There are two subtypes of this toxin:
- Imperatoxin A (activator): a peptide toxin which enhances the influx of Ca2+ from the sarcoplasmatic reticulum into the cell.
- Imperatoxin I (inhibitor): a peptide toxin which decreases the influx of Ca2+ from the sarcoplasmatic reticulum into the cell.
- 33 amino acids peptide.
- the formula is C148H260N58O45S6.
- shares the structure and function of the dihydropiridine receptor (DHPR). It corresponds to the II-III loop of the α1s subunit.
- three cysteine residues that form disulfide bridges to stabilize the three-dimensional structure.
IpTxa acts on the Ryanodine receptors (RyR), which are intracellular Ca2+ release channels mainly known for their role in regulating Ca2+ release from the sarcoplasmatic reticulum of striated muscles. The peptide acts better on RyR type 1 than on type 3. RyR type 2 seems to be insensitive to IpTxa.
The part of the peptide that looks like the II-III loop of the (DHPR) binds directly to RyR and enhances ryanodine binding to trigger Ca2+ release.
- Two polypeptides. A large subunit of 104 amino acids and a smaller one of 27 amino acids.
- Subunits are linked by a disulfide bond.
- Phospholipase A2 (PLA2) activity on the large subunit.
Like IpTxa, IpTxi acts on RyR.
When an action potential reaches the muscle, RyR channels open and Ca2+ becomes available in the cell to induce contraction. The presence of Ca2+ induces the large subunit of IpTxi to hydrolyze the Sn2 fatty acyl bond from the membrane of the sarcoplasmatic reticulum. This process is executed by PLA2 activity. The freed fatty acids bind to the RyR itself or to a closely associated protein linked to gating. Binding of the RyR induces blocking of the channel. When the concentration of free fatty acids is low there will be an incomplete block of RyR; higher concentrations will give a complete block.
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