A model of the tertiary structure of human IL-6, derived from the crystal-structure of granulocyte-colony stimulating factor, reveals a 5th helical region in the loop between the first and second α-helix. To investigate the importance of this region for biological activity of IL-6, residues Glu-52, Ser-53, Ser-54, Lys-55, Glu-56, Leu-58 and Glu-60 were individually replaced by alanine. IL-6·Leu-58Ala displayed a 5-fold reduced biological activity on the IL-6-responsive human cell lines XG-1 and A375. This reduction in bioactivity was shown to be due to a decreased capacity of the mutant protein to trigger IL-6 receptor-α-chain-dependent binding to the IL-6 signal transducer, gp130.
