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Characterization of a Peptide Inhibitor of Janus Kinase 2 That Mimics Suppressor of Cytokine Signaling 1 Function¹

Department of Microbiology and Cell Science, University of Florida, Gainesville, FL 32611

Positive and negative regulation of cytokines such as IFN- are key to normal homeostatic function. Negative regulation of IFN- in cells occurs via proteins called suppressors of cytokine signaling (SOCS)1 and -3. SOCS-1 inhibits IFN- function by binding to the autophosphorylation site of the tyrosine kinase Janus kinase (JAK)2. We have developed a short 12-mer peptide, WLVFFVIFYFFR, that binds to the autophosphorylation site of JAK2, resulting in inhibition of its autophosphorylation as well as its phosphorylation of IFN- receptor subunit IFNGR-1. The JAK2 tyrosine kinase inhibitor peptide (Tkip) did not bind to or inhibit tyrosine autophosphorylation of vascular endothelial growth factor receptor or phosphorylation of a substrate peptide by the protooncogene tyrosine kinase c-src. Tkip also inhibited epidermal growth factor receptor autophosphorylation, consistent with the fact that epidermal growth factor receptor is regulated by SOCS-1 and SOCS-3, similar to JAK2. Although Tkip binds to unphosphorylated JAK2 autophosphorylation site peptide, it binds significantly better to tyrosine-1007 phosphorylated JAK2 autophosphorylation site peptide. SOCS-1 only recognizes the JAK2 site in its phosphorylated state. Thus, Tkip recognizes the JAK2 autophosphorylation site similar to SOCS-1, but not precisely the same way. Consistent with inhibition of JAK2, Tkip inhibited the ability of IFN- to induce an antiviral state as well as up-regulate MHC class I molecules on cells at a concentration of 10 µM. This is similar to the K_d of SOCS-3 for the erythropoietin receptor. These data represent a proof-of-concept demonstration of a peptide mimetic of SOCS-1 that regulates JAK2 tyrosine kinase function.

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