The collapse of the spindle following ablation in S. pombe is mediated by microtubules and the motor protein dynein

Abstract

A microtubule-based machine called the mitotic spindle segregates chromosomes when eukaryotic cells divide. In the fission yeast S. pombe, which undergoes closed mitosis, the spindle forms a single bundle of microtubules inside the nucleus. During elongation, the spindle extends via antiparallel microtubule sliding by molecular motors. These extensile forces from the spindle are thought to resist compressive forces from the nucleus. We probe the mechanism and maintenance of this force balance via laser ablation of spindles at various stages of mitosis. We find that spindle pole bodies collapse toward each other following ablation, but spindle geometry is often rescued, allowing spindles to resume elongation. While this basic behavior has been previously observed, many questions remain about this phenomenon’s dynamics, mechanics, and molecular requirements. In this work, we find that previously hypothesized viscoelastic relaxation of the nucleus cannot fully explain spindle shortening in response to laser ablation. Instead, spindle collapse requires microtubule dynamics and is powered at least partly by the minus-end directed motor protein dynein. These results suggest a role for dynein in redundantly supporting force balance and bipolarity in the S. pombe spindle.STATEMENT OF SIGNIFICANCES. pombe serves as an important model organism for understanding cell division. Its structurally simple mitotic spindle is especially suited for mechanical perturbation. Since S. pombe undergoes a process of closed cell division, without breakdown of the nuclear envelope, force may be exerted between its nuclear envelope and spindle. Here, we mechanically sever spindles via laser ablation to probe this force balance. Following ablation, S. pombe spindle fragments collapse toward each other. We find that, contrary to prior expectations, forces from the chromosomes and nuclear envelope are not responsible for this collapse. Instead, it is microtubule-dependent, and is powered at least in part by the minus-end directed microtubule motor protein dynein.