Selection against toxic aggregation-prone protein sequences in bacteria.

Despite genetic variation has the potential to arise new protein functions, spontaneous mutations usually destabilize the native fold. Misfolded proteins tend to form cytotoxic intracellular aggregates, decreasing cell fitness and leading to degenerative disorders in humans. Therefore, it is thought that selection against protein misfolding and aggregation constrains the evolution of protein sequences. However, obtaining experimental data to validate this hypothesis has been traditionally difficult. Here we exploit bacteria as a model organism to address this question. Using variants of the Alzheimer's related Aβ42 peptide designed to exhibit different in vivo aggregation propensities we show here that, in cell competition experiments, the most aggregation-prone variants are always purged out from the growing population. Flow cytometry analysis of cellular metabolism and viability demonstrates that this purifying effect responds to a clear correlation between physiological burden and intrinsic aggregation propensity. Interestingly, the fitness cost of aggregation appears to be associated with aggregation rates rather than with overall protein solubility. Accordingly, we show that, by reducing in vivo aggregation rates, the model osmolyte proline is able to buffer the metabolic impact of protein aggregation. Overall, our data provide experimental support for the role of toxic protein aggregation on the cell fitness landscape and the evolution of natural protein sequences.