Three-dimensional structure and function study on the active region in theextracellular ligand-binding domain of human IL-6 receptor

In this study the three-dimensional (3-D) model of the ligand-binding domai n (V106-P322) of human interleukin-6 receptor (hIL-6 R) was constructed by computer-guided homology modeling technique using the crystal structure of the ligand-binding domain (K52-L251) of human growth hormone receptor (hGHR ) as templet. Furthermore, the active binding region of the 3-D model of hI L-6R with the ligand (hIL-6) was predicted. In light of the structural char acteristics of the active region, a hydrophobic pocket shielded by two hydr ophilic residues (E115 and E505) of the region was identified by a combinat ion of molecular modelling and the site-directed or double-site mutation of the twelve crucial residues in the ligand-binding domain of hIL-6R (V106-P 322), We observed and analyzed the effects of these mutants on the spatial conformation of the pocket-like region of hIL-6 R, The results indicated th at any site-directed mutation of the five Cys residues (four conservative C ys residues: Cys121, Cys132, Cys165, Cys176; near membrane Cys residue: Cys 193) or each double-site mutation of the five residues in WSEWS motif of hI L-6R (V106-P322) makes the corresponding spatial conformation of the pocket region block the linkage between hIL-6 R and hIL-6, However, the influence of the site-directed mutation of Cys211 and Cys277 individually on the con formation of the pocket region benefits the interaction between hIL-6R and hIL-6. Our study suggests that the predicted hydrophobic pocket in the 3-D model of hIL-6R (V106-P322) is the critical molecular basis for the binding of hIL-6R with its ligand, and the active pocket may be used as a target f or designing small hIL-6R-inhibiting molecules in our further study.