Mapping of a Region within the N Terminus of Jak1 Involved in Cytokine Receptor Interaction

Janus kinase 1 (Jak1) is a cytoplasmic tyrosine kinase that noncovalently associates with a variety of cytokine receptors. Here we show that the in vitro translated N-terminal domains of Jak1 are sufficient for binding to a biotinylated peptide comprising the membrane-proximal 73 amino acids of gp130, the signal-transducing receptor chain of interleukin-6-type cytokines. By the fold recognition approach amino acid residues 36–112 of Jak1 were predicted to adopt a β-grasp fold, and a structural model was built using ubiquitin as a template. Substitution of Tyr107 to alanine, a residue conserved among Jaks and involved in hydrophobic core interactions of the proposed β-grasp domain, abrogated binding of full-length Jak1 to gp130 in COS-7 transfectants. By further mutagenesis we identified the loop 4 region of the Jak1 β-grasp domain as essential for gp130 association and gp130-mediated signal transduction. In Jak1-deficient U4C cells reconstituted with the loop 4 Jak1 mutants L80A/Y81A and Δ(Tyr81–Ser84), the interferon-γ, interferon-α, and interleukin-6 responses were similarly impaired. Thus, loop 4 of the β-grasp domain plays a role in the association of Jak1 with both class I and II cytokine receptors. Taken together the structural model and the mutagenesis data provide further insight into the interaction of Janus kinases with cytokine receptors.