Fig. 2. Q646C, H647C, and A648C mutants exhibit increased in vitro kinase activity. Equal amounts of protein lysates (1000 µg) from PA317 cells that stably express wild-type ErbB4 or the ErbB4 mutants (Q646C, H647C, and A648C) were immunoprecipitated with an anti-ErbB4 rabbit polyclonal antibody. Lysates from PA317 cells that express the LXSN vector served as the negative control. Kinase reactions were performed on the immunoprecipitates in the presence of [-32P]ATP. The products were resolved by SDS-PAGE. The gel was dried overnight and exposed to X-ray film for 20 h to visualize the products of the kinase reactions.