Ectatomin (Ea) is a water soluble toxic component from Ectatomma tuberculatum ant venom. It contains two homologous polypeptide chains with 37 and 34 residues, respectively. The chains are linked together by a disulfide bond.[1] Ectatomin in a cell leads to an irreversible increase in ion leakage, a decrease in membrane resistance, and eventually cell death. Ectatomin is found to be the most potent toxic peptide from ant venom because it blocks the calcium channel in the victims. [5] Ectatomin is responsible for about 90% of the total toxicity of the venom produced by the ants. [2]
Structure
Ectatomin is a dimeric peptide. Both subunits, A and B, are formed by two alpha-helixes and four residues forming a hinge region which all combine to form a hairpin structure. The two subunits are then connected by a disulfide bond. All of this combines to form a four alpha-helical bundle structure in aqueous solution. [3] The toxin has a molecular weight of 7,928 Da. [2]
Subunits
The peptide sequences are listed below. There are homologies between the two side chains, along with homologies to other proteins. [5]
Subunit A
1ECI.A has a 37 amino acid peptide sequence as followed: GVIPKKIWETVCPTVEPWAKKCSGDIATYIKRECGKL [4]
Subunit B
1ECI.B has a 34 amino acid peptide sequence as followed: WSTIVKLTICPTLKSMAKKCEGSIATMIKKKCDK [4]
Function
A toxin that blocks sodium or calcium channels are considered to be neurotoxins. [5] A study done on rat cardiac ventricular myocytes showed that the smallest presence of ectatomin decreases calcium currents in the body within seconds. After a few seconds the amount of calcium currents were reduced by half.
When inserted into an artificial membrane, Ea forms an iron pore. Although it only requires 2 Ea molecules to form the pore, the formation of the pore requires reorganization of its helices. [4] [2]
There are two proposals as to how Ea forms pores. The first one, which is the most studied so far, is that a positively charged Ea molecule is concentrated close to a membrane, which then lowers the membrane's affinity. The membrane then breaks apart the two subunits and the hairpin sticks into the membrane, forming a pore. The second proposal is that the pore is formed by arranged whole Ea molecules in the membrane itself. [2]
Spectroscopy
1ECI produces 20 H1 NMR structures. The 20 structures include the N- and c-termini. The rms deviations are 0.75 A for the heavy backbone atoms, and 1.25 A for all of the heavy atoms. [3]
References
1. Arseniev, A S, et al. "Toxic Principle of Selva Ant Venom Is a Pore-Forming Protein Transformer." FEBS Letters, vol. 347, no. 2-3, 27 June 1994, pp. 112-116. EBSCOhost, proxy.library.maryville.edu/login?url=https://search.ebscohost.com/login.aspx?direct=true&db=mdc&AN=8033986&site=eds-live&scope=site.
2. Arseniev A.S, et al.(1994), Toxic principle of selva ant venom is a pore-forming protein transformer, FEBS Letters, 347, doi: 10.1016/0014-5793(94)00518-4
3. Nolde DE, Sobol AG, Pluzhnikov KA, Grishin EV, Arseniev AS (January 1995). "Three-dimensional structure of ectatomin from Ectatomma tuberculatum ant venom". J. Biomol. NMR. 5 (1): 1–13. doi:10.1007/BF00227465. PMID 7881269.
4. Pluzhnikov, Kirill, et al. “Analysis of Ectatomin Action on Cell Membranes.” European Journal of Biochemistry 262.2 (1999):501. Academic Search Complete. Web. 13 Feb. 2017
5. Touchard, Axel, Samira R. Aili, Eduardo G. Fox, Pierre Escoubas, Jerome Orivel, Graham M. Nicholson, and Alain Dejuan. “The Biochemical Toxin Arsenal from Ant Venoms.” MDPI. N.p., 20 Jan. 2016. Web. 13 Feb. 2017.
