(A) Diagram of the gp55 constructs. The EcoRI andFok I restriction sites were used to divide the coding sequence into three segments as described by Chung et al.22 P, sequences derived from gp55-P; A, sequences derived from gp55-A. gp55-APP was abbreviated as gp55-P because it is coded by SFFVAP-L, which induces polycythemic effects indistinguishable from those of SFFV-P.14 Construct gp55-AAP contains the entirety of gp55-A except for the membrane-spanning domain and one unique residue (Ser 376) in the exoplasmic domain derived from gp55-P. (B) Induction of erythroid colony formation from day-12.5 fetal liver CFU-E progenitors. Fetal liver cells were infected with retroviruses encoding gp55-P, gp55-A, gp55-AAP, or gp55-APA or with virus encoding β-galactosidase (control). Formation of CFU-E colonies was scored in the absence of Epo after 72 hours by staining with benzidine. Data represent the mean of the indicated number of assays (N) ± 1 standard deviation. Treatment with 1 U/mL Epo induced 957 ± 99 CFU-E colonies/50,000 fetal liver cells (N = 6).
