Psalmotoxin

Unknown parameter name Unknown parameter name Psalmotoxin (PcTx1) is a spider toxin from the venom of the Trinidad tarantula Psalmopoeus cambridgei. It selectively blocks Acid Sensing Ion Channel 1-a (ASIC1a), which is a proton-gated sodium channel. Psalmotoxin is a toxin produced in the venom glands of the South American tarantula Psalmopoeus cambridgei. The psalmotoxin structure can be classified as an inhibitor cystine knot (ICK) protein. Many ion channel effectors from snail, spider, and scorpion venoms share a similar ICK structure, although they possess very different pharmalogical profiles. Among ICK toxins, psalmotoxin is the only peptide known to act on homomeric ASIC1 channels. Psalmotoxin is a 40-amino acid peptide, possessing 6 cysteines linked by three disulfide bridges. The three-dimensional structure consists of a compact disulfide-bonded core from which three loops and the N and C termini emerge. The main element of the structure is a three-stranded antiparallel β-sheet. Psalmotoxin can bind to a particular isoform of the Acid Sensing Ion Channel, the Acid Sensing Ion Channel 1 (ASIC1). The binding of psalmotoxin has an effect on both of the two splice variants known of ASIC1, ASIC1a and ASIC1b.ASIC1 has two transmembrane components. After the first transmembrane component it forms a large extracellular bridge with the second transmembrane component, an extracellular loop. This extracellular loop contains cysteine rich domains. Psalmotoxin specifically binds these cysteine rich domains in the extracellular loop of ASIC1. This implicates this domain is the receptor site of ASIC1 for psalmotoxin. ASICs are proton-gated sodium channels. ASICs open when H+ binds. This occurs when the H+-concentration in the environment of the neuron is slightly higher compared to resting H+-concentrations (pH = 7.4). The expression of ASIC1a is high in both the central nervous system and in the sensory neurons of the dorsal root ganglia. ASIC1b is only expressed in sensory neurons. Expression of ASIC1a in the central nervous system relates to the involvement of ASIC1a in higher brain functions, such as learning, memory and fear conditioning. Expression of ASIC1a and ASIC1b in sensory neurons relates to their involvement in nociception and taste. Binding of psalmotoxin to ASIC1a is reported to increase the affinity of ASIC1a for H+. This increase in affinity for H+ results in the shift of ASIC1a into the desensitized state at resting H+-concentrations (pH = 7.4). The channel being desensitized means that the ion channel is bound to its ligand, H+, but is not able to let ions pass through the ion channel. The underlying mechanism of how this increase in affinity for H+ accounts for a shift of the ASIC1a channels into the desensitized state is not yet specified.