Ring Constriction Research Articles

Contractile Ring Constriction and Cell Wall Growth are Regulated by Mechanical Feedback and Destabilized by Mutations in Fission Yeast

Contractile Ring Constriction and Cell Wall Growth are Regulated by Mechanical Feedback and Destabilized by Mutations in Fission Yeast

Myosins in Cytokinesis.

Nearly five decades of research have established myosin as the main motor responsible for cytokinesis in organisms on the branch of the phylogenetic tree that includes amoebas, fungi and animals. This research has grown to be more mechanistic over the past decade, so we now have computer simulations of physically reasonable models that explain how myosins contribute to the assembly and constriction of contractile rings that pinch dividing cells into two daughter cells. Isoforms of myosin-II, from the same family as muscle myosins, are the main myosins for cytokinesis, but other myosins contribute to cytokinesis in fission yeast. Progress has been made on how animal cells use Rho-GTPases to control the accumulation and activity of myosin-II at the site of cleavage, but the regulatory mechanisms are less clear in other systems.

Insight into the Mechanisms Underlying the Tracheorelaxant Properties of the Sideritis raeseri Extract.

Sideritis raeseri Boiss. and Heldr. (Lamiaceae), known as “mountain tea,” is a native plant from the Mediterranean region, which is widely used in traditional medicine. This study evaluates the effects of the ethanol extract of Sideritis raeseri (SR) on airway smooth muscle activity and identifies the underlying mechanism. The S. raeseri extract (SRE) was extracted from air-dried parts of the shoot system of SR. The SRE (0.3–2 mg/mL) was tested in isolated rabbit tracheal rings, suspended in the organ bath, filled with Krebs solution, and bubbled with the carbogen mixture (95% O2/5% CO2) under a resting tension of 1 g in 37°C. In in vitro experiments, the SRE relaxed against acetylcholine-induced constriction in tracheal rings. Furthermore, SRE inhibited Ca2+-induced contractions in carbachol (CCh, 1 μM) as well as in the K+-depolarized trachea (80 mM). Our findings showed the NO/cGMP involvement in tracheorelaxant effects of SR. To this end, the effect of the SRE was potentiated by bradykinin (nitric oxide (NO) synthase activator, 100 nM), whereas it was inhibited by ODQ (inhibitor of NO-sensitive guanylyl cyclase, 10 μM) and L-NAME (NO synthase inhibitor, 100 μM), as well as indomethacin (cyclooxygenase inhibitor, 10 μM). These data suggest that the tracheorelaxant effect of the SRE is mediated at least partly by NO/cyclic guanosine monophosphate and cyclooxygenase-1-prostaglandin E2-dependent signaling. These findings indicate that the SRE may be used in various respiratory disorders.

Acquired left diaphragmatic hernia after orthotopic liver transplantation in adults

Background: Acquired diaphragmatic hernia (ADH) is a rare complication following surgery, with an incidence ranging between 0.8-5%. Regarding liver transplantation, only case series have been described in relation to pediatric and living donor transplants. We present a case of diaphragmatic hernia after orthotopic liver transplantation in adult. Material & Methods: A 68-year-old man was admitted to our hospital 18 months after liver transplantation for hepatocellular carcinoma, with a 2-day history of abdominal pain, vomiting and intestinal closure. On chronic treatment with Tacrolimus, Mycophenolic acid and Prednisone. He had no fever. Bloodtest showed PCR 2.28 without leukocytosis. He underwent a chest X-ray without findings. Due to his poor health status, an abdominal CT was performed that reported a left diaphragmatic hernia including the colon and small intestine together with an important gastric-duodenal and proximal jejunal dilatation. Result: Our patient underwent surgery finding dilated small bowel in left upper hypochondrium. A large left diaphragmatic hernia was found, with an orifice independent of the esophageal hiatus, whereby 80 cm of the middle jejunum, omentum, and transverse colon were herniated into the thoracic cavity. The hernia was reduced, observing a constriction ring. No intestinal resection was necessary. The diaphragmatic defect was closed with two opposing absorbable sutures. The patient presented a satisfactory evolution and he was discharged at home 6 days later. Follow-up at 13 months showed the patient is well with normal graft function. Conclusion: AHD after liver transplantation is a very rare and potentially fatal complication, occurring mainly in the pediatric recipient. In the last 10 years about 40 cases of AHD have been described after liver transplantation in pediatrics, being exceptional its presentation in the adult, with a single case published up to now, being fundamental the early diagnosis and the surgical treatment.

Retained Penile Ring: Case Reports

Background: Penile constriction rings have been used to prolong erection with vacuum devices in the treatment of erectile dysfunction. Some have also employed it for recreational purposes with the attendant risks of penile edema, strangulation and necrosis. We present two cases of entrapped phallus by penile rings. Case reports: 16 years old with pain and swelling of the phallus of 12 hours duration. This started after placement of a phallic ring to purportedly prolong erection which he was later unable to remove the ring despite several attempts and developed progressive swelling and pain in the phallus. He was also unable to pass urine. Examination revealed a circumcised phallus that was detumescent but grossly edematous, tender with shiny skin. Initial simple manoeuvres in the casualty proved abortive and ring had to be cut with an orthopaedic instrument. He was subsequently able to void without difficulty or any haematuria. 47 year old Asian male with a 12 hour history of penile pain and swelling following entrapment of a penile ring he had inserted for sexual gratification about 24 hours before presentation. There was a history of haematuria. The removal of the penile ring was achieved with a combination of cold compress squeeze, corporal aspiration and lubrication under general anaesthesia. Conclusion: Successful management of retained penile ring depends on early presentation by the patient to avoid adverse outcomes. It may require instruments not normally found in the urologist armamentarium.

Using the Four-Cell C. elegans Embryo to Study Contractile Ring Dynamics During Cytokinesis.

Cytokinesis is the process that completes cell division by partitioning the contents of the mother cell between the two daughter cells. It involves the highly regulated assembly and constriction of an actomyosin contractile ring, whose function is to pinch the mother cell in two. Research on the contractile ring has particularly focused on the signaling mechanisms that dictate when and where the ring is formed. In vivo studies of ring constriction are however scarce and its mechanistic understanding is therefore limited. Here we present several experimental approaches for monitoring ring constriction in vivo, using the four-cell C. elegans embryo as model. These approaches allow for the ring to be perturbed only after it forms and include the combination of live imaging with acute drug treatments, temperature-sensitive mutants and rapid temperature shifts, as well as laser microsurgery. In addition, we explain how to combine these with RNAi-mediated depletion of specific components of the cytokinetic machinery.

F-BAR Cdc15 Promotes Cdc42 Activation During Cytokinesis and Cell Polarization in Schizosaccharomyces pombe.

Cdc42, a Rho-family GTPase, is a master regulator of cell polarity. Recently, it has been shown that Cdc42 also facilitates proper cytokinesis in the fission yeast Schizosaccharomyces pombe Cdc42 is activated by two partially redundant GEFs, Gef1 and Scd1. Although both GEFs activate Cdc42, their deletion mutants display distinct phenotypes, indicating that they are differentially regulated by an unknown mechanism. During cytokinesis, Gef1 localizes to the division site and activates Cdc42 to initiate ring constriction and septum ingression. Here, we report that the F-BAR protein Cdc15 promotes Gef1 localization to its functional sites. We show that cdc15 promotes Gef1 association with cortical puncta at the incipient division site to activate Cdc42 during ring assembly. Moreover, cdc15 phospho-mutants phenocopy the polarity phenotypes of gef1 mutants. In a hypermorphic cdc15 mutant, Gef1 localizes precociously to the division site and is readily detected at the cortical patches and the cell cortex. Correspondingly, the hypermorphic cdc15 mutant shows increased bipolarity during interphase and precocious Cdc42 activation at the division site during cytokinesis. Finally, loss of gef1 in hypermorphic cdc15 mutants abrogates the increased bipolarity and precocious Cdc42 activation phenotype. We did not see any change in the localization of the other GEF Scd1 in a Cdc15-dependent manner. Our data indicate that Cdc15 facilitates Cdc42 activation at the division site during cytokinesis at the cell cortex to promote bipolarity and this is mediated by promoting Gef1 localization to these sites.

Who Needs a Contractile Actomyosin Ring? The Plethora of Alternative Ways to Divide a Protozoan Parasite.

Cytokinesis, or the division of the cytoplasm, following the end of mitosis or meiosis, is accomplished in animal cells, fungi, and amoebae, by the constriction of an actomyosin contractile ring, comprising filamentous actin, myosin II, and associated proteins. However, despite this being the best-studied mode of cytokinesis, it is restricted to the Opisthokonta and Amoebozoa, since members of other evolutionary supergroups lack myosin II and must, therefore, employ different mechanisms. In particular, parasitic protozoa, many of which cause significant morbidity and mortality in humans and animals as well as considerable economic losses, employ a wide diversity of mechanisms to divide, few, if any, of which involve myosin II. In some cases, cell division is not only myosin II-independent, but actin-independent too. Mechanisms employed range from primitive mechanical cell rupture (cytofission), to motility- and/or microtubule remodeling-dependent mechanisms, to budding involving the constriction of divergent contractile rings, to hijacking host cell division machinery, with some species able to utilize multiple mechanisms. Here, I review current knowledge of cytokinesis mechanisms and their molecular control in mammalian-infective parasitic protozoa from the Excavata, Alveolata, and Amoebozoa supergroups, highlighting their often-underappreciated diversity and complexity. Billions of people and animals across the world are at risk from these pathogens, for which vaccines and/or optimal treatments are often not available. Exploiting the divergent cell division machinery in these parasites may provide new avenues for the treatment of protozoal disease.

Coordinating septum formation and the actomyosin ring during cytokinesis in Schizosaccharomyces pombe.

During cytokinesis, animal and fungal cells form a membrane furrow via actomyosin ring constriction. Our understanding of actomyosin ring-driven cytokinesis stems extensively from the fission yeast model system. However, unlike animal cells, actomyosin ring constriction occurs simultaneously with septum formation in fungi. While the formation of an actomyosin ring is essential for cytokinesis in fission yeast, proper furrow formation also requires septum deposition. The molecular mechanisms of spatiotemporal coordination of septum deposition with actomyosin ring constriction are poorly understood. Although the role of the actomyosin ring as a mechanical structure driving furrow formation is better understood, its role as a spatiotemporal landmark for septum deposition is not widely discussed. Here we review and discuss the recent advances describing how the actomyosin ring spatiotemporally regulates membrane traffic to promote septum-driven cytokinesis in fission yeast. Finally, we explore emerging questions in cytokinesis, and discuss the role of extracellular matrix during cytokinesis in other organisms.

Comparison of structural progression between ciliopathy and non-ciliopathy associated with autosomal recessive retinitis pigmentosa

BackgroundTo evaluate and compare the progression of ciliopathy and non-ciliopathy autosomal recessive Retinitis Pigmentosa patients (arRP) by measuring the constriction of hyperautofluorescent rings in fundus autofluorescence (FAF) images and the progressive shortening of the ellipsoid zone line width obtained by spectral-domain optical coherence tomography (SD-OCT).ResultsFor the ciliopathy group, the estimated mean shortening of the ellipsoid zone line was 259 μm per year and the ring area decreased at a rate of 2.46 mm2 per year. For the non-ciliopathy group, the estimated mean shortening of the ellipsoid zone line was 84 μm per year and the ring area decreased at a rate of 0.7 mm2 per year.ConclusionsOur study was able to quantify and compare the loss of EZ line width and short-wavelength autofluorescence (SW-AF) ring constriction progression over time for ciliopathy and non-ciliopathy arRP genes. These results may serve as a basis for modeling RP disease progression, and furthermore, they could potentially be used as endpoints in clinical trials seeking to promote cone and rod survival in RP patients.

One stage release of circumferential constriction band in upper limb with multiple Z-plasty technique: a case report

INTRODUCTION: Congenital constriction band is an uncommon congenital abnormality with multiple disfiguring and disabling manifestations. The widely known management for the correction of congenital constriction band especially in circumferential constriction band is done in a multistage release of constriction band to eluding vascular and lymphatic embarrassment. The purpose of this case report was to present the outcome of one-stage release in terms of safety and aesthetic appearance. CASE DESCRIPTION:Women 28 year old came to outpatient plastic surgery clinic presented with circumferential constriction with no functional disability in her right arm since birth. Excision based on design of multiple Z-Plasty is started 2 cm proximal and distal from constricted band. The multiple Z- Plasty flaps are transposed around each other, changing the direction of the constriction band. Postoperatively, the extremity circulation was normal, and no edema was observed. The range of motion is within normal limits and aesthetically acceptable for the patient. DISCUSSION: Patterson classified constriction ring syndrome into four types, and type II is the most common. In type II Paterson , there are normal full-layer skin structures and no important nerve or blood vessel was affected so an incision completely encircling an extremity would not interfere with distal circulation and provided strong evidence that one-stage resection of constriction band could be performed. Using multiple Z-Plasty for constriction band release leads to improvement of functional and cosmetic appearance of the defect. CONCLUSION: A one-stage release of circumferential constriction band in the upper limb using multiple Z-Plasty is considered as a safe treatment with an aesthetically acceptable result for the patient

Long-Term Reproductive Outcomes after Hysteroscopic Treatment of Dysmorphic Uteri in Women with Reproductive Failure: An European Multicenter Study

To evaluate the long-term reproductive outcomes in patients with dysmorphic uterus treated by hysteroscopic metroplasty with miniaturized instruments. Retrospective multicenter cohort study. Tertiary care university hospitals. The study was conducted on 214 women with a dysmorphic uterus (T-shaped, infantilis, or other type of dysmorphic uterus according to the European Society of Human Reproduction and Embryology and the European Society for Gynaecological Endoscopy classification system) with history of primary unexplained infertility (group 1) or repeated (>2) early miscarriages (group 2). Dysmorphic uteri were diagnosed by office hysteroscopy and 3-dimensional transvaginal ultrasound (3D-TVS). All patients underwent in office hysteroscopic metroplasty using a continuous-flow hysteroscope with a 5 Fr operating channel introduced into the uterine cavity using the vaginoscopic approach. Longitudinal incisions were performed on the fibromuscular constriction rings in the isthmic area and in some cases on the other uterine walls with a 5 Fr bipolar electrode or scissors. At the end of the procedure, an antiadhesive gel was applied into the uterine cavity to minimize adhesion formation. Postsurgical assessment of the uterine cavity was carried out through office hysteroscopy and 3D-TVS. All patients were followed for at least 24 months. The metroplasty was completed in all cases, resulting in a significant increase of uterine cavity volume (100%) and optimization of uterine morphology in 211 of 214 women (98.6%). After 60 months, the overall clinical pregnancy rate was 72.9% (n = 156/214), and the live birth rate was 80.1% (n = 125/156). Specifically, 74 of 156 women (47.4%) conceived spontaneously (with a median time to pregnancy of 5.5 months), of whom 32.4% had previously failed 1 or more attempts at in vitro fertilization/intracytoplasmic sperm injection. Our long-term follow-up data demonstrate that the hysteroscopic correction of dysmorphic uteri may result in a high live birth rate in women suffering from unexplained infertility or repeated miscarriages.

Correlation between B-scan optical coherence tomography, en face thickness map ring and hyperautofluorescent ring in retinitis pigmentosa patients.

To evaluate and compare the B-scan OCT loss of ellipsoid zone, OCT en face thickness map constriction, and hyperautofluorescent ring constriction in RP patients. Retrospective case series study. Forty-eight eyes of 24 RP patients with a parafoveal hyperautofluorescent ring were studied. The diagnosis of RP was established by the presence of rod response impairment and a prevalent decrease of scotopic over photopic responses on electroretinography. The FAF and spectral-domain optical coherence tomography (SD-OCT) images were obtained from 24 patients with RP. The measurements of the EZ line width on B-scan OCT, hyperautofluorescent ring area on FAF, and hyperautofluorescent ring area on en face thickness map were performed by two independent graders. The measurements of these three parameters were correlated. The mean age of study patients was 46years old (sd = 19). The external and internal FAF rings involving the fovea were identified in all study eyes. The area of the thickness ring decreased at an average rate of 0.5 (sd 0.4) mm2 per year (P < 0.001). The average rate of EZ-line constriction was estimated to be 123 (sd 63) μm per year (P < 0.001). The hyperautofluorescent ring area decreased at an average rate of 0.9 (sd 0.98) mm2 per year (P < 0.001). The strongest correlation was observed between hyperautofluorescent ring area and EZ-line width (r = 0.78). We observed that the hyperautofluorescent ring area exhibits a faster progression rate than the area of the thickness ring. In addition, we found that the EZ-line width had a high positive correlation with the hyperautofluorescent ring area and a moderate positive correlation with area of the thickness ring.

Actomyosin contraction during cellularization is regulated in part by Src64 control of Actin 5C protein levels.

Src64 is required for actomyosin contraction during cellularization of the Drosophila embryonic blastoderm. The mechanism of actomyosin ring constriction is poorly understood even though a number of cytoskeletal regulators have been implicated in the assembly, organization, and contraction of these microfilament rings. How these cytoskeletal processes are regulated during development is even less well understood. To investigate the role of Src64 as an upstream regulator of actomyosin contraction, we conducted a proteomics screen to identify proteins whose expression levels are controlled by src64. Global levels of actin are reduced in src64 mutant embryos. Furthermore, we show that reduction of the actin isoform Actin 5C causes defects in actomyosin contraction during cellularization similar to those caused by src64 mutation, indicating that a relatively high level of Actin 5C is required for normal actomyosin contraction and furrow canal structure. However, reduction of Actin 5C levels only slows down actomyosin ring constriction rather than preventing it, suggesting that src64 acts not only to modulate actin levels, but also to regulate the actomyosin cytoskeleton by other means.

Mid- to long-term outcomes of the ‘anatomical approach’ to congenital megaprepuce repair

The understanding of the aetiopathogenesis and the long-term outcomes of correctional repair of congenital megaprepuce (CMP) remains unestablished. Different techniques have been described; however, optimum results have been difficult to achieve. The aim of our study was to review our experience of the 'anatomical approach' to the correction of CMP. This is a retrospective review of boys who underwent CMP repair between 2003 and 2014. All of them had the anatomical approach: A curved incision is made between the scrotal and penile skin. Circumferential dissection superficial to Buck's fascia frees the penis and allows the scrotum to assume a more caudal position. The dissection distally lifts the preputial sac that is opened ventrally. The redundant inner preputial skin is excised. After full degloving, the dorsal flap of skin is thinned. The base of the flap is anchored, and the rectangle of skin is wrapped around the shaft. The remaining diamond-shaped incision is closed in a vertical line. Postoperatively, foam dressings and urethral catheter are left in place. Symptomatology, hospital stay, postoperative complications, redosurgery, voiding and cosmesis were evaluated. Sixty-one patients had surgery at 5-151 months (median 17 months). Medianhospital stay was 24h (12-168h). Foam dressings and urethral catheter were used in 47 boys and removed in outpatient clinic after 3-7 days. There were two immediate postoperative complications requiring surgery. One due to severe oedema and constriction ring, and the other had wound infection with wound breakdown, both in patients with no dressings (P<0.05). Follow-up was available for 58 patients, with a median of 22 months (3-79 months). Two required redosurgery (3.5%), one with poor cosmetic result and one recurrence. In all patients, a normal circumcised appearance had been achieved and in 47, cosmetic result was excellent (81%). Five patients have residual suprapubic fat, and four, excess ventral skin. One with residual suprapubic fat has reached puberty and it has resolved. All declared good stream. The same technique was used for six patients with poor cosmetic result after surgery elsewhere. The outcome is excellent for all, with median follow-up of 20.5 months (5-31 months). The 'anatomical' repair provides the ability to create the appearance of a standard circumcision, which is generally accepted as normality. Apart from the penis itself, this technique facilitates correction of the penoscrotal transposition, giving the external genitalia the best appearance. It seems to provide good functional and cosmetic outcomes, and it holds true at mid- to long-term follow-up. Dressings and catheter can potentially prevent postoperative complications. However, a prospective study with standardised parameters is required to confirm these findings.

Fission yeast type 2 node proteins Blt1p and Gef2p cooperate to ensure timely completion of cytokinesis

BackgroundThe conserved NDR-family kinase Sid2p localizes to the contractile ring during fission yeast cytokinesis to promote ring constriction, septation, and completion of cell division. Previous studies have found that the Type 2 interphase node proteins Blt1p and Gef2p contribute to localization of Sid2p and its regulatory protein Mob1p at the division site. However, their relative contributions and whether they operate in the same or parallel pathways has been unclear. In this study, we quantify the respective roles of Blt1p and Gef2p in Sid2p/Mob1p recruitment and characterize the effect of single and double deletion mutants on contractile ring dynamics and completion of cell division.ResultsUsing quantitative confocal fluorescence microscopy, we measured Sid2p and Mob1p recruitment to the division site in blt1∆, gef2∆, and blt1∆/gef2∆ mutant cells. We observed an equivalent decrease in Sid2p/Mob1p localization for both single and double mutants. Though assembly of the contractile ring is normal in these mutants, the reduction in Sid2p/Mob1p at the division site delayed the onset of contractile ring constriction and completion of division. We quantified localization of Blt1p and Gef2p at the medial cortex throughout the cell cycle and found that Blt1p localization to interphase nodes and the contractile ring is independent of Gef2p. However, Gef2p localization to the contractile ring is decreased in blt1∆ mutants.ConclusionsBlt1p and Gef2p work in the same pathway, rather than in parallel, to localize the NDR-family kinase Sid2p and its regulatory partner Mob1p to the division site, thereby promoting timely completion of cell division. Future studies are necessary to understand how additional fission yeast cytokinesis proteins work with these Type 2 interphase node components to promote Sid2p/Mob1p recruitment.

Fundus autofluorescence and ellipsoid zone (EZ) line width can be an outcome measurement in RHO-associated autosomal dominant retinitis pigmentosa.

To evaluate the progression of retinitis pigmentosa (RP) due to mutations in rhodopsin (RHO) by measuring the short-wavelength autofluorescence (SW-AF) increased autofluorescence ring and ellipsoid zone (EZ)-line width. Fundus autofluorescence (FAF) and spectral domain optical coherence tomography (SD-OCT) images were obtained from 10 patients with autosomal dominant RP due to mutations in the RHO gene. Measurements of ring area on FAF images, as well as the EZ line width on SD-OCT images and horizontal, vertical diameter, were performed by two independent masked graders. The ring area decreased by a rate of 0.6 ± 0.2mm2 per year. We observed that the EZ line width decreased by an average of 152 ± 37μm per year, while the horizontal and vertical diameters decreased by 106 ± 35μm and 125 ± 29μm per year, respectively. Progression rates were similar between eyes. We observed SW-AF ring constriction and a progressive loss of EZ line width over time.

Amniotic Band Syndrome with holoprosencephaly in Palestinian neonate: case report

Amniotic band syndrome (ABS) is a rare congenital anomaly. It is associated with different limb deformities or amputation. Severe cases have multiple constriction rings and it could be associated with disfigured face, brain and spine. Our case was a male newborn and he was severely affected. The baby had multiple constriction bands, amputated fingers, and toes. He had cleft lip and palate, malformed nose, hypertelorism, exophthalmos, red-sided absent eyelids, and low set ears and holoprosencephaly.

A Cdc42 GEF, Gef1, through endocytosis organizes F-BAR Cdc15 along the actomyosin ring and promotes concentric furrowing.

During cytokinesis, fission yeast coordinates actomyosin ring constriction with septum ingression, resulting in concentric furrow formation by a poorly defined mechanism. We report that Schizosaccharomyces pombe cells lacking the Cdc42 activator Gef1, combined with an activated allele of the formin, Cdc12, display non-concentric furrowing. Non-concentrically furrowing cells display uneven distribution of the scaffold Cdc15 along the ring. This suggests that, after ring assembly, uniform Cdc15 distribution along the ring enables proper furrow formation. We find that, after assembly, Cdc15 is recruited to the ring in an Arp2/3 complex-dependent manner and is decreased in the activated cdc12 mutant. Cdc15 at cortical endocytic patches shows increased levels and extended lifetimes in gef1 and activated cdc12 mutants. We hypothesize endocytosis helps recruit Cdc15 to assembled rings; uneven Cdc15 distribution at the ring occurs when endocytic patches contain increased Cdc15 levels and the patch-association rate is slow. Based on this, we developed a mathematical model that captures experimentally observed Cdc15 distributions along the ring. We propose that, at the ring, Gef1 and endocytic events promote uniform Cdc15 organization to enable proper septum ingression and concentric furrow formation.

Crosslinking activity of non-muscle myosin II is not sufficient for embryonic cytokinesis in C. elegans.

ABSTRACTCytokinesis in animal cells requires the assembly and constriction of a contractile actomyosin ring. Non-muscle myosin II is essential for cytokinesis, but the role of its motor activity remains unclear. Here, we examine cytokinesis in C. elegans embryos expressing non-muscle myosin motor mutants generated by genome editing. Two non-muscle motor-dead myosins capable of binding F-actin do not support cytokinesis in the one-cell embryo, and two partially motor-impaired myosins delay cytokinesis and render rings more sensitive to reduced myosin levels. Further analysis of myosin mutants suggests that it is myosin motor activity, and not the ability of myosin to crosslink F-actin, that drives the alignment and compaction of F-actin bundles during contractile ring assembly, and that myosin motor activity sets the pace of contractile ring constriction. We conclude that myosin motor activity is required at all stages of cytokinesis. Finally, characterization of the corresponding motor mutations in C. elegans major muscle myosin shows that motor activity is required for muscle contraction but is dispensable for F-actin organization in adult muscles.This article has an associated ‘The people behind the papers’ interview.