Increased cell surface expression and enhanced folding in the endoplasmic reticulum of a mutant erythropoietin receptor.

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Abstract
In both transfected and normal hematopoietic cells, the majority of newly made erythropoietin receptor (EPO-R) subunits are retained in the endoplasmic reticulum (ER), destined for degradation. Only a small fraction exit the ER and are competent to bind EPO, suggesting that the EPO-R folds inefficiently. The EPO-R contains a 5-amino acid motif, WSXWS, in the extracellular domain that is conserved among members of the cytokine receptor family. We describe a mutant EPO-R with a change in the middle residue of this motif, A234E, that is transported from the ER more efficiently than the wild-type (wt) receptor and is expressed in elevated numbers at the cell surface. This mutant polypeptide is processed more efficiently in the ER than its wt counterpart, suggesting that it folds better than the wt EPO-R. Inefficient folding and processing of the wt EPO-R in the ER may be one mechanism for controlling the number of plasma membrane receptors.