Cofilin Protects Contractile Rings from Bridging Instabilities

Abstract

Significant experimental evidence from fission yeast studies suggests the major structural challenge faced by the cytokinetic contractile ring is bridging instability. When uncontrolled, this instability causes sections of the actomyosin ring to pull away from the membrane into straight bridges. Such straight bridges were observed in rings with the myosin-II mutant myo2-E1, which binds actin weakly (Laplante et al., 2015). Bridges have also been observed in rings with the ADF/cofilin mutants adf1-M2 and adf1-M3 (Cheffings et al., 2019), which impair cofilin-mediated severing of filamentous actin. Bridges contained actin and myosin II Myp2, leaving myosin II Myo2 behind on the plasma membrane. More bridging occurred in regions of lower Myo2 density. To examine the origin of bridging, and the role of ADF/cofilin in stabilizing the ring, we used simulations of the fission yeast ring. In simulations, the ∼400-800 pN ring tensions generated high centripetal (“Laplace”) forces tending to unanchor the ring from the membrane. These were opposed primarily by binding of membrane-anchored Myo2 to the tense actin filaments, drastically weakened in the myo2-E1 mutant. Simulations of myo2-E1 rings accurately reproduced the bridging phenotype (Laplante et al., 2015). Simulations also reproduced the bridging phenotype in rings with the ADF/cofilin mutants. Impaired cofilin-mediated severing increased actin filament lengths, thus increasing the ring tension and hence the Laplace forces. Bridging occurred when Laplace forces exceeded the Myo2-actin unbinding threshold. The filament tension was proportional to its length, as longer filaments were pulled by more myosin II heads. In ADF/cofilin mutants filaments grew ∼3-fold longer and had ∼ 3-fold normal tension. Bridges peeled away from the plasma membrane at locations where inward Laplace forces exceeded the local anchoring force from Myo2. Bridging was therefore most likely in regions with low Myo2 density, as observed (Cheffings et al., 2019).