Tyrosine kinase of the non-receptor type. Appears to be required in early development for specific cell migrations (epiboly), expression of homeobox protein goosecoid and formation of anterior structures. 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. Present in the unfertilized egg through to the blastula stage where it is distributed uniformly. Levels drop rapidly at four hours development, remain very low until 10 hours, then gradually increase from 12 hours with a rapid increase at 48 hours. At 48 hours it is concentrated in the region of the gill arches. Also present in the adult. 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.