Spindle alignment is uncoupled with Kar9 symmetry breaking

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 early spindle alignment. We report the majority of asymmetric cells are not stably aligned. Rather, stable alignment, a state we define as perfect alignment, is correlated with Kar9 localization to the bud compartment, regardless of symmetry state. Our findings suggest that Kar9 asymmetry alone is insufficient for perfect alignment and reveal a possible role for Swe1 in regulating spindle alignment efficiency.