Tyr-28 is the major site of phosphorylation on p73 upon Hck co-expression. (A, B) Cos1 cells transfected with indicated combinations of p73 and Hck plasmids were analyzed by western blotting for pTyr. The same blot was reprobed for p73 and Hck. (C) Src tyrosine kinase phosphorylates p73α on Tyr-28 residue. Cos1 cells transfected with p73α, Y28F-p73α or c-Src and KD-Src were subjected to western blotting for pTyr, p73 and Src.