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Science  12 Feb 1999:
Vol. 283, Issue 5404, pp. 901l
DOI: 10.1126/science.283.5404.901l

Erythropoietin (EPO) is a peptide hormone that regulates the growth of red blood cells. It acts by bridging the extracellular portions of two membrane-bound EPO receptors; the juxtaposition of the cytoplasmic portions leads to activation of an intracellular phosphorylation pathway. Livnah et al. (p. 987) describe the crystal structure of an unliganded dimer of the EPO receptor in which many of the interfacial contacts are those that would normally be used to bind EPO. In this dimer, the intracellular domains point away from each other. Could this inactive dimer exist on the surfaces of intact cells, exquisitely responsive to the low circulating concentrations of EPO? Remy et al. (p. 990) present evidence for these preformed dimers by measuring the separation of the intracellular domains with an assay for recovery of enzyme activity.

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