Tyrosine kinase of the non-receptor type. ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Mn(2+) was used in the in vitro kinase assay but Mg(2+) is likely to be the in vivo cofactor. Wholly intracellular, possibly membrane associated. Possesses two phosphotransferase domains. The second one probably contains the catalytic domain, while the presence of slight differences suggest a different role for domain 1. The protein kinase 1 domain is also called the pseudokinase domain and has a regulatory role through the transactivation of other JAK kinases associated with signaling receptors. The protein kinase 2 domain is the catalytically active domain. Belongs to the protein kinase superfamily. Tyr protein kinase family. JAK subfamily.