Microtubule dynamic instability is sensitive to specific biological viscogens in vitro

Abstract

Cytoplasm is a viscous, crowded, and heterogeneous environment, and its local viscosity and degree of macromolecular crowding have significant effects on cellular reaction rates. Increasing viscosity slows down diffusion and protein conformational changes, while increasing macromolecular crowding speeds up reactions. As a model system for cellular reactions, microtubule dynamics are slowed down in vivo when cytoplasm concentration is increased by osmotic shifts, indicating a dominant role for viscosity in microtubule reaction pathways. In the cell, viscosity is determined by diverse species of "biological viscogens", including glycerol, trehalose, intermediate metabolites, proteins, polymers, organelles, and condensates. Here we show in vitro that microtubule dynamic instability is sensitive to specific viscogen species, particularly glycerol. We found that increasing viscosity with glycerol or trehalose slowed microtubule growth, slowed microtubule shrinkage, and increased microtubule lifetimes, similar to the "freezing" observed previously in vivo. Increasing viscosity with a globular protein, bovine serum albumin, increased microtubule growth rates, as its viscous effects may be balanced against its macromolecular crowding effects. At matched viscosities, glycerol had an outsized effect on microtubule lifetimes, rescues, and nucleation compared to other viscogens. Increasing viscosity did not, however, increase the intensity of EB3-GFP comets, indicating that GTP hydrolysis is unaffected by buffer conditions. We propose that glycerol exerts its distinct effect on microtubule dynamic instability by stabilizing the microtubule lattice after phosphate release. Effects of specific viscogens may modulate many cellular reaction rates within local environments of cytoplasm.