Mutational analysis of the alpha subunit of the human interleukin-3 receptor

The alpha subunit of the human interleukin-3 receptor (IL-3Rα) is a 70- kD glycoprotein member of the hematopoietin receptor superfamily. This protein associates with a β subunit common to the receptors for IL-5 and granulocyte-macrophage colony-stimulating factor (GM-CSF) to form a high- affinity receptor for IL-3. To identify regions of IL-3Rα critical for ligand binding and receptor function, cDNAs encoding mutant receptors were generated and expressed in COS cells along with the p subunit. Mutant receptors lacking almost the entire cytoplasmic domain of IL-3Rα [IL- 3Rα(CD)] or carrying a substitution of trp for leu in the membrane proximal leu-ser-x-trp-ser (LSXWS) box bound ¹²⁵I-IL-3 with nearly the same affinity as wild-type IL-3Rα. In contrast, a mutant lacking the entire 'LSXWS' motif failed to bind ¹²⁵I-IL-3 with high affinity despite showing surface expression. In addition, hybrid receptors composed of the first 104 amino acids (aa) of IL-3Rα joined to aa 118 through 400 of the α subunit of the GM-CSF receptor (GM-Rα) [IL-3Rα/GM-Rα] or the first 118 aa of GM-Rα joined to aa 104 through 378 of IL-3Rα [GM-Rα/IL-3Rα] failed to bind ¹²⁵I-IL-3 in the presence of the β subunit. A third hybrid receptor composed of the first 281 residues of IL-3Rα fused to residues 306 through 379 of GM-Rα [IL-3Rα/GM-Rα-DS] also failed to bind ¹²⁵I-IL-3 in the presence of the β subunit but, in contrast to the IL-3Rα/GM-Rα hybrid, demonstrated weak surface expression. Mutant receptors lacking the N-terminal 30 aa and the N-terminal 9 aa also did not bind ¹²⁵I-IL-3 with high affinity, although both were expressed on the cell surface. These data suggest that although the cytoplasmic domain and the leucine residue of the 'LSXWS' box are not critical for ligand binding or β-subunit association, the 'LSXWS' motif and amino-terminal sequences are important for these functions.