Cell http://doi.org/qrz (2013)

Credit: © 2013 ELSEVIER

For intracellular processes to work, ions, proteins and bigger components — such as protein complexes and protein filaments — have to be able to move around the crowded environment of the cytoplasm, either through active transport (mediated by cytoskeletal motor proteins) or passive diffusion. Although understanding cytoplasmic passive transport in bacteria — which lack both a proper cytoskeleton and active transport mechanisms for nucleic acids and proteins — is crucial in cellular physiology, seemingly conflicting reports of subdiffusive and normally diffusive transport from both experiments and theory have hindered progress. Now, Christine Jacobs-Wagner and colleagues show that for components of less than 30 nm in size the bacterial cytoplasm behaves as a simple viscous liquid, but that for bigger macromolecules it increasingly behaves as a glass-forming liquid that displays dynamical features akin to those of colloidal glasses. The authors thus provide an explanation for past observations of both normal and anomalous diffusion. They also demonstrate that cellular metabolism fluidizes the cytoplasm by causing perturbations that allow the macromolecules to escape the cages formed by their neighbours.