Novel bangle lectin from Photorhabdus asymbiotica: sugar-binding specificity, structure and interaction with host immune system

Photorhabdus asymbiotica is gram-negative bioluminescent bacteria living in a symbiotic relationship with nematodes from the genus Heterorhabditis. Together with nematode it forms a complex that is highly pathogenic for insects. However, while other three recognized species of the Photorhabdus genus are strictly entomopathogenic, P. asymbiotica is unique in its ability to act as an emerging human pathogen as well. Analysis of the P. asymbiotica genome identified a novel lectin designated PHL. Recombinant protein was purified and characterized. It exhibited high affinity for fucosylated carbohydrates including saccharides from bacterial cell wall or human blood epitopes. It inhibits the production of reactive oxygen species in human blood and antimicrobial activity both in human blood, serum and insect haemolymph. The structure analysis of these complexes revealed an unusual organization of binding sites that was not observed in any other lectin so far. The presence of high number of binding sites per monomer together with protein dimerization enables high affinity of the lectin towards potential interacting surfaces, e.g. bacteria, immune cells or host epithelia. These results suggest that PHL might play a crucial role in the interaction of P. asymbiotica with both human and insect hosts.