Effect of tunicamycin treatment on ligand binding to the erythropoietin receptor: conversion from two classes of binding sites to a single class.

Scatchard analyses of erythropoietin (EPO) binding to its receptor (EPO-R) have shown that some erythroid cells display a biphasic nature of the ligand-saturation curve, indicating the presence of two classes of binding sites with different affinities. The biochemical basis accounting for this observation is unknown. We found that the culture of a human erythroleukemia cell line with tunicamycin, an inhibitor of N-glycosylation, converted the biphasic Scatchard plot to a single phase with high-affinity sites. Scatchard plots of baby hamster kidney (BHK) cells that had been engineered to express cloned mouse EPO-R were also biphasic and the plots of cells cultured with tunicamycin became a single phase with high-affinity sites. Mouse EPO-R is glycosylated at one asparagine residue in the extracellular region. The mutant EPO-R, in which asparagine residue responsible for N-glycosylation was replaced with glutamine residue, was expressed on BHK cells. Unexpectedly, mutant EPO-R was similar in ligand binding to wild-type EPO-R. BHK cells that expressed mutant EPO-R showed biphasic Scatchard plots that were converted to single-phase plots with only high-affinity sites by tunicamycin treatment. These results indicate that the N-linked sugar of EPO-R is not involved in the manifestation of two classes of binding sites, and that there is a yet unidentified glycoprotein crucial for the ligand-saturation characteristics of EPO-R.