Purification of the saxitoxin receptor of the sodium channel from rat brain

The saxitoxin (STX) receptor has been purified 740-fold from rat brain by a combination of ion exchange chromatography, wheat germ agglutinin chromatography, and sedimentation on sucrose gradients to a specific activity of 1488 pmol/mg of protein. The best fractions were estimated to be 47% pure from their specific activity or 66% pure on the basis of NaDodSO4 gel electrophoresis. Two polypeptides, alpha (Mr approximately equal to 270,000 +/- 10,000) and beta (Mr approximately equal to 38,300 +/- 2000) (mean +/- SD) copurify with STX binding activity. Two polypeptides of the same apparent Mr are specifically covalently labeled by photoreactive derivatives of 125I-labeled scorpion toxin in rat brain synaptosomes and are likely to be identical to alpha and beta. The solubilized STX receptor has a Mr of 316,000 +/- 63,000, limiting its composition to one alpha polypeptide and one or more beta polypeptides per soluble receptor. Our results suggest that the alpha and beta polypeptides contain both the STX binding site and the scorpion toxin binding site of the mammalian sodium channel.