JAK2, a third member of the JAK family of protein tyrosine kinases.

We have isolated cDNA clones encoding a third, widely expressed, member of the JAK family of protein tyrosine kinases (PTKs). The anticipated amino acid sequence of JAK2 predicts the presence of two kinase-related domains, a feature characteristic of this family of PTKs. The structural similarity of JAK2 to the other members of this family extends towards their N-termini, beyond the two kinase-related domains, and reveals five further domains of substantial amino acid similarity. The C-terminal portion of one of these domains, the JH4 domain, bears an intriguing, albeit tenuous, similarity to the core element of the SH2 domain, whereas the remaining JAK homology domains do not appear to be a feature of other known proteins.