Lon-mediated proteolysis of the FeoC protein prevents Salmonella enterica from accumulating the Fe(II) transporter FeoB under high-oxygen conditions.

The Salmonella Feo system consists of the FeoA, FeoB, and FeoC proteins and mediates ferrous iron [Fe(II)] import. FeoB is an inner membrane protein that, along with contributions from two small hydrophilic proteins, FeoA and FeoC, transports Fe(II). We previously reported that FeoC binds to and protects the FeoB transporter from FtsH-mediated proteolysis. In the present study, we report proteolytic regulation of FeoC that occurs in an oxygen-dependent fashion. While relatively stable under low-oxygen conditions, FeoC was rapidly degraded by the Lon protease under high-oxygen conditions. The putative Fe-S cluster of FeoC seemed to function as an oxygen sensor to control FeoC stability, as evidenced by the finding that mutation of the putative Fe-S cluster-binding site greatly increased FeoC stability under high-oxygen conditions. Salmonella ectopically expressing the feoB and feoC genes was able to accumulate FeoB and FeoC only under low-oxygen conditions, suggesting that FeoC proteolysis prevents Salmonella from accumulating the FeoB transporter under high-oxygen conditions. Finally, we propose that Lon-mediated FeoC proteolysis followed by FtsH-mediated FeoB proteolysis helps Salmonella to avoid uncontrolled Fe( II) uptake during the radical environmental changes encountered when shifting from low-iron anaerobic conditions to high-iron aerobic conditions.