Bacterial lactoferrin-binding protein A binds to both domains of the human lactoferrin C-lobe

Bacterial Pathogenesis Research Group, Department of Microbiology and Infectious Diseases, Faculty of Medicine, University of Calgary, Rm 274, Heritage Medical Research Building, 3330 Hospital Drive NW, Calgary, Alberta, Canada T2N 4N1 Correspondence Anthony B. Schryvers schryver{at}ucalgary.ca Pathogenic bacteria in the family Neisseriaceae express surface receptors to acquire iron from the mammalian iron-binding proteins. Transferrins and lactoferrins constitute a family of iron-binding proteins highly related in both sequence and structure, yet the bacterial receptors are able to distinguish between these proteins and uphold a strict binding specificity. In order to understand the molecular basis for this specificity, the interaction between human lactoferrin (hLf) and the lactoferrin-binding protein A (LbpA) from Moraxella catarrhalis was studied. A periplasmic expression system was designed for the heterologous expression of LbpA, which enabled the investigation of its binding activity in the absence of lactoferrin-binding protein B (LbpB). To facilitate delineation of the LbpA-binding regions of hLf, chimeric proteins composed of hLf and bovine transferrin were made. Binding studies performed with the chimeric proteins and recombinant LbpA identified two binding regions within the C-terminus of hLf. Furthermore, native LbpA from Moraxella and Neisseria spp. bound the identical spectrum of hybrid proteins as the recombinant receptor, demonstrating a conserved binding interaction with the C-lobe of hLf. Abbreviations: bTf, bovine transferrin; hLf, human lactoferrin; HRP, horseradish peroxidase