Critical role of the periplasm in copper homeostasis in Gram-negative bacteria

Copper is essential for life, but is toxic in excess; that is, cells must keep an optimal internal copper concentration. Under aerobic conditions, less toxic Cu(Ⅱ) taken up by bacterial cells is reduced to more toxic Cu(I) in the cytoplasm. Copper homeostasis is achieved in the cytoplasm and the periplasm as a unique feature of Gram-negative bacteria. The copper efflux pumps, CopA and CusCBA export Cu(I) from the cytoplasm or the periplasm to outside of the cells in Escherichia coli. In addition, the periplasmic proteins, such as a multi-copper oxidase CueO, play a role in the periplasmic detoxification. While the efflux pumps are highly conserved in Gram-negative bacteria, the periplasmic proteins are diversified, indicating that copper homeostasis in the periplasm could contribute to adaptation to various living environments. However, the role of the periplasm and periplasmic proteins in regard to whole-cell copper homeostasis remains unknown. In this study, we addressed the role of the periplasm and periplasmic proteins in copper homeostasis to adapt to various ecological niches. We have used a systems approach, alternating rounds of experiments and models, to further elucidate the dynamics of copper efflux system. We measured the response to copper of the main specific copper export systems in the wild type E. coli strain, and a series of deletion mutant strains. We interpreted these data using a detailed mathematical model and Bayesian model fitting routines, and verified copper homeostasis. Compared with the simulation and the growth in response to copper, we found that the growth was associated with copper abundance in the periplasm. In particular, CueO unique to Gram-negative bacteria contributes both to protection against Cu(I) toxicity and to incorporating copper into the periplasmic components/proteins, resulting in maximizing the growth. These results suggest that Gram-negative bacteria have evolved to utilize the periplasm as a sensor and store for copper, in order to enable Gram-negative bacteria to adapt to a wide range of environmental copper concentrations.