Equatorial Non-muscle Myosin II and Plastin Cooperate to Align and Compact F-actin Bundles in the Cytokinetic Ring

Abstract

Cytokinesis is the last step of cell division that physically partitions the mother cell into two daughter cells. Cytokinesis requires the assembly and constriction of a contractile ring, a circumferential array of filamentous actin (F-actin), non-muscle myosin II motors (myosin), and actin-binding proteins that forms at the cell equator. Cytokinesis is accompanied by long-range cortical flows from regions of relaxation toward regions of compression. In the C. elegans one-cell embryo, it has been suggested that anteriordirected cortical flows are the main driver of contractile ring assembly. Here, we use embryos co-expressing motor-dead and wild-type myosin to show that cortical flows can be severely reduced without major effects on contractile ring assembly and timely completion of cytokinesis. Fluorescence recovery after photobleaching in the ingressing furrow reveals that myosin recruitment kinetics are also unaffected by the absence of cortical flows. We find that myosin cooperates with the F-actin crosslinker plastin to align and compact F-actin bundles at the cell equator, and that this cross-talk is essential for cytokinesis. Our results thus argue against the idea that cortical flows are a major determinant of contractile ring assembly. Instead, we propose that contractile ring assembly requires localized concerted action of motor-competent myosin and plastin at the cell equator.

Plastin and spectrin cooperate to stabilize the actomyosin cortex during cytokinesis

“…In plst-1(prt89) embryos, F-actin bundles took longer to align circumferentially (Figures 5A and 5C). 16 In plst-1(prt89);sma-1(RNAi) embryos, equatorial F-actin bundles initially attempted to align circumferentially, similar to F-actin bundles in plst-1(prt89) embryos, but became increasingly disorganized around the time of equatorial deformation (Figures 5B and 5C). At this time, bright actin clusters started to appear in the equatorial region, and their number increased over time (Figures 5A, 5B, 6C, S5A).…”

“…Ring constriction rate (the slope of the linear region between $75% and 10% furrow ingression) was not affected after plst-1(RNAi) (0.19 ± 0.04 mm/s versus 0.19 ± 0.02 mm/s in controls), confirming that PLST-1 acts primarily at the early stages of cytokinesis (ring assembly and furrow initiation) (Figure 2D). 13,16 We conclude that among the nine conserved crosslinkers examined, only PLST-1 depletion impacts cytokinesis in the C. elegans one-cell embryo.…”

“…Inhibition of the plastin homolog PLST-1, which has previously been implicated in cytokinesis, delays contractile ring assembly but does not affect the speed of ring closure. 13,16 In the sensitized plastin inhibition background, the bH-spectrin SMA-1, but none of the other crosslinkers we examined, becomes essential for cytokinesis. Our findings indicate that interplay between plastin and bH/a-spectrin occurs via their actin-binding activities and does not involve SMA-1's plasma membrane anchoring function.…”

“…6,12,13 Plastin has been described to localize in cytokinetic rings, neurons, intestinal microvilli, hair cell stereocilia, and fibroblast filopodia, and to contribute to cytokinesis, invasion by pathogenic bacteria, endocytosis, and epidermal morphogenesis. [13][14][15][16] Spectrins are large and flexible crosslinkers that have not been previously implicated in cytokinesis. The functional form of spectrin consists of two antiparallel heterodimers of b-spectrin and a-spectrin.…”

Plastin 3 in X-Linked Osteoporosis: Imbalance of Ca2+-Dependent Regulation Is Equivalent to Protein Loss

“…In Caenorhabditis elegans zygotes, plastin increases cortical contractility and helps to stabilize myosin at the equatorial cortex (Ding et al, 2017 ; Leite et al, 2020 ), whereas the role of the fission yeast ortholog fimbrin in cytokinesis is controversial (Laporte et al, 2012 ; Christensen et al, 2019 ). Interestingly, plastin and myosin cooperate in cytokinesis (Leite et al, 2020 ) and epidermal morphogenesis/basement membrane assembly (Dor-On et al, 2017 ), while demonstrating mutually exclusive subcellular localization (Garbett et al, 2020 ), which can result from direct competition between myosin and plastin for binding sites on actin (Behrmann et al, 2012 ; Schwebach et al, 2020 ). Additional explanation for this antagonism is competition with tropomyosin, which has been demonstrated for yeast fimbrin (Christensen et al, 2019 ), but not yet for mammalian plastin and tropomyosin isoforms.…”