Proteolytic Cascade for the Activation of the Insect Toll Pathway Induced by the Fungal Cell Wall Component

The insect Toll signaling pathway is activated upon recognition of Gram-positive bacteria and fungi, resulting in the expression of antimicrobial peptides via NF-kappa B-like transcription factor. This activation is mediated by a serine protease cascade leading to the processing of Spatzle, which generates the functional ligand of the Toll receptor. Recently, we identified three serine proteases mediating Toll pathway activation induced by lysine-type peptidoglycan of Gram-positive bacteria. However, the identities of the downstream serine protease components of Gram-negative-binding protein 3 (GNBP3), a receptor for a major cell wall component beta-1,3-glucan of fungi, and their order of activation have not been characterized yet. Here, we identified three serine proteases that are required for Toll activation by beta-1,3-glucan in the larvae of a large beetle, Tenebrio molitor. The first one is a modular serine protease functioning immediately downstream of GNBP3 that proteolytically activates the second one, a Spatzle-processing enzyme-activating enzyme that in turn activates the third serine protease, a Spatzle-processing enzyme. The active form of Spatzle-processing enzyme then cleaves Spatzle into the processed Spatzle as Toll ligand. In addition, we show that injection of beta-1,3-glucan into Tenebrio larvae induces production of two antimicrobial peptides, Tenecin 1 and Tenecin 2, which are also inducible by injection of the active form of Spatzle-processing enzyme-activating enzyme or processed Spatzle. These results demonstrate a three-step proteolytic cascade essential for the Toll pathway activation by fungal beta-1,3-glucan in Tenebrio larvae, which is shared with lysine-type peptidoglycan-induced Toll pathway activation.