Enhances the high-affinity desensitization of human P2RX3 purinoceptors (PubMed:22842000). At 50 nM, the toxin decreases the IC(50) for ambient ATP from 2.67 nM to 0.77 nM in human P2RX3 (PubMed:22842000). Expressed by the venom gland. The toxin is composed of 2 domains: a highly rigid N-terminal inhibitor cystine knot (knottin) domain and a rather flexible C-terminal linear cationic cytotoxin domain that forms amphiphilic alpha-helices. The presence of a 'disulfide through disulfide knot' structurally defines this protein as a knottin. Amidation at Leu-64 is not mandatory for activity on P2RX3. The toxin does not affect mouse P2RX2 or P2RX2/P2RX3 purinoceptors (PubMed:22842000). The toxin (and its derivatives composed of N-terminal or C-terminal domains) are inactive against a number of bacterial strains (PubMed:27412961). Belongs to the neurotoxin 19 (CSTX) family. 05 (U4-Lctx) subfamily.