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Aggregates of misfolded proteins are a hallmark of many age-related diseases.Recently, they have been linked to aging of Escherichia coli (E. coli) whereprotein aggregates accumulate at the old pole region of the aging bacterium.Because of the potential of E. coli as a model organism, elucidating aging andprotein aggregation in this bacterium may pave the way to significant advancesin our global understanding of aging. A first obstacle along this path is todecipher the mechanisms by which protein aggregates are targeted to specificintercellular locations. Here, using an integrated approach based onindividual-based modeling, time-lapse fluorescence microscopy and automatedimage analysis, we show that the movement of aging-related protein aggregatesin E. coli is purely diffusive (Brownian). Using single-particle tracking ofprotein aggregates in live E. coli cells, we estimated the average size anddiffusion constant of the aggregates. Our results evidence that the aggregatespassively diffuse within the cell, with diffusion constants that depend ontheir size in agreement with the Stokes-Einstein law. However, the aggregatedisplacements along the cell long axis are confined to a region that roughlycorresponds to the nucleoid-free space in the cell pole, thus confirming theimportance of increased macromolecular crowding in the nucleoids. We thus used3d individual-based modeling to show that these three ingredients (diffusion,aggregation and diffusion hindrance in the nucleoids) are sufficient andnecessary to reproduce the available experimental data on aggregatelocalization in the cells. Taken together, our results strongly support thehypothesis that the localization of aging-related protein aggregates in thepoles of E. coli results from the coupling of passive diffusion- aggregationwith spatially non-homogeneous macromolecular crowding. They further supportthe importance of "soft" intracellular structuring (based on macromolecularcrowding) in diffusion-based protein localization in E. coli.

Localization of protein aggregation in Escherichia coli is governed by diffusion and nucleoid macromolecular crowding effect