Tyk2, a member of the Janus Kinase (JAK) family of protein tyrosine kinases, is required for interferon-α/β binding and signaling in higher vertebrates. Currently, little is known about the role of the different JAKs in signaling responses to interferon (IFN) in lower vertebrates including fish. In this paper we report the identification and characterization of Atlantic salmon (Salmo salar) Tyk2. Four cDNA sequences, two containing an open reading frame encoding full-length Tyk protein and two with an up-stream in frame stop codon, were identified. The deduced amino acid sequences of the salmon full-length Tyk2 proteins showed highest identity with Tyk2 from other species and their transcripts were ubiquitously expressed. Like in mammals the presented data suggests that salmon Tyk2 is auto-phosporylated when ectopically expressed in cells. In our experiments, full-length salmon Tyk2 overexpressed in CHSE-cells phosphorylated itself, while both a kinase-deficient mutant and the truncated Tyk2 (Tyk-short) were inactive. Interestingly, the overexpression of full length Tyk2 was shown to up-regulate the transcript levels of the IFN induced gene Mx, thus indicating the involvement of salmon Tyk2 in the salmon IFN I pathway.

Suppressor of cytokine signaling (SOCS) proteins are crucially involved in the control of inflammatory responses through their impact on various signaling pathways including the JAK/STAT pathway. Although all SOCS protein family members are identified in teleost fish, their functional properties in non-mammalian vertebrates have not been extensively studied. To gain further insight into SOCS functions in bony fish, we have identified and characterized the Atlantic salmon (Salmo salar) SOCS1, SOCS2 and CISH genes. These genes exhibited sequence conservation with their mammalian counterparts and they were ubiquitously expressed. SOCS1 in mammalian species has been recognized as a key negative regulator of interferon (IFN) signaling and recent data for the two model fish Tetraodon (Tetraodon nigroviridis) and zebrafish (Danio rerio) suggest that these functions are conserved from teleost to mammals. In agreement with this we here demonstrate a strong negative regulatory activity of salmon SOCS1 on type I and type II IFN signaling, while SOCS2a and b and CISH only moderately affected IFN responses. SOCS1 also inhibited IFNγ-induced nuclear localization of STAT1 and a direct interaction between SOCS1 and STAT1 and between SOCS1 and the Tyk2 kinase was found. Using SOCS1 mutants lacking either the KIR domain or the ESS, SH2 and SOCS box domains showed that all domains affected the ability of SOCS1 to inhibit IFN-mediated signaling. These results are the first to demonstrate that SOCS1 is a potent inhibitor of IFN-mediated JAK-STAT signaling in teleost fish.