The N-terminus of gp130 is critical for the formation of the high-affinity interleukin-6 receptor complex

The high-affinity human interleukin-6 (IL-6) receptor complex consists of IL-6 and two cell membrane-associated receptor components, the IL-6 receptor and the high-affinity converter and signal transducer, gp130. Using recombinant IL-6 and the extracellular ('soluble') domains of the IL-6 receptor (sIL-6R) and gp130 (sgpl30), we have shown previously that the high-affinity complex (IL·6-sIL·6R-sgpl30) is hexameric, consisting of two molecules each of IL-6, sIL-6R and sgp130 [Ward et al., (1994), J. Biol Chem. 269, 23286-23289]. Biosensor analysis of the N-terminally extended form of sgp130 (sgp130-FLAG) indicates a failure of sgp130FLAG to form a similar stable high-affinity hexameric complex, yet a low-affinity complex is formed. Analysis of the IL-6·IL-6R·sgpl30-FLAG complex using size-exclusion chromatography and SDS-PAGE indicates that this complex is consistent with a stable lower order complex consisting of IL-6, sIL-6R, and sgp130-FLAG. These data support our model of the hexameric IL-6 receptor complex, which postulates the critical involvement of the N-terminal Ig-like domain of gp130 in the formation of a stable high-affinity complex and IL-6 signaling sgp130 [Simpson et al. (1997), Protein Sci. 6, 929-955].