Binding of LynA and LynB to various mutant EpoRs in COS7 cells. (A) Schematic representation of EpoR mutants used in binding studies in COS7 cells. The transmembrane domain (TM) is represented with a hatched box, whereas the conserved sequence motifs, Box1 and Box2, are represented with solid boxes. The positions of eight tyrosine residues in the cytoplasmic region are indicated with vertical lines. Numbers in parentheses indicate the amino acid number at the carboxy terminus of the EpoR or at the sites of truncation. EC, extracellular region; IC, intracellular region. (B and C) The wild-type (W) or mutant EpoRs (H, S, or PB) were coexpressed with both LynA and LynB in COS7 cells, as indicated. Cells were lysed and immunoprecipitated with anti-Lyn or anti-EpoR, as indicated. Immunoprecipitates were subjected to immunoblotting with indicated antibodies. (D and E) LynA (A) or LynB (B) was coexpressed with the wild-type EpoR in COS7 cells, as indicated, and examined as described above.