A protein that takes part in the formation of active interleukin-1 receptor complex. It binds specifically to interleukin-1 and the interleukin-1 RECEPTOR TYPE I 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.