Roles of the N and C terminal domains of the interleukin-3 receptor α chain in receptor function

Abstract

The interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor, and IL-5 receptor α chains are each composed of three extracellular domains, a transmembrane domain and a short intracellular region. Domains 2 and 3 constitute the cytokine receptor module (CRM), typical of the cytokine receptor superfamily; however, the function of the N-terminal domain is not known. We have investigated the functions of the N-terminal and C-terminal domains of the IL-3 receptor (IL-3R) α chain. We find that cells transfected with the receptor β chain (hβc) and a truncated IL-3Rα that is devoid of the intracellular region fail to proliferate or to activate STATS in response to human IL-3, despite binding the IL-3 with affinity indistiguishable from that of full-length receptor. In addition, IL-3-induced phosphorylation of hβc was not detected. Thus, the IL-3Rα intracellular region does not contribute detectably to stabilization of the receptor/ligand complex, but is essential for signal propagation. In contrast, a truncated IL-3Rα with the N-terminal domain deleted interacts functionally with the β chain; mouse cells transfected with these receptor chains proliferate in response to human IL-3 and STAT5 transcription factor is activated. High- and low-affinity binding sites are retained, although the affinity for IL-3 is decreased 15- fold, indicating a significant role for the N-terminal domain in IL-3 binding.