CSK binds with and phosphorylates c-Jun in vitro. A, the interaction between the two test proteins, as GAL4-CSK (pBIND-csk) and VP16-c-Jun (pACT-c-jun) fusion constructs (lane 4), resulted in an increase in luciferase expression compared to the negative controls (lanes 1, 2, and 3). JNK1 (pBIND-jnk1; lane 5) was used as a positive control. Data are represented as means ± S.D. of 3 independent experiments. Significant differences were evaluated using the Student’s t test (*, p < 0.01). B, CSK phosphorylates c-Jun in the in vitro kinase assay in the presence of 32P as visualized by autoradiography. C, CSK phosphorylates c-Jun at tyrosine sites as determined by an in vitro kinase assay with detection by Western blot.