Comparison of activation loop conformation for Jak3, Lck, and IRTK. The 3 panels show ribbon representation diagrams for the activation loops of Jak3, IRTK,31 and Lck.³⁰  The Jak3 activation loop is phosphorylated on 2 adjacent tyrosines, Tyr980 and Tyr981. pTyr981 is in a similar location to phosphorylated tyrosines in the previously determined activated crystal structures of Lck³⁰  (PDB entry 3LCK) and insulin receptor tyrosine kinase31 (PDB entry 1IR3). Lck is phosphorylated on a single tyrosine, in the equivalent position to Tyr981; however, insulin receptor is phosphorylated on 3 tyrosines on the activation loop, 2 of which are the equivalent residues to Tyr980 and Tyr981 in Jak3. pTyr980 in Jak3 is in a similar conformation to that of pTyr1162 in IRTK. In Jak3, Arg870 of the C-helix coordinated pTyr981. Arg866 also extends toward pTyr981 but is poorly ordered; therefore, its side chain is not illustrated. Figures were prepared using the program SETOR.³⁵