Control of receptor-induced signaling complex formation by the kinetics of ligand/receptor interaction

Tumor necrosis factor (TNF) exists both as a membrane-integrated type II precursor protein and a soluble cytokine that have different bioactivities on TNFR2 (CD120b) but not on TNFR1 (CD120a). To identify the molecular basis of this disparity, we have investigated receptor chimeras comprising the cytoplasmic part of Fas (CD95) and the extracellular domains of the two TNF receptors. The membrane form of TNF, but not its soluble form, was capable of inducing apoptosis as well as activation of c-Jun N-terminal kinase and NF-kappaB via the TNFR2-derived chimera. In contrast, the TNFR1-Fas chimera displayed strong responsiveness to both TNF forms. This pattern of responsiveness is identical to that of wild type TNF receptors, demonstrating that the underlying mechanisms are independent of the particular type of the intracellular signaling machinery and rather are controlled upstream of the intracellular domain. We further demonstrate that the signaling strength induced by a given ligand/receptor interaction is regulated at the level of adaptor protein recruitment, as shown for FADD, caspase-8, and TRAF2. Since both incidents, strong signaling and robust adapter protein recruitment, are paralleled by a high stability of individual ligand-receptor complexes, we propose that half-lives of individual ligand-receptor complexes control signaling at the level of adaptor protein recruitment.