Omega-grammotoxin SIA

Omega-grammotoxin SIA is a protein toxin that inhibits P, Q and N voltage-gated calcium channels (Ca2+ channels) in neurons.

Source
The source of omega-grammotoxin SIA is the venom of a   tarantula  spider (Grammostola rosea).

Chemistry
Amino Acid Sequence: Asp-Cys-Val-Arg-Phe-Trp-Gly-Lys-Cys-Ser-Gln-Thr-Ser-Asp-Cys-Cys-Pro-His-Leu-Ala-Cys-Lys-Ser-Lys-Trp-Pro-Arg-Asn-Ile-Cys-Val-Trp-Asp-Gly-Ser-Val Molecular Formula: C177H268N52O50S6 Omega-grammotoxin SIA can be purified from Grammostola rosea venom by reverse phase high performance liquid chromatography.

Target
Omega-Grammotoxin SIA is a 36 amino acid residue protein toxin from spider venom that inhibits P, Q and N-type  voltage-gated calcium channels in neurons. It binds to the channels with high affinity (if closed). It also binds to potassium channels but with lower affinity than to the calcium channels. The toxin binding site has high affinity when channels are in closed states and low affinity when channels are activated. (4)

Mode of action
It is believed that omega-grammotoxin SIA inhibits channel function by binding with high affinity to closed, resting states of the channel and that bound toxin makes it more difficult for channels to be opened by depolarization, so much larger   depolarizations are required for channel activation.