Properties of a Human Alpha-Interferon Purified fromE. ColiExtracts

Abstract

A human alpha interferon, designated HuIFN-γA, produced in E. coli by direct expression of cloned cDNA [Goeddel et al., Nature 287, 411–416 (1980)] has been purified from bacterial extracts and characterized. The protein has a molecular weight (19,400 by SDS/PAGE) and amino acid composition consistent with the DNA sequence. The pI was determined to be 6.1. The molecule has a specific activity of 1.5 × 10⁸ NIH reference units/mg of protein. The sequence of the first 35 amino acids is identical to that expected from the nucleotide sequence. About 50% of the molecules begin with the expected cysteine, and 50% begin with the initiator methionine which E. coli apparently did not remove efficiently. Analysis of a trypsin digest of the native molecule showed that all four of the molecule's cysteines are involved in disulfide bonds: Cys¹ is bonded to Cys⁹⁸, and Cys²⁹ is bonded to Cys.¹³⁸