HUMAN INTERLEUKIN-3 (IL-3) INDUCES DISULFIDE-LINKED IL-3 RECEPTOR ALPHA-CHAIN AND BETA-CHAIN HETERODIMERIZATION, WHICH IS REQUIRED FOR RECEPTOR ACTIVATION BUT NOT HIGH-AFFINITY BINDING

The human interleukin-3 receptor (IL-3R) is a heterodimer that compris es an IL-3-specific alpha chain (IL-3R alpha) and a common beta chain (beta(c)) that is shared with the receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF) and IL-5. These receptors belong t o the cytokine receptor superfamily, but they are structurally and fun ctionally more related to each other and thus make up a distinct subfa mily. Although activation of the normal receptor occurs only in the pr esence of ligand, the underlying mechanisms are not known. We show her e that human IL-3 induces heterodimerization of IL-3R alpha and beta(c ) and that disulfide linkage of these chains is involved in receptor a ctivation but not high-affinity binding. Monoclonal antibodies (MAb) t o IL-3R alpha and beta(c) were developed which immunoprecipitated, in the absence of IL-3, the respective chains from cells labelled with I- 125 On the cell surface. However, in the presence of IL-3, each MAb im munoprecipitated both IL-3R alpha and beta(c). IL-3-induced receptor d imers were disulfide and nondisulfide linked and were dependent on IL- 3 interacting with both IL-3R alpha add beta(c). In the presence of IL -3 and under nonreducing conditions, MAb to either IL-3R alpha or beta (c) immunoprecipitated complexes with apparent molecular weights of 21 5,000 and 245,000 and IL-3R alpha and beta(c) monomers. Preincubation with iodoacetamide prevented the formation of the two high-molecular-w eight complexes without affecting noncovalent dimer formation or high -affinity IL-3 binding. Two-dimensional gel electrophoresis and Wester n blotting (immunoblotting) demonstrated the presence of both IL-3R al pha and beta(c) in the disulfide-linked complexes. IL-3 could also be coimmunoprecipitated with anti-IL-3R alpha or anti-beta(c) MAb, but it was not covalently attached to the receptor. Following IL-3 stimulati on, only the disulfide-linked heterodimers exhibited reactivity with a ntiphosphotyrosine antibodies, with beta(c) but not IL-3R alpha being the phosphorylated species. A model of IL-3R activation is proposed wh ich may be also applicable to the related GM-CSF and IL-5 receptors.