Analysis of the human interleukin-6/human interleukin-6 receptor binding interface at the amino acid level: proposed mechanism of interaction.

The interaction between interleukin-6 (IL-6) and IL-6 receptor (IL-6R) is the initial and most specific step in the IL-6 signaling pathway. Understanding its mechanism at the amino acid level is the basis for developing small IL-6-inhibiting molecules. We studied the human IL-6 (hIL-6)/hIL-6R binding interface by a combination of molecular modelling and site-directed mutagenesis. Our model suggests that the center of the interface between the two molecules consists of hydrophobic contacts predicted to account for most of the binding-free energy. These contacts can be regarded as a hydrophobic core shielded by hydrophilic residues that are also needed for recognition. Following this hypothesis, we altered in hIL-6 and hIL-6R residues predicted to reside in the contact region and to interact with each other. We studied the capacity of these mutants to form an IL-6/IL-6R complex and their ability to transduce the signal. This combined approach has led to the identification of certain residue-clusters in the binding interface and to a rational explanation of their specific interactions, suggesting therein a likely mechanism of complex formation. The results confirm the predictive model and strongly support our hypothesis. Comparison with other cytokines and their alpha-subunit receptors suggests that the structural location of certain binding sites are conserved.