Diffusive protein interactions in human versus bacterial cells

Abstract Random encounters between proteins in crowded cells are by no means passive, but found to be under selective control. This control enables proteome solubility, helps to optimise the diffusive search for interaction partners, to regulate the cell volume, and to allow for an adaptation to environmental extremes. Interestingly, the residues that modulate the random encounters act mesoscopically through protein net charge and exposed hydrophobicity, meaning that their impact and detailed signatures vary across organisms with different intracellular constraints. To examine such variations, we compare the diffusive behaviour of one bacterial and two human proteins in the E. coli and the human cytosols, using in-cell NMR relaxation. We find that proteins that ‘stick’ and whose signals become broadened beyond detection in E. coli are generally less restricted in mammalian cells. Also, the rotational correlation times in the mammalian cytosol are less sensitive to surface-charge mutations. This shows that, in terms of maintaining protein motions, the mammalian cytosol is more forgiving to surface alterations than E. coli cells. The cellular differences seem not linked to the proteome properties per se, but rather to a 6-fold difference in physiological protein concentrations. Taken together, the results outline a scenario in which the tolerant cytosol of mammalian cells, found in long-lived multicellular organisms, provides an enlarged evolutionary playground, where random protein-surface mutations are less deleterious than in short-generational bacteria.