Disulfide bond structure and N-glycosylation sites of the extracellular domain of the human interleukin-6 receptor

The high affinity interleukin-6 (IL-6) receptor is a hexameric complex consisting of two molecules each of IL-6, IL-6 receptor (IL-6R), and the high affinity converter and signaling molecule, gp130, The extracellular "soluble" part of the IL-GR (sIL-6R) consists of three domains: an amino-terminal Ig-like domain and two fibronectin-type III (FN III) domains. The two FN III domains comprise the cytokine-binding domain defined by a set of 4 conserved cysteine residues and a WSXWS sequence motif. Here, we have determined the disulfide structure of the human sIL-6R by peptide mapping in the absence and presence of reducing agent. Mass spectrometric analysis of these peptides revealed four disulfide bonds and two free cysteines. The disulfides Cys(102)-Cys(113) and Cys(146)-Cys(157),, consistent with known cytokine-binding domain motifs, and Cys(28)-Cys(77) With known Ig superfamily domains. An unusual cysteine connectivity between Cys(6)-Cys(174), which links the Ig-like and NH2-rminal FN III domains causing them to fold back onto each other, has not previously been observed among cytokine receptors, The two free cysteines (Cys(192) and Cys(258)) were detected as cysteinyl-cysteines, although a small proportion of Cys(258) was reactive with the alkylating agent 4-vinylpyridine. Of the four potential N-glycosylation sites, carbohydrate moieties were identified on Asn(36), Asn(74), and Asn(202), but not on Asn(226).