The JAK-binding protein JAB inhibits Janus tyrosine kinase activity through binding in the activation loop

The Janus family of protein tyrosine kinases (JAKs) regulate cellular proce sses involved in cell growth, differentiation and transformation through th eir association with cytokine receptors, However, compared with other kinas es, little is known about cellular regulators of the JAKs, We have recently identified a JAK-binding protein (JAB) that inhibits JAK signaling in cell s. In the studies presented here we demonstrate that JAB specifically binds to the tyrosine residue (Y1007) in the activation loop of JAK2, whose phos phorylation is required for activation of kinase activity. Binding to the p hosphorylated activation loop requires the JAB SH2 domain and an additional N-terminal 12 amino acids (extended SH2 subdomain) containing two residues (Ile68 and Leu75) that are conserved in JAB-related proteins. An additiona l N-terminal 12-amino-acid region (kinase inhibitory region) of JAB also co ntributes to high-affinity binding to the JAK2 tyrosine kinase domain and i s required for inhibition of JAK2 signaling and kinase activity. Our studie s define a novel type of regulation of tyrosine kinases and might provide a basis for the design of specific tyrosine kinase inhibitors.