The receptor for the human granulocyte-macrophage colony-stimulating factor (GM-CSF) (GM-R) is a heterodimeric complex consisting of two subunits, GM-R alpha and GM-R beta. Structural analyses have shown a number of highly conserved amino acid motifs present in both GM-R alpha and GM-R beta. 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-R alpha by examining the effects of specific motif mutations on ligand binding and surface expression. Transient expression of these mutant GM-R alpha subunits in COS cells shows that these extracellular motis are essential for ligand binding. Alterations of the cytoplasmic region of GM-R alpha do not alter GM-CSF binding or the reconstitution of high-affinity receptors when coexpressed with GM-R beta. Permeabilization and immunostaining of cells transfected with mutant GM-R alpha 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-R alpha s showed that extracellular domain mutants accumulated in the cytoplasm and were not efficiently transported to the cell surface.
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