Two Modes of β-Receptor Recognition Are Mediated by Distinct Epitopes on Mouse and Human Interleukin-3

The cytokine interleukin-3 (IL-3) is a critical regulator of inflammation and immune responses in mammals. IL-3 exerts its effects on target cells via receptors comprising an IL-3-specific alpha-subunit and common beta-subunit (beta c; shared with IL-5 and granulocyte-macrophage colony-stimulating factor) or a beta-subunit that specifically binds IL-3 (beta(IL-3); present in mice but not humans). We recently identified two splice variants of the alpha-subunit of the IL-3 receptor (IL-3R alpha) that are relevant to hematopoietic progenitor cell differentiation or proliferation: the full length ("SP1" isoform) and a novel isoform (denoted "SP2") lacking the N-terminal Ig-like domain. Although our studies demonstrated that each mouse IL-3 (mIL-3) R alpha isoform can direct mIL-3 binding to two distinct sites on the beta(IL-3) subunit, it has remained unclear which residues in mIL-3 itself are critical to the two modes of beta(IL-3) recognition and whether the human IL-3R alpha SP1 and SP2 orthologs similarly instruct human IL-3 binding to two distinct sites on the human beta c subunit. Herein, we describe the identification of residues clustering around the highly conserved A-helix residue, Glu(23), in the mIL-3 A- and C-helices as critical for receptor binding and growth stimulation via the beta(IL-3) and mIL-3R alpha SP2 subunits, whereas an overlapping cluster was required for binding and activation of beta(IL-3) in the presence of mIL-3R alpha SP1. Similarly, our studies of human IL-3 indicate that two different modes of beta c binding are utilized in the presence of the hIL-3R alpha SP1 or SP2 isoforms, suggesting a possible conserved mechanism by which the relative orientations of receptor subunits are modulated to achieve distinct signaling outcomes.