Physicochemical characterization of the .alpha.-peptide of the sodium channel from rat brain

Abstract

The .alpha.-peptide of the rat brain sodium channel of apparent molecular weight 260K has been purified to homogeneity in order to determine its structural and chemical properties. By negative-stain electron microscopy, the molecule morphology of the solubilized channel protein appears as a stack of disks or rouleaux whose dimensions are 40 .ANG. .times. 200 .ANG.. Measurement of the secondary structure by circular dichroism shows that the .alpha.-peptide is a conformationally flexible polypeptide that contains mostly .beta.-sheet and random-coil in mixed detergent-phospholipid micelles and folds into a conformation that has approximately 65% .alpha.-helix after reconstitution into phosphatidylcholine vesicles. Preparative polyacrylamide gel electrophoresis was used to obtain chemically homogeneous peptide to analyze the amino acid and carbohydrate composition. The amino acid composition shows a reasonably high content of acidic amino acids with no striking excess of hydrophobic amino acids, while carbohydrate analyses show that carbohydrate is 31% by weight of the protein with sialic acid representing over 50% of the total carbohydrates. The high .alpha.-helical content, the amino acid composition, and the large carbohydrate mass are similar to those of the eel electroplax sodium channel and appear to be general features of the sodium channels which have been analyzed structurally and chemically to date.