Strikingly, we found that DIAPH3 preferentially nucleates β-actin filaments, suggesting that the cytokinetic ring is comprised of a specialized β-actin filament network that is primarily regulated by DIAPH3 activity. DIAPH3 is recruited to the cytokinetic furrow as the cell exits metaphase by the scaffolding protein anillin, and requires activation through the binding of both RhoA and anillin to release it from its autoinhibited state 9. The contractile ring is built by the de novo assembly of β-actin filaments at the cell equator through the action of the formin, DIAPH3 8, 9, 10. In this model, microtubules of the central spindle position a region of active RhoA-GTP at the adjacent plasma membrane that directs the assembly of a contractile actomyosin ring where furrow ingression occurs 7. The observation that microtubules position contractile forces at the equatorial site of furrow ingression challenged this view 6, and led to an equator-centric model that has gained widespread support. In the pole-centric view, furrow ingression is proposed to result from membrane relaxation at the cortical poles of the cell while membrane tension is maintained at the equator 5. The different models focus on the primary site of activity on the plasma membrane required for furrow ingression. In metazoans, the mechanisms underlying the breaking of symmetry within the plasma membrane that lead to cytokinetic furrow ingression have been hotly debated for decades. Consequently, plasma membrane stiffness becomes asymmetric as there is greater tension at the site of furrow ingression 3. As the cell exits mitosis, it elongates and an actomyosin ring, the contractile ring, forms at the cell equator between the segregating chromosomes 1, 2. As a cell enters mitosis it rounds up, a process driven by the remodelling of the actin cytoskeleton to generate a plasma membrane with uniformly higher stiffness 3, 4. It follows that an acute spatio-temporal control over mitotic events must be enforced to ensure that the furrow forms at the correct time, after chromosome segregation begins and at the right place, between the segregating chromosomes.ĭuring mitosis the plasma membrane undergoes dramatic remodelling. As each complete copy of the genetic material is segregated to the opposite poles of the cell, a furrow of plasma membrane, the cytokinetic furrow, ingresses between them ultimately fusing and generating two new cells. The contemporaneous production of a β-actin contractile ring at the cell equator and loss of γ-actin from the poles is required to generate a stable cytokinetic furrow and for the completion of cell division.Ĭytokinesis is the final stage of the cell cycle when a cell partitions its genetic and cytosolic components into two new cells that then physically separate 1, 2. During anaphase, CLIP170 is delivered by astral microtubules and displaces IQGAP1 from DIAPH1, leading to formin autoinhibition, a decrease in cortical stiffness and localized membrane blebbing. While activation of the formin DIAPH3 at the cytokinetic furrow underlies β-actin filament production, we show that the γ-actin network is specifically depleted at the cell poles through the localized deactivation of the formin DIAPH1. Here we demonstrate that the opposing regulation of the β- and γ-actin networks is required for successful cytokinesis. During cytokinesis, the actin cytoskeleton is partitioned into two spatially distinct actin isoform specific networks: a β-actin network that generates the equatorial contractile ring, and a γ-actin network that localizes to the cell cortex.
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