RTX-III (neurotoxin-III,δ-SHTX-Hcr1a) is a neurotoxin peptide derived from the Sebae anemone Radianthus crispa. The toxin targets voltage-dependent sodium channels by preventing its complete inactivation, which can lead to a prolonged influx of sodium ions and depolarization of the cell’s membrane.
Source
editRTX-III is secreted by the sea anemone Radianthus crispa, also known as Heteractis crispa or Radianthus macrodactylus, which inhabits the Indian and Pacific Oceans.[2]
Structure
editPrimary structure
editThe RTX-III neuropeptide consists of 48 amino acids cross-linked by three disulfide bridges.[3]
The amino acid sequence of the neurotoxin-III is:
- GNCKCDDEGPYVRTAPLTGYVDLGYCNEGWEKCASYYSPIAECCRKKK[3]
and its molecular mass is 5378.33 Da.[3]
Secondary structure
editDue to RTX-III's structural characteristics, this toxin is categorized as a type II sea anemone neurotoxin. The toxin has a guanidine group of Arg13 residues,[2] as well as disulfide bridges, which may be important in maintaining its active conformation.[5]
Homology
editRTX-III is highly homologous with ShI, also a type II toxin, from the sea anemone Stichodactyla helianthus, whose sequence is 88% identical.[3][6][7] RTX-III also shares significant homology with other toxins in the type II family, including RpII and RTX-VI.[8]
Target
editRTX-III is a Nav activator (also known as a sodium channel opener), which elicits changes in the functioning voltage-gated sodium channels of arthropods, insects and mammals. Research has shown evidence of affinity binding with various types of sodium channels.[8] The toxin modulates the BgNav1 subtype of insects and the VdNav1 subtype of arachnoids. In mammals, it selectively modulates Nav 1.3 and Nav1.6 sodium channels.[8]
All sea anemone toxins are thought to bind within binding site 3 of voltage-dependent sodium channels. The binding site for RTX-III, in particular, is proposed to overlap with that of the channel-inactivating scorpion α-toxins and spider δ-toxins, though it is not entirely identical.[8]
Mode of action
editRTX-III prevents or reduces the speed with which sodium channels are inactivated. The toxin inhibits the inactivation of the voltage-dependent sodium channels in a selective manner. The sodium channels may stay open for longer than normal, and consequently, the influx of sodium is prolonged.[2] In turn, the influx of sodium may depolarize the membrane potential value towards a more positive membrane potential. Therefore, inactivation will be incomplete and less sensitive to any potential changes, slowing down the kinetics of sodium inactivation.[8]
RTX-III differs from the conventional way in which sea anemones operate – an arginine residue being the center of binding with a sodium channel.[2] In the case of neurotoxin-III, it is hypothesized that Arg13 may play a role in selecting specific sodium channel isoforms.[2][9][10] However, these findings might only partially apply to RTX-III since a different, homologous toxin was investigated – RTX-VI.[9][10]
Toxicity and potency
editRTX-III presents a high toxicity in mammals. The LD₅₀ for mice varies from 25 to 40 µg/kg, while the LD100 is 82 µg/kg in arthropods.[3] Specific amino acid substitutions in the RTX-III sequence occur at the positions most toxic for mice.[3]
The EC50 values of RTX-III also differ between mammals (381.8 nM) and insects/arthropods (978.1 nM).[8] RTX-III displays a lower potency in arachnid and insect channels, with relatively high EC50 values. However, in mammalian channels the toxin may be more potent, showing smaller EC50 values.[8] Since RTX-III is produced by a sea anemone, its main role is the effective modulation of arthropod sodium channels, so that the prey is immobilized but not necessarily killed.[8]
| LD50 (µg/kg) | LD100(µg/kg) | EC50 (nM) | |
|---|---|---|---|
| Mammals | 25-40 | x | 381.8 |
| Insects / Arthropods | x | 82 | 978.1 |
RTX-III’s toxic properties are distributed between its many functional groups, such as the Arg-13 guanidine group[11] and the Gly-1 amino group.[2]
References
edit- ^ Karelj (2011-07-29), English: Sea anemone Sebae anemone, (Heteractis crispa) in Prague sea aquarium "Sea world", Czech Republic, retrieved 2024-10-23
- ^ a b c d e f Mahnir, Vladimir M.; Kozlovskaya, Emma P. (1991-01-01). "Structure-toxicity relationships of neurotoxin RTX-III from the sea anemone Radianthus macrodactylus: Modification of amino groups". Toxicon. 29 (7): 819–826. Bibcode:1991Txcn...29..819M. doi:10.1016/0041-0101(91)90218-G. ISSN 0041-0101. PMID 1681602.
- ^ a b c d e f g h Zykova, Tatiana A.; Vinokurov, Leonid M.; Kozlovskaya, Emma P.; Elyakov, Georgy B. (1985). "Amino acid sequence of neurotoxin III from the sea anemone Radianthus macrodactylus". Bioorganicheskaya Khimiya. 11: 302–310.
- ^ Monastyrnaya, Margarita Mikhailovna; Kalina, Rimma Sergeevna; Kozlovskaya, Emma Pavlovna (2023). "The Sea Anemone Neurotoxins Modulating Sodium Channels: An Insight at Structure and Functional Activity after Four Decades of Investigation". Toxins. 15 (1): 8. doi:10.3390/toxins15010008. ISSN 2072-6651. PMC 9863223. PMID 36668828.
- ^ Nabiullin, A. A.; Odinokov, Stanislav E.; Vozhova, E. I.; Kozlovskaya, Emma. P.; Elyakov, Georgy B. (1982). "A circular-dichroism study on the conformational stability of toxin-I from sea anemone Radianthus macrodactylus". Bioorganicheskaya Khimiya. 8 (12): 1644–1648.
- ^ Kem, William R.; Parten, Benne; Pennington, Michael W.; Price, David A.; Dunn, Ben M. (1989-04-18). "Isolation, characterization, and amino acid sequence of a polypeptide neurotoxin occurring in the sea anemone Stichodactyla helianthus". Biochemistry. 28 (8): 3483–3489. doi:10.1021/bi00434a050. ISSN 0006-2960. PMID 2568126.
- ^ Wilcox, G R; Fogh, R H; Norton, R S (1993). "Refined structure in solution of the sea anemone neurotoxin ShI". Journal of Biological Chemistry. 268 (33): 24707–24719. doi:10.1016/s0021-9258(19)74523-2. ISSN 0021-9258. PMID 7901218.
- ^ a b c d e f g h i Kalina, Rimma S.; Peigneur, Steve; Zelepuga, Elena A.; Dmitrenok, Pavel S.; Kvetkina, Aleksandra N.; Kim, Natalia Y.; Leychenko, Elena V.; Tytgat, Jan; Kozlovskaya, Emma P.; Monastyrnaya, Margarita M.; Gladkikh, Irina N. (2020). "New Insights into the Type II Toxins from the Sea Anemone Heteractis crispa". Toxins. 12 (1): 44. doi:10.3390/toxins12010044. ISSN 2072-6651. PMC 7020476. PMID 31936885.
- ^ a b Moran, Yehu; Cohen, Lior; Kahn, Roy; Karbat, Izhar; Gordon, Dalia; Gurevitz, Michael (2006-07-01). "Expression and Mutagenesis of the Sea Anemone Toxin Av2 Reveals Key Amino Acid Residues Important for Activity on Voltage-Gated Sodium Channels". Biochemistry. 45 (29): 8864–8873. doi:10.1021/bi060386b. ISSN 0006-2960. PMID 16846229.
- ^ a b Honma, Tomohiro; Shiomi, Kazuo (2006-01-01). "Peptide Toxins in Sea Anemones: Structural and Functional Aspects". Marine Biotechnology. 8 (1): 1–10. Bibcode:2006MarBt...8....1H. doi:10.1007/s10126-005-5093-2. ISSN 1436-2236. PMC 4271777. PMID 16372161.
- ^ Mahnir, Vladimir M.; Kozlovskaya, Emma P.; Elyakov, Georgy B. (1989-01-01). "Modification of arginine in sea anemone toxin RTX-III from Radianthus macrodactylus". Toxicon. 27 (10): 1075–1084. Bibcode:1989Txcn...27.1075M. doi:10.1016/0041-0101(89)90001-9. ISSN 0041-0101. PMID 2573177.
