The three carboxy-terminal amino acids of human interleukin-6 are essential for its biological activity.

We have constructed on the cDNA level deletion mutants of human interleukin-6 lacking one, two, three or four amino acids from the carboxy-terminus of the molecule. After in vitro transcription and translation the biological activity of these deletion mutants was determined by two independent bioassays. Both, the mouse B9 cell proliferation assay and the fibrinogen induction assay with the human hepatoma cell line HepG2 led to the following result: already the removal of the last amino acid resulted in a five-fold loss of biological activity. An additional slight reduction was seen when two amino acids were removed from the carboxy-terminus. Interleukin-6 lacking three or four C-terminal amino acids were completely inactive. The presented results emphasize the extreme importance of the carboxy-terminus of interleukin-6 for its biological function.