Collective dynein transport of the nucleus by pulling on astral microtubules 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. Although 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 spindle pole body (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 that 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 that 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 clustering. Stochastic simulations also suggest that a higher number of force generators than predicted by force balance may be required to ensure the robustness of spindle positioning. Take Away• Saccharomyces cerevisiaea cytoplasmic viscosity is estimated by quantifying the in vivo mobility of the nucleus and comparing to simulations.• The in vivo length dynamics of microtubules (MTs) interacting with daughter bud cortex during nuclear positioning in mitosis are measured.• We developed a model of astral MT interactions with cortical dynein in the process of nuclear positioning during mitosis.• A force balance model predicts the minimal force required for nuclear positioning. Kunalika Jain and Neha Khetan contributed equally to the manuscript.

Evolutionary divergence of anaphase spindle mechanics in nematode embryos constrained by antagonistic pulling and viscous forces

“…Previous models have proposed that modulation of the force can be achieved by changes in the number of motors, the individual force per motor, the attachment and detachment rate of motors, and microtubule dynamics, as seen in other systems ( Sutradhar et al. , 2015 ; Jain et al. , 2021 ).…”

Detecting directed motion and confinement in single-particle trajectories using hidden variables

“…Long-range directed motion of the SPB is expected during mitosis when the nucleus orients towards the bud and the spindle elongates to separate the chromosome masses of mother and daughter cell (Fig. 7a), but some directed motion of the SPB may also occur in other phases of the cell cycle due to the forces exerted on it by microtubules (42, 43). When fitting individual tracks, we observe a significant fraction of persistently moving SPB (13% with a confidence interval of 95%, Fig.…”

Evolutionary divergence of anaphase spindle mechanics in nematode embryos constrained by antagonistic pulling and viscous forces

“…We found variations in the net pulling forces, by a factor of ~10, between species. Previous models have proposed that modulation of the force can be achieved by changes in the number of motors, the individual force per motor, the attachment and detachment rate of motors, and microtubule dynamics, as seen in other systems (Sutradhar et al, 2015, Jain et al, 2021. Establishing transgenic lines, in particular to follow live microtubules, in these different species is a necessary step towards more quantitative measurements of these parameters.…”