The cytoplasm of a cell is stuffed with large molecules and organelles. Such crowding may cause some molecules to aggregate, which could in turn affect their biological function. To monitor cytoplasmic crowding, Delarue et al. made a probe from bacterial proteins that form large scaffolds linked to fluorescent proteins. In yeast and human cells, the probes revealed that the protein kinase complex mTORC1 (mechanistic target of rapamycin complex 1) enhanced cytoplasmic viscosity. It did so by increasing production of ribosomes while inhibiting their degradation. Because ribosomes occupy about 20% of a cell’s volume, mTORC1 activity tunes cytoplasmic crowding and thus influences cell physiology (L. Bryan Ray).

Cell 10.1016/j.cell.2018.05.042 (2018)