The two proteins of the erythropoietin receptor are structurally similar

The structure of the erythropoietin receptor has been identified in this laboratory as two proteins of 100 and 85 kDa by cross-linking 125I-erythropoietin (125I-EP) to the surface of erythroid cells purified from the spleens of mice infected with the anemia strain of Friend virus. This study investigates the relatedness of these two proteins and the possibility that these proteins are subunits of the functional receptor for EP. Other workers have claimed that the 100- and 85-kDa proteins are bridged by disulfide bonds. This most likely is an artifact due to the insolubility of the cross-linked membrane. Proteolytic digestion by the method of Cleveland (Cleveland, D. W., Fischer, S. G., Kirschner, M. W., and Laemmli, U. K. (1977) J. Biol. Chem. 252, 1102-1106) resulted in identical fragments from the 100- and 85-kDa proteins, which strongly suggests that the primary amino acid sequence of these two proteins is similar if not identical. Increasing the number of protease inhibitors during the preparation of membranes and the binding and cross-linking steps increased the ratio of 100-kDa protein labeled compared to the 85-kDa protein. Together these results suggest that the 85-kDa protein is derived by proteolytic cleavage of the 100-kDa receptor for EP. It is not clear whether the 100-kDa protein can bind EP in the absence of the 85-kDa protein.