THE MEMBRANE PROXIMAL CYTOKINE RECEPTOR DOMAIN OF THE HUMAN INTERLEUKIN-6 RECEPTOR IS SUFFICIENT FOR LIGAND-BINDING BUT NOT FOR GP130 ASSOCIATION

Interleukin-6 (IL-6) belongs to the family of the ''four-helix bundle' ' cytokines. The extracellular parts of their receptors consist of sev eral Ig- and fibronectin type III-like domains. Characteristic of thes e receptors is a cytokine-binding module consisting of two such fibron ectin domains defined by a set of four conserved cysteines and a trypt ophan-serine-X-tryptophan-serine (WSXWS) sequence motif. On target cel ls, IL-6 binds to a specific IL-6 receptor (IL-6R), and the complex of IL-6 IL-GR associates with the signal transducing protein gp130. The IL-GR consists of three extracellular domains. The NH2-terminal Ig-lik e domain is not needed for ligand binding and signal initiation. Here we have investigated the properties and functional role of the third m embrane proximal domain. The protein can be efficiently expressed in b acteria, and the refolded domain is shown to be sufficient for IL-6 bi nding. When complexed with IL-6, however, it fails to associate with t he gp130 protein. Since the second and the third domain together with IL-6 can bind to gp130 and induce signaling, our data demonstrate the ligand binding function of the third domain and point to an important role of the second domain in complex formation with gp130 and signalin g.