Gade, V. R.; Heinrich, S.; Paloni, M.; Gomez-Garcia, P. A.; Dzanko, A.; Oswald, A.; Marchand, D.; Khawaja, S.; Barducci, A.; Weis, K.

The organization and biophysical properties of the cytoplasm influence all cellular reactions, including molecular interactions and the mobility of biomolecules. It has become clear that the cytoplasm does not behave like a simple fluid but instead is a densely crowded and highly organized environment. Yet, the detailed properties of the cytoplasm, the molecular mechanisms that control them and how they influence the biochemistry of cells remain poorly understood. Here, we investigate the diffusive properties of the cytoplasm in silico and in vivo, employing mRNPs (messenger ribonucleoprotein) and GEM (genetically encoded multimeric) particles as rheological probes in proliferating cells. We demonstrate that cytoplasmic diffusivity increases upon polysome disassembly due to translation inhibition or upon a reduction in mRNA levels. Reducing ribosome concentration by up to 20-25% without a change in polysome levels has no effect in vivo. In addition, we show that upon polysome disassembly, mRNA condensation into P-bodies does not affect cytosolic diffusion in budding yeast. Altogether, our results show that mRNAs and their organization into polysomes control the biophysical properties of the eukaryotic cytoplasm.