Special conditions allow binding of the siderophore salmochelin to siderocalin (NGAL-lipocalin)

Siderocalin is part of the innate immune system and is secreted by epithelial cells in the early stages of inflammation. This protein binds catecholate siderophores such as enterobactin with high affinity. As a consequence, the iron supply and growth of the siderophore-producing bacteria is disturbed. Recombinant siderocalin isolated from strains of Escherichia coli K-12 contained bound enterobactin. Very low amounts of siderocalin were isolated from another K-12 strain unable to produce enterobactin, which indicated that enterobactin might stabilize the recombinant protein. Siderocalin isolated from E. coli strain Nissle 1917, which produces the glucosylated enterobactin salmochelin, contained a mixture of bound salmochelin (55%) and enterobactin (45%). The growth of an enterobactin-producing E. coli K-12 strain but not of the same strain carrying a plasmid encoding iroBCDEN and therefore able to produce salmochelin was suppressed when siderocalin was added to the medium, which indicated that salmochelin is bound by siderocalin before siderocalin folds into its final conformation in the cell, and that binding of salmochelin to matured siderocalin is not possible. Salmochelin is mainly produced by pathogenic enterobacteria. Glucosylation of enterobactin may be a mechanism by which these bacteria evade trapping of the enterobactin by siderocalin.