Figure 1.

Tyr-479 in the EPO-R is sufficient to promote EPO-dependent proliferation and differentiation. (A) Proliferation of parental BaF3 cells and BaF3 cells expressing the EPO-R or the mutant EPO-Rs F8 or F7Y479 in different concentrations of EPO. The number of cells was determined using a Coulter counter. The average cell numbers per milliliter in four independent determinations (±SD) are plotted against the concentration of EPO (units/ml) added. (B) Tyr-479 in the EPO-R is sufficient to support EPO-dependent differentiation of erythroid progenitors. Fetal liver cells harvested from day 12.5 EPO-R^−/− mice were infected with retroviruses designed to express either the wild-type EPO-R or the point mutant EPO-Rs F7Y479, F8, or Y479F. The number of benzidine-positive colonies (CFU-Es) formed in the presence of EPO and stem cell factor was counted after 2 days. The results plotted represent the mean (±SD) of three independent experiments. All values are expressed per 10⁴ nucleated fetal liver cells. The morphology of CFU-E colonies formed in the presence of EPO and stem cell factor in cultures infected with retroviruses expressing the wild-type EPO-R (wt) or the mutant EPO-R F7Y479 (F7-Y479) is the same.