Molecular Mechanism That Induces Activation of Spätzle, the Ligand for the Drosophila Toll Receptor

Abstract

The Drosophila Toll receptor is activated by an endogenous cytokine ligand Spätzle. Active ligand is generated in response to positional cues in embryonic dorso-ventral patterning and microbial pathogens in the insect immune response. Spätzle is secreted as a pro-protein and is processed into an active form by the serine endoproteases Easter and Spätzle-processing enzyme during dorso-ventral patterning and infection, respectively. Here, we provide evidence for the molecular mechanism of this activation process. We show that the Spätzle prodomain masks a predominantly hydrophobic region of Spätzle and that proteolysis causes a conformational change that exposes determinants that are critical for binding to the Toll receptor. We also gather that a conserved sequence motif in the prodomain presents features of an amphipathic helix likely to bind a hydrophobic cleft in Spätzle thereby occluding the putative Toll binding region. This mechanism of activation has a striking similarity to that of coagulogen, a clotting factor of the horseshoe crab, an invertebrate that has changed little in 400 million years. Taken together, our findings demonstrate that an ancient passive defense system has been adapted during evolution and converted for use in a critical pathway of innate immune signaling and embryonic morphogenesis.

Structure of the Toll-Spätzle complex, a molecular hub in Drosophila development and innate immunity

“…We and others (27,34,40,41) have proposed that the prodomains cover a hydrophobic flank of the dimer between the base of the molecule and the wing. Assuming that the (covalently linked) uncleaved prodomain in hcSpätzle is not influenced by removal of the opposing prodomain, geometric constraints demand that the leading protomer juxtaposing LRRNT1 must be in the processed form, and that the remaining prodomain of hcSpätzle must be attached to the otherwise exposed trailing protomer.…”

Antiviral responses of arthropod vectors: an update on recent advances

“…Upon recognition of PAMPs, for example lipoteichoic acid of gram-positive bacteria, fungal peptidoglycan and possibly virus debris, the cytokine spaetzle is cleaved into its active form and binds to the Toll receptor [4]. A downstream signalling cascade similar to those of mammalian TLRs results in degradation of the IjB orthologue Cactus and activation of the transcription factor REL1 [108], an orthologue of NFjB, which translocates to the nucleus and initiates transcription of AMPs such as defensins.…”

Airway Fibrinogenolysis and the Initiation of Allergic Inflammation

“…In the absence of T and B cells, these organisms mount a limited set of responses against a wide variety of infectious organisms. In response to gram-negative bacterial invasion, lipopolysaccharide (LPS) activates a series of proteolytic events within the circulating hemolymph of these organisms to activate two defensive outcomes: (1) the hemolymph coagulates, preventing further dissemination of the infection, and (2) a fragment of the main coagulant protein (pro-Späetzele in Drosophila and coagulogen in horseshoe crabs [26,27]) signals through the main arthropod immune receptor, Toll, to induce additional antibacterial defensive mechanisms (28). The identical pathway is triggered upon invasion by fungi, but instead of LPS, fungal proteinases directly activate the proteolytic cascade, leading to hemolymph coagulation and activation of Toll through the endogenous proteinase Persephone (29).…”