Binding properties of the regulatory domains in Manduca sexta hemolymph proteinase-14, an initiation enzyme of the prophenoloxidase activation system.

Pathogen recognition and rapid initiation of defense responses are essential for the survival of host insects. In Manduca sexta, hemolymph proteinase-14 precursor (proHP14) senses non-self presence and triggers a branched serine proteinase pathway which leads to prophenoloxidase activation and melanin formation around the invading organisms. To understand functions of individual domains in HP14, we have produced a series of HP14 domains and truncation mutants and studied their interactions with microbial polysaccharides and beta-1,3-glucan recognition protein-1 (betaGRP1)-a biosensor for fungal and bacterial infection. These include: the low-density lipoprotein receptor class A repeats 1-5 (LDL(1-5)), Sushi domain, Wonton domain, and proteinase catalytic domain of HP14, as well as proHP14 missing 1-4 LDL repeats (DeltaLDL(1), DeltaLDL(12), DeltaLDL(1-3) and DeltaLDL(1-4)). LDL(1-5), Sushi, and Wonton domains specifically recognized Lys-type PG, whereas the latter two also bound betaGRP1. Wonton in addition bound to lipopolysaccharide (LPS), lipoteichoic acid (LTA), and meso-diaminopimelic acid (DAP)-type peptidoglycan (PG). The four N-terminally truncated proHP14 (DeltaL(x)) further confirmed specific interactions with LPS, LTA, DAP-PG, Lys-PG, laminarin, and betaGRP1. These binding data suggest a broad specificity of proHP14 in pattern recognition. Its role in mediating immune responses is anticipated to be influenced by other plasma factors and surface structures of invading pathogens.