Effects of IL-1 receptor accessory protein on IL-1 binding.

Interleukin-1 (IL-1) is an important pro-inflammatory cytokine that exerts its diverse biological functions by binding to a receptor complex consisting of two transmembrane proteins, type I IL-1 receptor (IL-1RI) and IL-1 receptor accessory protein (IL-1RAcP). Both receptor chains are indispensable for IL-1 signaling, but only IL-1RI is able to bind the cytokine. The effects of IL-1RAcP on IL-1 binding are unclear. This study shows that although expression of IL-1RAcP does not alter the equilibrium dissociation constant of IL-1, it has an effect on the non-equilibrium binding modus, most likely due to changes in on/off rates. This defines an additional function of IL-1RAcP: it stabilizes the active IL-1 receptor complex.