A protein that takes part in the formation of active interleukin-1 receptor complex. It binds specifically to $INTERLEUKIN-$1 and the TYPE-I INTERLEUKIN-1 RECEPTOR at the cell surface to form a heterotrimeric complex that brings its cytoplasmic domain into contact with the cytoplasm domain of the TYPE-I INTERLEUKIN-1 RECEPTOR. Activation of intracellular signal transduction pathways from the receptor is believed to be driven by this form of cytoplasmic interaction.