(A) Buffer (Cont), active GST-tagged- PDGFRβ (PDGFRβ) or EGFR (EGFR) were subjected to a cold kinase assay in the presence (+PKA-C) or absence (−PKA-C) of purified PKA-Cα. The samples were then processed for immunoblot analysis using an antibody that recognizes pan phosphotyrosine. * denotes the position of PKA-C (mw 38 kD). The upper band migrating near the 98 kD marker corresponds to the molecular weights of the active, tyrosine-phosphorylated, bacterially purified, GST-receptors. (B) Recombinant PKA-Cα was incubated with buffer only (Cont) or bacterially purified, GST-tagged PDGFRβ or PDGFR α under cold kinase assay conditions and samples were processed as described in (A).