Protein folding dynamics in the cell.

Protein folding is a remarkably fast unimolecular reaction, spanning microseconds to hours at room temperature. Thus, free energy differences and activation barriers on the free energy landscape of proteins are rather small. This opens up the possibility of living cells modulating their protein's landscapes, providing cells another way to control the function of their proteomes after transcriptional control, translational control, and post-translational modification. In this Feature Article, we discuss advances in physicochemical studies of protein stability and folding inside living cells. We focus in particular on our studies using fast relaxation imaging (FREI). Although the effect of the cell on protein free energy landscapes is only a few kT, the strong cooperativity of many folding and binding processes allows small modulation of the energy and entropy to produce a large population modulation. Lastly, we discuss some biomolecular processes that are particularly likely to be affected by in-cell modulation of the proteome, and thus of interest for quantitative physical chemistry studies.