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Institut für Biochemie, Rheinisch-Westfälische Technische Hochschule Aachen, Aachen, Germany
Cytokines are key mediators for the regulation of hemopoiesis and
the coordination of immune responses. They exert their various
functions through activation of specific cell surface receptors,
thereby initiating intracellular signal transduction cascades which
lead to defined cellular responses. As the common signal-transducing
receptor subunit of at least seven different cytokines, gp130 is an
important member of the family of hemopoietic cytokine receptors which
are characterized by the presence of at least one cytokine-binding
module. Mutants of gp130 that either lack the Ig-like domain D1 (
D1)
or contain a distinct mutation (F191E) within the cytokine-binding
module have been shown to be severely impaired with respect to IL-6
induced signal transduction. After cotransfection of COS-7 cells with a
combination of both inactive gp130 mutants, signal transduction in
response to IL-6 is restored. Whereas cells transfected with
D1 do
not bind IL-6/sIL-6R complexes, cells transfected with the F191E mutant
bind IL-6/sIL-6R with low affinity. Combination of
D1 and F191E,
however, leads to high-affinity ligand binding. These data suggest that
two different gp130 epitopes, one on each receptor chain, sequentially
cooperate in asymmetrical binding of IL-6/IL-6R in a tetrameric
signaling complex. On the basis of our data, a model for the mechanism
of IL-6-induced gp130 activation is proposed.
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