How proteins sense and navigate the cellular interior to find their functional partners remain poorly understood. An intriguing aspect of this search is that it relies on diffusive encounters with the crowded cellular background, made up by protein surfaces that are largely non-conserved. The question is then if/how this protein search is amenable to selection and biological control? To shed light on this issue, we examined the motions of three evolutionary divergent proteins in the E. coli cytoplasm by in-cell NMR. The results show that the diffusive in-cell motions, after all, follows simplistic physical-chemical rules: the proteins reveal a common dependence on (i) net-charge density, (ii) surface hydrophobicity and (iii) the electric dipole moment. The bacterial protein is here biased to move relatively freely in the bacterial interior, whereas the human counterparts more easily stick. Even so, the in-cell motions respond predictably to surface mutation, allowing us to tune and intermix the protein’s behaviour at will. The findings show how evolution can swiftly optimise the diffuse background of protein-encounter complexes by just single point mutations, and provide a rational framework for adjusting the cytoplasmic motions of individual proteins, e.g. for rescuing poor in-cell NMR signals and for optimising protein therapeutics.

Physicochemical code for protein interactions in live cells

255th National Meeting (2018)

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