Three conserved motifs in the extracellular domain of the human granulocyte-macrophage colony-stimulating factor receptor subunit are essential for ligand binding and surface expression

Abstract

The receptor for the human granulocyte-macrophage colony-stimulating factor (GM-CSF) (GM-R) is a heterodimeric complex consisting of two subunits, GM-Rα and GM-Rβ. Structural analyses have shown a number of highly conserved amino acid motifs present in both GM-Rα and GM-Rβ. These motifs include QYFLY, CXW, XW, and WSXWS motifs in the extracellular domain; a conserved cysteine in the transmembrane domain; and the entire cytoplasmic domain, including the LXVLX box in the carboxy terminal region of the cytoplasmic domain. We have investigated the role of these motifs in GM-Rα by examining the effects of specific motif mutations on ligand binding and surface expression. Transient expression of these mutant GM-Rα subunits in COS cells shows that these extracellular motifs are essential for ligand binding. Alterations of the cytoplasmic region of GM-Rα do not alter GM-CSF binding or the reconstitution of high-affinity receptors when coexpressed with GM-Rβ. Permeabilization and immunostaining of cells transfected with mutant GM-Rα subunits yields data suggesting that each of the mutant subunits is present in the cytoplasm. Immunostaining of both intact and permeabilized COS cells transiently transfected with wild-type or mutant GM- Rαs showed that extracellular domain mutants accumulated in the cytoplasm and were not efficiently transported to the cell surface.