Kar9 symmetry breaking alone is insufficient to ensure spindle alignment

Miram Meziane,Rachel Genthial,Jackie Vogel

doi:10.1038/s41598-021-83136-w

Abstract

Spindle positioning must be tightly regulated to ensure asymmetric cell divisions are successful. In budding yeast, spindle positioning is mediated by the asymmetric localization of microtubule + end tracking protein Kar9. Kar9 asymmetry is believed to be essential for spindle alignment. However, the temporal correlation between symmetry breaking and spindle alignment has not been measured. Here, we establish a method of quantifying Kar9 symmetry breaking and find that Kar9 asymmetry is not well coupled with stable spindle alignment. We report the spindles are not aligned in the majority of asymmetric cells. Rather, stable alignment is correlated with Kar9 residence in the bud, regardless of symmetry state. Our findings suggest that Kar9 asymmetry alone is insufficient for stable alignment and reveal a possible role for Swe1 in regulating Kar9 residence in the bud.

Highlights

Spindle positioning must be tightly regulated to ensure asymmetric cell divisions are successful
Two consecutive Kar[9] spots in a track can define a line in 3D tracing the motion of the dynamic astral MTs (aMTs) + end, eliminating the need for aMT labelling. This method relies on the fact that over the course of a short time step (5 s), the spindle poles move slowly in comparison to highly dynamic aMT + ends and assumes the majority of aMTs contours are linear in this regime
In order to examine the functional relevance of the observed symmetry state diversity, we explored the relationship between various traditional parameters of interest with alignment index (AI) in wild type (WT) cells

Summary

Introduction

Spindle positioning must be tightly regulated to ensure asymmetric cell divisions are successful. Crucial to the success of an asymmetric cell division is the precise regulation of spindle positioning. In this context, the spindle can be positioned off center such that the cytokinesis event produces daughters of different sizes. Cortical cues are translated into spindle motions to ensure that the daughters inherit the appropriate factors Both cases are illustrated in the first cell division of the C. elegans embryo. A unifying feature of spindle positioning is the coupling of cortical cues and spindle movements This crucial element is mediated by spindle pole nucleated microtubules (MTs) that project into the cytoplasm known as astral MTs (aMTs). The proteins at the cortex allow the aMTs, and by extension the spindle pole bodies, to read polarity cues and translate them into spindle pole motions leading to the alignment of the spindle axis with the polarity axis of the cell[2]

Methods

Results

Conclusion