Low affinity binding of interleukin-1β and intracellular signaling via NF-κB identify Fit-1 as a distant member of the interleukin-1 receptor family

The fit-1 gene gives rise to two different mRNA isoforms, which code for soluble (Fit-1S) and membrane-bound (Fit-1M) proteins related to the type I interleukin (IL)-1 receptor. To investigate IL-1 binding, we have synthesized and purified histidine-tagged polypeptides corresponding to Fit-1S and the extracellular domain of the type I IL-1 receptor using a vaccinia expression system. Fit-1S is shown to interact with IL-1β, but not with IL-1α. However, Fit-1S binds IL-1β only with low affinity in contrast to the IL-1 receptor, suggesting that IL-1β is not a physiological ligand of Fit-1S. Moreover, expression of the membrane-bound protein Fit-1M in transiently transfected Jurkat cells did not result in activation of the transcription factor NF-κB following IL-1β treatment. However, a chimeric protein consisting of the extracellular domain of the type I IL-1 receptor and of the transmembrane and intracellular regions of Fit-1M stimulated NF-κB-dependent transcription as efficiently as the full-length type I IL-1 receptor. These data indicate that Fit-1M is a signaling molecule belonging to the IL-1 receptor family.