Physical properties of the cytoplasm modulate the rates of microtubule polymerization and depolymerization

Abstract

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.

Microtubules form by progressively faster tubulin accretion, not by nucleation–elongation

“…S2 C ), although an outlier curve also limited the concentration range available for fitting. The net rate of microtubule elongation predicted from these parameters is ∼1 µm/min at 13 µM αβ-tubulin, which is comparable to but slightly lower than the plus- and minus-end elongation rates of 1.5 µm/min and 0.4 µm/min, respectively (combined growth rate of 1.9 µm/min), observed in a recent study ( Strothman et al, 2019 ; note also that the presence of glycerol in our assay is expected to reduce microtubule elongation rates compared with standard, glycerol-free buffers; Molines et al, 2020 ). More importantly, the overall concentration dependence of the assembly curves was still being underestimated.…”

Cortical tension overrides geometrical cues to orient microtubules in confined protoplasts

“…However, such high osmolarity may also induce artifacts, or even trigger slowed life mechanisms, as in dormant seeds for instance ( 23 ). In fact, high osmolarity has been proposed to slow microtubule dynamics in yeast because of cytoplasmic crowding ( 24 ). We thus focused on the comparison between 600 and 280 mOsm/L mannitol, even if, in these conditions, we could not induce such high aspect-ratios.…”

Glycogen-dependent demixing of frog egg cytoplasm at increased crowding

“…Exploration in the crowded but still mixed regime may facilitate study of the effect of crowding on biochemical processes. Recent work examined how crowding affects microtubule polymerization using osmotic perturbation of fission yeast (Molines et al, 2020). It will be interesting to ask similar questions in cytoplasmic extracts.…”