Assignment of the disulphide bonds of leukocyte interferon

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Abstract
Nucleotide sequencing of cloned c[complementary]DNA can lead to rapid and precise prediction of the primary amino acid sequence of gene products, but it cannot establish such post-translational modifications as the existence and arrangement of disulfide bonds. The complete sequences of at least 1 fibroblast and 7 leukocyte interferon genes are already known, but knowledge derived from analysis of the proteins is confined to information on the N-terminal and some tryptic fragments. The presence of at least 1 disulfide 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 4 highly conserved cysteines. Of these genes 1 has been engineered for efficient direct expression in Escherichia coli and the gene product, leukocyte interferon A (IFN-.alpha.A) was purified from bacterial extracts to a single species of MW 19,400 I. The determination of the disulfide bonds of the purified protein by analysis of tryptic fragments is reported, results indicate that Cys 1 is bonded to Cys 98, and Cys 29 is bonded to Cys 138.