FIGURE 6.
ErbB4 preferentially serves as the activator kinase when paired with ErbB2. A, Western blots of whole cell lysates from S2R+ cells transiently transfected with wild-type and kinase-deficient (KD) forms of ErbB1 (B1), ErbB2 (B2), and ErbB4 (B4) and treated with EGF or NRG1β (+) or left untreated (−). Antibodies against phospho-Tyr1068 (pY1068; ErbB1), phospho-Tyr1221/1222 (pY1221/2; ErbB2), phospho-Tyr1284 (pY1284; ErbB4), and the ErbB1 (B1-Cyto), ErbB2 (B2-Cyto), and ErbB4 (BCyto) cytoplasmic regions were used as indicated. B, schematic representation of phosphorylation patterns observed in A. Phosphorylation is indicated by black dots. C, phylogenetic tree of human ErbB proteins based on sequence conservation generated by the maximum likelihood method (51). The scale bar indicates probability of amino acid change as an indicator of divergence time.