Receptor for the cleaved activated form of spz, spaetzle C-106 (PubMed:12872120). Binding to spaetzle C-106 activates the Toll signaling pathway and induces expression of the antifungal peptide drosomycin (PubMed:12872120, PubMed:8808632, PubMed:10973475). Component of the extracellular signaling pathway that establishes dorsal-ventral polarity in the embryo (PubMed:3931919). Promotes heterophilic cellular adhesion (PubMed:2124970). Involved in synaptic targeting of motoneurons RP5 and V to muscle 12 (M12); functions as a repulsive cue inhibiting motoneuron synapse formation on muscle 13 (M13) to guide RP5 and V to the neighboring M12, where its expression is repressed by tey (PubMed:20504957). May also function in embryonic neuronal survival and the synaptic targeting of SNa motoneurons (PubMed:19018662). In the absence of ligand, forms a low-affinity disulfide-linked homodimer (PubMed:24733933). In the presence of ligand, crystal structures show one Tl molecule bound to a spaetzle C-106 homodimer (PubMed:24282309, PubMed:24733933). However, the active complex probably consists of two Tl molecules bound to a spaetzle C-106 homodimer (PubMed:24282309, PubMed:24733933). This is supported by in vitro experiments which also show binding of the spaetzle C-106 dimer to 2 Tl receptors (PubMed:12872120). Ligand binding induces conformational changes in the extracellular domain of Tl (PubMed:24282309). This may enable a secondary homodimerization interface at the C-terminus of the Tl extracellular domain (PubMed:24282309). In the fat body, detected in puntate structures along the cell periphery. Becomes evenly distributed in the cytoplasm 1 hour after bacterial infection. In early embryos, concentrated in the pseudocleavage furrows that form transiently between nuclei before cellularization and in the cleavage furrows during cellularization (at protein level) (PubMed:1879347). Later, found on cells in the mesectoderm, stomodeum, proctodeum, anterior and posterior midguts, splanchnopleura, salivary gland placode and adjacent to the segmentally repeated tracheal placodes (at protein level) (PubMed:1879347). During and after germ band shortening, localized in a number of cell types, including the salivary gland, foregut, hindgut, Malpighian tubules and epidermis (at protein level) (PubMed:1879347). In embryos, high expression in M13 with comparatively low expression in M12 (PubMed:20504957). Expressed both maternally and zygotically (PubMed:2449285, PubMed:1879347, PubMed:12617819). Maternal Tl increases in syncytial embryos, reaching a peak at the syncytial blastoderm stage and then decreases and is almost undetectable by the time of ventral furrow formation at gastrulation (at protein level) (PubMed:1879347). Expressed throughout development, with highest levels of expression in the embryo (PubMed:10973475). Expressed in all embryonic central nervous system axons (PubMed:19018662). In larvae, detected in the blood cells and fat body (PubMed:12617819). Up-regulated during vesicular stomatitis virus (VSV) infection. Belongs to the Toll-like receptor family. In some plant proteins and in human SARM1, the TIR domain has NAD(+) hydrolase (NADase) activity (By similarity). However, despite the presence of the catalytic Asp residue, the isolated TIR domain of human TLR4 lacks NADase activity (By similarity). Based on this, it is unlikely that Toll-like receptors have NADase activity.