Voltage-regulated sodium channel molecules.

In summary, TTX and STX binding have been used to follow the purification of sodium channel proteins from electric organ, mammalian skeletal muscle, and brain. In each instance they were proteins somewhat larger than the acetylcholine receptor and exhibited stability properties that reflect an intimate interaction with membrane lipids. The principal peptide constituent seems to be a large glycopeptide of approximately 250,000 daltons. Because this is evidently the only constituent of the electroplax protein, it must contain the TTX receptor site, probably forming the ion pathway itself. Photo-labeling with ScTX and studies with Tityus gamma toxin (58a) indicate a site involved with gating processes, also associated with the large peptide of the neuronal and electroplax proteins. The smaller peptides are not consistent features of all of the preparations, but may contribute to the molecular ensemble. If, however, the entire channel were formed from a single extremely large peptide, there would be interesting mechanistic implications, because the ion transporting and voltage-sensing mechanisms would be accounted for by domains within the folded polypeptide chain. Clearly, the prospect for combining biochemical isolation and reconstitution with the new biophysical technology offers an exciting experimental conjunction.