The Functional Antagonism between Eg5 and Dynein in Spindle Bipolarization Is Not Compatible with a Simple Push-Pull Model

Abstract

During cell division, the molecular motor Eg5 crosslinks overlapping antiparallel microtubules and pushes them apart to separate mitotic spindle poles. Dynein has been proposed as a direct antagonist of Eg5 at the spindle equator, pulling on antiparallel microtubules and favoring spindle collapse. Some of the experiments supporting this hypothesis relied on endpoint quantifications of spindle phenotypes rather than following individual cell fates over time. Here, we present a mathematical model and proof-of-principle experiments to demonstrate that endpoint quantifications can be fundamentally misleading because they overestimate defective phenotypes. Indeed, live-cell imaging reveals that, while depletion of dynein or the dynein binding protein Lis1 enables spindle formation in presence of an Eg5 inhibitor, the activities of dynein and Eg5 cannot be titrated against each other. Thus, dynein most likely antagonizes Eg5 indirectly by exerting force at different spindle locations rather than through a simple push-pull mechanism at the spindle equator. On the basis of fixed-sample-based microscopy, molecular motors Eg5 and dynein are believed to directly push against each other at the mitotic spindle equator and determine pole distance. Florian and Mayer show that single-timepoint imaging often misestimates the frequency of spindle phenotypes. Therefore, they use time-lapse microscopy to show that the activities of dynein and Eg5 are not titratable against each other. They conclude that, in spindle formation, dynein antagonizes Eg5 through an indirect mechanism rather than a simple push-pull model. © 2012 The Authors.