The Erythropoietin Receptor: Biogenesis, Dimerization, and Intracellular

sential for the survival and proliferation of erythroid progenitor cells and their differentiation into erythro-cytes. Like most receptors for hematopoietic growth factors, the EPO-R (D'Andrea et al. 1989) is a type I transmembrane protein and member of the cytokine receptor superfamily, which includes the receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF), and interleukins IL-2,-3,-4,-5,-6, and-7. These receptors contain four conserved cysteines and a Trp-Ser-X-Trp-Ser motif in their extracellular domains (Cosman et al. 1990; Miyajima et al. 1992). The cyto-solic domains of these receptors are dissimilar, and none contains intrinsic enzymatic activity. Signaling through the EPO-R, like that of othe