SHTX

SHTX is a toxin derived from the sea anemone Stichodactyla haddoni; there are four different subtypes, SHTX I, II, III and IV. SHTX I, II and III can paralyze crabs by acting on potassium channels, while SHTX IV works on sodium channels, and is lethal to crabs.

SHTX I, II, III and IV are toxins derived from the sea anemone Stichodactyla haddoni.

SHTX I and II consist of 28 amino acids. SHTX I and II have been classified as members of the type IV potassium channel toxin family. The protein sequences of SHTX I and II differ only at amino acid position 6, where the SHTX I protein contains a hydroxyproline and SHTX II contains a proline. SHTX I contains two disulfide bridges between Cys-7 and -19 and between Cys-10 and -25., whereas most peptide toxins derived from sea anemones have three disulfide bridges. Because of the extensive homology between SHTX I and II, it is plausible that SHTX II will share the location of the disulfide bridges. SHTX I and II also share homology with Am I, a toxin isolated from the sea anemone Antheopsis maculata. These proteins have the same positions of cysteines in their amino acid sequences, which points towards a comparable structure. Am I is toxic to crabs and is lethal in high doses (LD₅₀ 830 µg/kg), but the target of this toxin is still unknown.

The protein SHTX III is a Kunitz-type protease inhibitor, with a length of 62 amino acids. It shows homology to other members of the Kunitz-type protease inhibitor family and the highest similarity is shown with AEPI-I, a toxin derived from the sea anemone Actinia equina.

SHTX IV is composed of 48 amino acids and is a member of the type 2 sea anemone sodium channel toxin family. The protein shares homology with the other members of this family, especially with the toxin Rp II from the sea anemone Radianthus paumotensis. The amino acid glycine at the C-terminus of the SHTX IV protein is deleted after completing translation. As a result, the mature protein has an amidated lysine at its C-terminus instead of a glycine.

SHTX II acts on voltage-gated potassium channels and was reported to be approximately 50 times less potent than ¹²⁵I-a-dendrotoxin to synaptosomal membranes, with an IC₅₀ of 270 nM. The affinity and target of SHTX I has not been assessed yet, however, due to the similarities of SHTX I with SHTX II in both sequence and crab paralyzing activity, it is considered to show the same affinity to potassium channels as SHTX II.

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