Site-specific religation of G-CSF fragments through a thioether bond.

A new approach is described for linking, through a thioether bond, the C-terminus of one unprotected polypeptide with the N-terminus of another. Homocysteine thiolactone is attached to the C-terminus of one polypeptide by reverse proteolysis and provides through hydroxylamine treatment a free sulfhydryl group. The alpha-amino group of a second polypeptide is selectively iodoacetylated by reaction with iodoacetic anhydride at pH 6.0 or the N-hydroxysuccinimide ester derivative at pH 7.0. Coupling of the two modified fragments occurs in a spontaneous alkylation reaction under mild conditions. After preliminary experiments with small peptides, this approach was extended to large protein fragments derived from recombinant analogs of G-CSF by enzymatic digestion. This approach provides a means of making head-to-tail protein chimeras or introducing noncoded structural elements into a protein.