Purification and characterization of recombinant human insulin-like growth factor II (IGF-II) expressed as a secreted fusion protein in Escherichia coli

Human insulin-like growth factor II (IGF-II) was produced in an Escherichia coli ompT strain as a 22.5-kDa fusion protein. IGF-II was fused to the carboxy-terminal of a synthetic 15-kDa IgG-binding protein, originating from staphylococcal protein A, via a unique methionine linker. During fermentation, the fusion protein was exported to the growth medium at levels exceeding 900 mg/liter and subsequently affinity purified on IgG Sepharose followed by ion exchange on S Sepharose. After chemical cleavage with CNBr, yielding an authentic IGF-II molecule, the recombinant IGF-II was purified to homogeneity by a two step procedure involving ion-exchange and reverse-phase HPLC. A substantial fraction of the secreted protein was found to be biologically active, eliminating the need for complex refolding procedures. The yield of highly purified and biologically active IGF-II was 5-7 mg/liter of fermenter broth. The IGF-II produced by this method displayed biochemical, immunological, receptor binding, and biological activity properties equal to those of native IGF-II isolated from human serum.