LACTOFERRIN BINDS TO PORINS OMPF AND OMPC IN ESCHERICHIA-COLI

Lactoferrin (Lf) is an iron-binding antimicrobial protein present in m ilk and on mucosal surfaces, with a suggested role in preimmune host d efense. Certain strains of Escherichia coli (bacterial whole cells) de monstrate specific interaction with I-125-labeled Lf. A band with a ma ss of similar to 37 kDa, which was reactive with horseradish peroxidas e-labeled Lf, was identified in the boiled cell envelope and outer mem brane preparations of an Lf-binding E. coli strain, E34663, and a non- Lf-binding strain, HH45, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting (immunoblotting). Such a band was not detected in the unboiled native cell envelope and oute r membrane preparations. The molecular mass and the property of heat m odifiability suggested that the Lf-binding proteins were porins. The n ative trimeric form of porin OmpF isolated from strain B6 and its diss ociated monomeric form both reacted with horseradish peroxidase-labele d if and with monoclonal antibodies specific for OmpF. Furthermore, by using E. coli constructs with defined porin phenotypes, OmpF and OmpC were identified as the Lf-binding proteins by urea-SDS-PAGE and Weste rn blotting and by I-125-Lf binding studies with intact bacteria. Thes e data establish that Lf binds to porins, a class of well-conserved mo lecules common in E. coli and many other gram-negative bacteria. Howev er, in certain strains of E. coli these pore-forming proteins are shie lded from Lf interaction.