Cell https://doi.org/10.1016/j.cell.2018.05.042 (2018)
Measuring macromolecular crowding in cells relies on the use of tracer particles that convey the relative diffusion coefficient within a cell, but existing approaches perturb the cell. Delarue et al. developed fluorescently labeled scaffolding domains that self-assemble inside cells into stable particles of a defined size, which they termed genetically encoded multimeric nanoparticles (GEMs). Loss of mTORC1 signaling through rapamycin treatment or amino acid starvation increased the mobility of GEMs, which mutant screens and cryo-electron tomography linked to decreased ribosome concentration. The strong correlation between diffusion coefficient and ribosome concentration enabled the authors to develop a predictive model. In addition, higher ribosome concentration increased the prevalence of multivalent proteins entering a liquid droplet phase, whereas inhibition of mTORC1 decreased this phase transition. Overall, this study revealed new cellular mechanisms that regulate molecular diffusion and phase separation.