Grammotoxin

Grammotoxin is a protein toxin that inhibits P-, Q- and N-type voltage-gated calcium channels (Ca 2+ channels) in neurons. Grammotoxin is also known as omega-grammotoxin SIA.

Source
The source of grammotoxin is the  venom of a   tarantula  spider (Grammostola spatulate).

Chemistry
Grammotoxin is a 36 Amino Acid protein toxin, which has the following 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 (1)

Its chemical formula is: C177H268N52O50S6 (1)

Grammotoxin can be purified from Grammostola spatulata venom by reverse phase high performance liquid chromatography.(2)

Target
Grammotoxin is a 36 amino acid residue protein toxin from spider venom that inhibits P-, Q- and N-type  voltage-gated calcium channels in  neurons.

Mode of action
The toxin binding site on the channels has high affinity for the toxins when they are closed and low affinity when channels are activated. (4) As a result, the toxin preferentially binds to the closed channels. It binds at a region which contains the voltage-sensing domains. When bound, the toxin makes it more difficult for channels to be opened by depolarization, so much larger depolarizations are required for channel activation. (4) Grammotoxin also binds to potassium channels but with lower affinity than to the calcium channels.(3)