Cytoplasm Biophysical Properties Madolute Microtubule Dynamics

The cytoplasm represents a crowded environment whose properties may change according to physiological or developmental states. Although the effects of crowding and viscosity on in vitro reactions have been well studied, if and how the biophysical properties of the cytoplasm impact cellular functions in vivo remain poorly understood. Here, we probed the effects of cytoplasmic concentration on microtubule (MT) dynamics by studying the effects of osmotic shifts in the fission yeast Schizosaccharomyces pombe. Increasing cytoplasmic concentration by hyperosmotic shock led to proportionate reductions in the rates of interphase MT growth and shrinkage. Conversely, dilution of the cytoplasm in hypoosmotic shifts led to proportionately faster rates. Numerous lines of evidence indicate that these effects were due to biophysical properties of the cytoplasm. These effects were recapitulated in in vitro reconstituted MT assays by modulating viscosity, not by crowding. Our findings suggest that even at normal conditions, the viscous properties of cytoplasm modulate the dynamic reactions of MT polymerization and depolymerization.