Initiating protease with modular domains interacts with β-glucan recognition protein to trigger innate immune response in insects

The autoactivation of an initiating serine protease upon binding of pattern recognition proteins to pathogen surfaces is a crucial step in eliciting insect immune responses such as the activation of Toll and prophenoloxidase pathways. However, the molecular mechanisms responsible for autoactivation of the initiating protease remains poorly understood. Here, we investigated the molecular basis for the autoactivation of hemolymph protease 14 (HP14), an initiating protease in hemolymph of Manduca sexta, upon the binding of beta-1,3-glucan by its recognition protein, beta GRP2. Biochemical analysis using HP14 zymogen (proHP14), beta GRP2, and the recombinant proteins as truncated forms showed that the amino-terminal modular low-density lipoprotein receptor class A (LA) domains within HP14 are required for proHP14 autoactivation that is stimulated by its interaction with beta GRP2. Consistent with this result, recombinant LA domains inhibit the activation of proHP14 and prophenoloxidase, likely by competing with the interaction between beta GRP2 and LA domains within proHP14. Using surface plasmon resonance, we demonstrated that immobilized LA domains directly interact with beta GRP2 in a calcium-dependent manner and that high-affinity interaction requires the C-terminal glucanase-like domain of beta GRP2. Importantly, the affinity of LA domains for beta GRP2 increases nearly 100-fold in the presence of beta-1,3-glucan. Taken together, these results present the first experimental evidence to our knowledge that LA domains of an insect modular protease and glucanase-like domains of a beta GRP mediate their interaction, and that this binding is essential for the protease autoactivation. Thus, our study provides important insight into the molecular basis underlying the initiation of protease cascade in insect immune responses.