Hainantoxin-IV (HNTX-IV) can specifically inhibit the neuronal tetrodotoxin-sensitive sodium channels and defines a new class of depressant spider toxin. The se-quence of native HNTX-IV is ECLGFGKGCNPSNDQCCKSSNLVCSRKHRWCKYEI-NH2. In the present study,to obtain further insight into the primary and tertiary structural requirements of neuronal sodium channel blockers, we determined the solution structure of HNTX-IV as a typical inhibitor cystine knot motif and synthesized four mutants designed based on the predicted sites followed by structural elucidation of two inactive mutants. Pharmacological studies indicated that the S12A and R26A mutants had activities near that of native HNTX-IV, while K27A and R29A demonstrated activities reduced by 2 orders of magnitude. 1H MR analysis showed the similar molecular conformations for native HNTX-IV and four synthetic mutants. Furthermore,in the determined structures of K27A and R29A,the side chains of residues 27 and 29 were located in the identical spatial position to those of native HNTX-IV.These results suggested that residues Ser12, Arg26, Lys27,and Arg29 were not responsible for stabilizing the distinct conformation of HNTX-IV, but Lys27 and Arg29 were critical for the bioactivities. The potency reductions produced by Ala substitutions were primarily due to the direct interaction of the essential residues Lys27 and Arg29 with sodium channels rather than to a conformational change. After comparison of these structures and activities with correlated toxins, we hypothesized that residues Lys27, Arg29, His28, Lys32, Phe5, and Trp30 clustered on one face of HNTX-IV were responsible for ligand binding.