Existence of abnormal protein aggregates in healthy Escherichia coli cells.

Protein aggregation is a phenomenon observed in all organisms and has often been linked with cell disorders. In addition, several groups have reported a virtual absence of protein aggregates in healthy cells. In contrast to previous studies and the expected outcome, we observed aggregated proteins in aerobic exponentially growing and "healthy" Escherichia coli cells. We observed overrepresentation of "aberrant proteins," as well as substrates of the major conserved chaperone DnaK (Hsp70) and the protease ClpXP (a serine protease), in the aggregates. In addition, the protein aggregates appeared to interact with chaperones known to be involved in the aggregate repair pathway, including ClpB, GroEL, GroES, and DnaK. Finally, we showed that the levels of reactive oxygen species and unfolded or misfolded proteins determine the levels of protein aggregates. Our results led us to speculate that protein aggregates may function as a temporary "trash organelle" for cellular detoxification.