Quantitative Characterization of Budding Yeast Polarization at the Mesoscale

Abstract

Cellular symmetry breaking or polarization is a key process that underlies many types of cellular behavior like division, migration, and differentiation. In budding yeast, polarization is required for bud site selection. Polarization of key regulator Cdc42 has been molecularly dissected for decades, yet current data provide insufficient constraints to quantitatively differentiate between pattern-forming mechanisms and models. Here we present a phenomenological description of polarization in terms of physical characteristics using live cell microscopy and quantitative analysis. We identify multiple phases of Cdc42 polarization and derive mesoscale observables that describe each phase, yielding a basal framework to systematically characterize symmetry-breaking phenotypes. We then apply this description to two case studies in which the wildtype polarity network is perturbed: a small perturbation after eliminating spatial cue communication and a larger perturbation involving removal of near-essential scaffold Bem1. This confirms the applicability of the framework in nonwildtype contexts while revealing differences between genetic backgrounds. Finally, we study the dynamics of other polarizing components by extending our analysis to the Gic2 PBD domain, a Cdc42-GTP biosensor, and to polarisome component Spa2, highlighting variation in spot dynamics between protein species while also showing similarities to Cdc42. Our approach enables standardized comparison of polarization between diverse molecular backgrounds at the mesoscale and helps constrain future modeling efforts.