(A) Schematic diagram of chimeric
erythropoietin/MLV envelope protein (Epo-env). The diagram
demonstrates the location of erythropoietin inserted in the Fr-MLV
envelope protein (Fig. 1B). (B) Rescue of
Fr-MLV (Epo-Env) infection by purified RBD. 293 cells or 293-derived
cell lines expressing EpoR and/or mCAT1 were treated with a 1:10
dilution of viral supernatant containing Fr-MLV (Epo-env) in the
presence or absence of Fr-RBD (40 nM). On cells that expressed both
EpoR and mCAT1, Fr-MLV (Epo-env) infection as a function of RBD
concentration at 0, 4, 8, 20, and 40 nM RBD was examined. Cells were
assayed for acquired β-galactosidase expression 48 h
postinfection. Experiments were performed in triplicate, and standard
errors are indicated. (C) Competitive inhibition of
Fr-MLV (Epo-env) infection by erythropoietin. Either human 293-derived
cell lines expressing mCAT1 (EpoR−) or mCAT1 and EpoR (EpoR+) were
exposed to Fr-MLV (Epo-env) in the presence of RBD (40 nM) and
increasing concentrations of erythropoietin (0–20 units). Infection
was measured by staining for β-galactosidase expression 48 h
later.