Behaviors of microtubules depend on two critical concentrations

The concept of critical concentration is a central idea in understanding the behaviors of microtubules (MTs) and other steady-state (energy-utilizing, non-equilibrium) cytoskeletal polymers. Classically, the critical concentration (CC) is the concentration of subunits necessary to obtain polymer. However, the classical theory used to explain and predict CC is based on equilibrium polymers and fails to account for dynamic instability (DI). It has been unclear how the classical theory should be adjusted to incorporate DI or how the behavior of an individual dynamically unstable filament relates to that of its population. To address these questions, we used previously established simulations to follow at multiple scales the behavior of systems of computationally modeled dynamic microtubules. We show that polymers such as microtubules that exhibit dynamic instability have not one but at least two critical concentrations: one above which growth phases of individual filaments can occur transiently, and another above which the population9s overall polymer mass will increase persistently. We propose that whether a steady-state polymer behaves like microtubules (displays dynamic instability) or like actin (can be modeled as an equilibrium polymer) depends on how far apart these critical concentrations are. This revised framework helps to explain and unify a wide range of experimental observations.