LAUSR

Contrary to eukaryotic cells, both bacteria and archaea (mostly) lack active cytoplasmic transport. The encounter rate between proteins in the cytoplasm thus depends (almost) exclusively on lateral diffusion. Previously, it was found that the cytoplasmic protein diffusion coefficient drops with an increase of (1) its molecular weight and (2) cytoplasmic excluded volume. Here we show the dependence of the protein diffusion coefficient on the strength of binding interactions. We have done FRAP measurements to obtain diffusion coefficients of surface-modified GFPs in Escherichia coli, Lactococcus lactis and Haloferax volcanii. The tested variants of GFP vary in net charge from −30 to +25. The examined organisms differ in their cytoplasmic ion concentration. The GFP diffusion coefficient drops with increasing positive charge, up to two orders of magnitude. This is caused by binding of the GFPs to ribosomes. The drop in the diffusion coefficient decreases with increased cytoplasmic ion concentration. Our findings suggest a limit to what the surface charge of a protein can be in the cytoplasm of a given cell.