The amino acid sequence of the tryptophan-containing subunit (alpha-subunit) of bovine brain S-100 protein.

The tryptophan-containing subunit (alpha-subunit) of bovine brain S-100 protein was purified from a S-aminoethyl derivative of S-100a protein, and its amino acid sequence was determined. The alpha-subunit contained 93 residues, including one tryptophan, and had a molecular weight of 10,400. The sequence shows an extensive homology (58% identity) to the sequence of another "tryptophan-free" subunit (beta-subunit) found in both S-100a and S-100b protein, and has a calcium binding site characteristic of the "E-F hand" proteins, such as calmodulin or troponin C. The tryptophan residue is located at position 90 which is presumably adjacent to the C-terminal end of the alpha-helix following the calcium binding loop, and thus appears likely to serve as a specific probe in structure-function studies of S-100 protein.