Collective Dynein Transport of the Nucleus by Pulling on Astral MTs during Saccharomyces cerevisiae Mitosis

Abstract

Positioning the nucleus at the bud‐neck during Saccharomyces cerevisiae mitosis involves pulling forces of cytoplasmic dynein localized in the daughter cell. While genetic analysis has revealed a complex network positioning the nucleus, quantification of the forces acting on the nucleus and the number of dyneins driving the process has remained difficult. To better understand the collective forces involved in nuclear positioning, we compare a model of dyneins driven microtubule (MT) pulling, MT pushing and cytoplasmic drag to experiments. During S. cerevisiae mitosis, MTs interacting with the cortex nucleated by the daughter SPB (SPB‐D) are longer than the mother SPB (SPB‐M), increasing further during spindle elongation in anaphase. Interphasic SPB mobility is effectively diffusive, while the mitotic mobility is directed. By optimizing a computational model of the mobility of the nucleus due to diffusion and MTs pushing at the cell membrane to experiment, we estimate the viscosity governing the drag force on nuclei during positioning. A force‐balance model of mitotic SPB mobility compared to experimental mobility, suggests even one or two dynein dimers are sufficient to move the nucleus in the bud‐neck. Using stochastic computer simulations of a budding cell, we find punctate dynein localization can generate sufficient force to reel in the nucleus to the bud‐neck. Compared to uniform motor localization, puncta involve fewer motors suggesting a functional role for motor cluster‐ ing. Stochastic simulations also suggest a higher number of force generators than predicted by force‐balance may be required to ensure the robustness of spindle positioning.