Mastoparan

Mastoparan
Solution structure of mastoparan from Vespa simillima xanthoptera.[1]
Identifiers
Symbol	Mastoparan_2
Pfam	PF08251
InterPro	IPR013214
Available protein structures: Pfam structures PDB RCSB PDB; PDBe PDBsum structure summary

Mastoparan is a peptide toxin from wasp venom. It has the chemical structure Ile-Asn-Leu-Lys-Ala-Leu-Ala-Ala-Leu-Ala-Lys-Lys-Ile-Leu-NH2.^[2]

The net effect of mastoparan's mode of action depends on cell type, but seemingly always involves exocytosis. In mast cells, this takes the form of histamine secretion, while in platelets and chromaffin cells release serotonin and catecholamines are found, respectively. Mastoporan activity in the anterior pituitary gland leads to prolactin release.

In the case of histamine secretion, the effect of mastoparan takes place via its interference with G protein activity. By stimulating the GTPase activity of certain subunits, mastoparan shortens the lifespan of active G protein. At the same time, it promotes dissociation of any bound GDP from the protein, enhancing GTP binding. In effect, the GTP turnover of G proteins is greatly increased by mastoparan. These properties of the toxin follow from the fact that it structurally resembles activated G protein receptors when placed in a phospholipid environment. The resultant G protein-mediated signaling cascade leads to intracellular IP₃ release and the resultant influx of Ca²⁺.

In an experimental study conducted by Tsutomu Higashijima and his counterparts, mastoparan was compared to melittin, which is found in bee venom.^[2] Mainly, the structure and reaction to phosphate was studied in each toxin. Using Circular Dichroism (CD), it was found that when mastoparan was exposed to methanol, an alpha helical form existed. It was concluded that strong intramolecular hydrogen bonding occurred. Also, two negative bands were present on the CD spectrum. In an aqueous environment, mastoparan took on a nonhelical, unordered form. In this case, only one negative band was observed on the CD spectrum. Adding phosphate buffer to mastoparan resulted in no effect.

Melittin produced a different conformational change than mastoparan. In an aqueous solution, melittin went from a nonhelical form to an alpha helix when phosphate was added to the solution. The binding of melittin to the membrane was believed to result from electrostatic interactions, not hydrophobic interactions.

References

1. ^ PDB 2CZP; Todokoro Y, Yumen I, Fukushima K, Kang SW, Park JS, Kohno T, Wakamatsu K, Akutsu H, Fujiwara T (August 2006). "Structure of Tightly Membrane-Bound Mastoparan-X, a G-Protein-Activating Peptide, Determined by Solid-State NMR". Biophys. J. 91 (4): 1368–79. doi:10.1529/biophysj.106.082735. PMC 1518647. PMID 16714348. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1518647. 
2. ^ ^{a b} Higashijima T, Uzu S, Nakajima T, Ross EM (May 1988). "Mastoparan, a peptide toxin from wasp venom, mimics receptors by activating GTP-binding regulatory proteins (G proteins)". J. Biol. Chem. 263 (14): 6491–4. PMID 3129426.