In Fission Yeast the Constriction Rate is not Set by the Cytokinetic Ring, but by the Septum Growth Machinery

Abstract

Constriction of the actomyosin contractile ring is often thought to drive division of the cell into two and to set the constriction rate. We measured a tension in the cytokinetic ring of fission yeast, where ring constriction is accompanied by septation, the growth of new cell wall in the wake of the constricting ring. Using a cell wall modulus ∼ 30 MPa, we conclude ring tension produces negligible septum strains ∼ 0.01% for a ring of radius and width ∼0.1 μm. Thus the inward septum growth rate determines the constriction rate, not the ring tension.Instead, ring tension maintains the circular shape of the septum, ensuring proper closure, using a mechanism that exploits the mechanosensitivity of septum growth. Hypothesizing mechanonsensitivity of the septum growth machinery, a mathematical model showed that ring tension suppresses septum edge roughness. Measurements of septum edge roughness in live cells agreed with model predictions, implying a mechanosensitivity per beta glucan synthase (Bgs) complex, the relative synthesis rate increase per force. Estimating the density of active Bgs complexes, a ring at constriction onset () therefore exerts centripetal force per Bgs, giving a small rate of synthesis increase. A rigorous calculation, from our mathematical model, shows little effect of tension on the constriction rate throughout constriction: ring radius versus time is almost linear with a small downward curvature.Thus, ring tension has little effect on the constriction rate, which is set by the intrinsic rate of synthesis by the cell's septum synthesis machinery.