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Bacterial outer membrane channel for divalent metal ion acquisition

The prevailing model of bacterial membrane function predicts that the outer membrane is permeable to most small solutes because of pores with limited selectivity based primarily on size. Here, we identified mnoP in the Gram-negative bacterium Bradyrhizobium japonicum as a gene coregulated with the inner membrane Mn2+ transporter gene mntH. MnoP is an outer membrane protein expressed specifically under manganese limitation. MnoP acts as a channel to facilitate the tranlocation of Mn2+, but not Co2+ or Cu2+, into reconstituted proteoliposomes. An mnoP mutant is defective in high-affinity Mn2+ transport into cells and has a severe growth phenotype under manganese limitation. We suggest that the outer membrane is a barrier to divalent metal ions that requires a selective channel to meet the nutritional needs of the cell.