. Cell. 117: 361–372.