Structural Properties of Bovine Brain S-100 Protein

The S-100 protein was purified from bovine brain by ammonium sulfate precipitation, ion exchange chromatography on DEAE-Sephadex, and gel filtration through Sephadex G-100. Aggregation by disulfide bond formation results in the appearance of more than one peak of the S-100 protein on DEAE-Sephadex and Sephadex G-100. Aggregation can be reversed by incubation with 0.01 m 2-mercaptoethanol for 1 hour at 60°. At least one sulfhydryl group is exposed to Ellman's reagent by calcium or, less effectively, by magnesium and strontium. The protein contains three subunits. One of these can be separated from the other two by ion exchange chromatography on DEAE-Sephadex in 8 m urea. The separated subunit has the only tryptophan in the S-100 protein and does not contain arginine. The amino acid composition suggests the other two subunits are different. When the lysines of these two subunits are blocked by maleylation, trypsin cleaves only at the two arginines in these two subunits. Only two fragments of 2000 and 5000 molecular weight result, suggesting the two subunits are structurally similar.