The diffusion coefficient for PGK folding in eukaryotic cells.

We compare the folding kinetics of a fluorescent phosphoglycerate kinase construct in 30 mammalian cells with that in aqueous buffer. In both environments, the kinetics can be fitted to the functional form exp[-(t/τ)(β)]. A histogram of τ shows that the average folding relaxation time in cells is only twice as long as in aqueous buffer. Consideration of the folding free energy and of β reveals that only some of the variation in τ arises from perturbation of the protein's energy landscape. Thus, the diffusion that controls barrier crossing during protein folding is nearly as fast in cells as in vitro, even though translational diffusion of phosphoglycerate kinase in the cell is slow compared to in vitro.