A global, myosin light chain kinase-dependent increase in myosin II contractility accompanies the metaphase–anaphase transition in sea urchin eggs
Movie 3: The global rise in cortical contractility initiates at the metaphase-anaphase transition (6.344Mb)
Movie 4: Anaphase and cytokinesis in a spherical sea urchin imaged with polarization optics (8.941Mb)
Movie 5: The global rise in cortical contractility precedes the shift in spindle birefringence (9.362Mb)
Movie 10: Changes in cortical contractility in an egg treated with the ROCK inhibitor H-1152 (5.589Mb)
Movie 11: Changes in cortical contractility in an egg injected with MLCK inhibitory peptide (5.869Mb)
Movie 12: Changes in cortical contractility in an egg treated with the MLCK inhibitor ML-7 (6.994Mb)
Bresnick, Anne R.
Shuster, Charles B.
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Myosin II is the force-generating motor for cytokinesis, and although it is accepted that myosin contractility is greatest at the cell equator, the temporal and spatial cues that direct equatorial contractility are not known. Dividing sea urchin eggs were placed under compression to study myosin II-based contractile dynamics, and cells manipulated in this manner underwent an abrupt, global increase in cortical contractility concomitant with the metaphase–anaphase transition, followed by a brief relaxation and the onset of furrowing. Prefurrow cortical contractility both preceded and was independent of astral microtubule elongation, suggesting that the initial activation of myosin II preceded cleavage plane specification. The initial rise in contractility required myosin light chain kinase but not Rho-kinase, but both signaling pathways were required for successful cytokinesis. Last, mobilization of intracellular calcium during metaphase induced a contractile response, suggesting that calcium transients may be partially responsible for the timing of this initial contractile event. Together, these findings suggest that myosin II-based contractility is initiated at the metaphase–anaphase transition by Ca2+-dependent myosin light chain kinase (MLCK) activity and is maintained through cytokinesis by both MLCK- and Rho-dependent signaling. Moreover, the signals that initiate myosin II contractility respond to specific cell cycle transitions independently of the microtubule-dependent cleavage stimulus.
Author Posting. © American Society for Cell Biology, 2006. This article is posted here by permission of American Society for Cell Biology for personal use, not for redistribution. The definitive version was published in Molecular Biology of the Cell 17 (2006): 4093-4104, doi:10.1091/mbc.E06-02-0119.