Assignment of the disulphide bonds of leukocyte interferon.

Nucleotide sequencing of cloned cDNA can lead to rapid and precise preduction of the primary amino acid sequence of gene products, but it cannot establish such post-translational modifications as the existence and arrangement of disulphide bonds. For example, the complete sequences of at least one fibroblast and seven leukocyte interferon genes are already known, but knowledge derived from analysis of the proteins is confined to information on the N-termini and some tryptic fragments. The presence of at least one disulphide bond in leukocyte interferon is suggested by that molecule's sensitivity to reducing agents. In addition, comparison of all leukocyte interferon gene sequences so far reported indicates four highly conserved cysteines. One of these genes has been engineered for efficient direct expression in Escherichia coli and we have purified the gene product, leukocyte interferon A (IFN-alpha A, Fig. 1) from bacterial extracts to a single species of molecular weight (MW) 19,400. I report here the determination of the disulphide bonds of the purified protein by analysis of tryptic fragments. The results indicate that Cys 1 is bonded to Cys 98, and Cys 29 is bonded to Cys 138.