Interleukin-11 (IL-11) is a member of the gp130 family of cytokines. These
cytokines drive the assembly of multisubunit receptor complexes, all of whi
ch contain at least one molecule of the transmembrane signaling receptor gp
130. IL-11 has been shown to induce gp130-dependent signaling through the f
ormation of a high affinity complex with the IL-11 receptor (IL-11R) and gp
130. Site-directed mutagenesis studies have identified three distinct recep
tor binding sites of IL-11, which enable it to form this high affinity rece
ptor complex. Here we present data from immunoprecipitation experiments, us
ing differentially tagged forms of ligand and soluble receptor components,
which show that multiple copies of IL-11, IL-11R, and gp130 are present in
the receptor complex. Furthermore, it is demonstrated that sites II and III
of IL-11 are independent gp130 binding epitopes and that both are essentia
l for gp130 dimerization. We also show that a stable high affinity complex
of IL-11, IL-11R, and gp130 can be resolved by nondenaturing polyacrylamide
gel electrophoresis, and its composition verified by second dimension dena
turing polyacrylamide gel electrophoresis. Results indicate that the three
receptor binding sites of IL-11 and the Ig-like domain of gp130 are all ess
ential for this stable receptor complex to be formed. We therefore propose
that IL-11 forms a hexameric receptor complex composed of two molecules eac
h of IL-11, IL-11R, and gp130.