The primary structure of the cytotoxin restrictocin

The complete amino acid sequence of the single polypeptide chain of cytotoxin restrictocin has been determined. Its structure was established by automated Edman degradation of the intact molecule reduced and [14C]carboxymethylated and of fragments obtained by chemical cleavage of the protein with cyanogen bromide and BNPS-skatole and by enzymatic cleavage of the polypeptide chain with trypsin. The molecule consists of 149 amino acid residues with a calculated relative molecular mass of 16836. The protein presents two disulfide bridges, one between cysteine residues at positions 5 and 147 and the other one formed by cysteine residues at positions 75 and 131. The amino acid sequence of restrictocin shows a high degree of homology (86%) with that of the cytotoxin named alpha-sarcin.