An astral simulacrum of the central spindle accounts for normal, spindle-less, and anucleate cytokinesis in echinoderm embryos.
Bottom Line: Although the nature, origin, and variety of such cues have long been obscure, one component is certainly the Rho activator Ect2.Here we describe the behavior and function of Ect2 in echinoderm embryos, showing that Ect2 migrates from spindle midzone to astral microtubules in anaphase and that Ect2 shapes the pattern of Rho activation in incipient furrows.In all these cases, the cell assembles essentially the same cytokinetic signaling ensemble—opposed astral microtubules decorated with Ect2 and Cyk4.
Affiliation: Oregon Institute of Marine Biology, Charleston, OR 97420 Cancer Research UK London Research Institute, Clare Hall Laboratories, South Mimms EN6 3LD, United Kingdom.Show MeSH
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Mentions: We took advantage of the fact that starfish oocytes can be cultured after injection for several days before fertilization to allow wt 3xGFP Ect2 to accumulate to visible levels in time for first cleavage. Despite mild cortical hyperactivity, starfish embryos better tolerated extra wt Ect2 while displaying identical localization as in urchins (Figure 4A and Supplemental Video S6). As in urchin embryos, we saw little evidence of an Ect2 gradient in the nascent furrow (unless Ect2 was expressed at pathological levels) until the early blastula stage. On the other hand, Ect2 was clearly detectable on astral microtubules even in the earliest divisions: coexpression of wt 3xGFP Ect2 with 2xmCh ensconsin microtubule-binding domain (EMTB) showed that Ect2 associated with distal portions of equatorially directed astral microtubules beginning in anaphase. Ect2-decorated astral microtubules reached progressively further toward the cortex and then became brighter as they were collected by the ingressing furrow (Figure 4, A′ and A″). Kymographs of the equatorial region illustrate redistribution of Ect2 from central spindle to the asters to the membrane over time as the furrow constricts (Figure 4B and Supplemental Figure S1D).
Affiliation: Oregon Institute of Marine Biology, Charleston, OR 97420 Cancer Research UK London Research Institute, Clare Hall Laboratories, South Mimms EN6 3LD, United Kingdom.