PARTICIPATION OF 2 SER-SER-PHE-TYR REPEATS IN INTERLEUKIN-6 (IL-6)-BINDING SITES OF THE HUMAN IL-6 RECEPTOR

The alpha-subunit of interleukin-6 (IL-6) receptor is a member of the hematopoietin receptor family. The alignment of its amino acid sequenc e with those of other members of this family (human somatotropin recep tor/murine IL-3 receptor beta and human IL-2 receptor beta) has sugges ted that amino acids included in two SSFY repeats found in each of its hematopoietin receptor domains, contribute to the binding of the liga nd. The involvement of these amino acids in IL-6 binding and signal tr ansduction was studied by site-directed mutagenesis and molecular mode lling. We present a computer-derived three-dimensional model of the IL -6/IL-6 receptor complex based on the structure of the human somatotro pin/human somatotropin receptor complex. This model allowed the locati on of distinct regions important for IL-6 and gp130 binding. We show t hat some of the residues included in the SSFY repeats located in our I L-6 receptor model in the loops between beta-strands E and F of domain -I and B' and C', of domain-II, participate in the formation of a majo r IL-6-binding site. These residues are necessary for IL-6 and gp130 b inding and for signal transduction. Using our IL-6 receptor mutants we mapped the epitopes of four anti-(IL-6 receptor) neutralising monoclo nal antibodies to these residues. Our results demonstrate that a gener ic hematopoietin receptor family structural module can be used for the study of both alpha and beta receptor subunits belonging to this fami ly.