We have examined the e ects of human papilloma virus (HPV) E6 proteins on interferon (IFN) signaling. Here we show that expression of the`malignant' HPV-18 E6 in human HT1080 cells results in inhibition of Jak-STAT activation in response to IFN-a but not IFN-g. This inhibitory e ect is not shared by the`benign' HPV-11 E6. The DNA-binding and transactivation capacities of the transcription factor ISGF3 are diminished in cells expressing HPV-18 E6 after IFN-a treatment as a result of decreased tyrosine phosphorylation of Tyk2, STAT2 and STAT1. However, HPV-18 E6 does not a ect the induction of tyrosine phosphorylation and DNA-binding of STAT1 by IFN-g. In addition, HPV E6 proteins physically interact with Tyk2. This interaction takes place preferably with HPV-18 E6 and to a lesser extent with HPV-11 E6. The E6/Tyk2 interaction requires the JH 6 -JH 7 domains of Tyk2, which are important for Tyk2 binding to the cytoplasmic portion of IFN-a receptor 1 (IFNAR1). These ®ndings demonstrate an inhibitory role of HPV-18 E6 in the IFN-a-induced Jak-STAT pathway, which may be explained, at least in part, by the ability of E6 to interact with and impair Tyk2 activation.

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Tyk2 belongs to the Janus kinase (JAK) family of receptor associated tyrosine kinases, characterized by a large N-terminal region, a kinase-like domain and a tyrosine kinase domain. It was previously shown that Tyk2 contributes to interferon-␣ (IFN-␣) signaling not only catalytically, but also as an essential intracellular component of the receptor complex, being required for high affinity binding of IFN-␣. For this function the tyrosine kinase domain was found to be dispensable. Here, it is shown that mutant cells lacking Tyk2 have significantly reduced IFN-␣ receptor 1 (IFNAR1) protein level, whereas the mRNA level is unaltered. Expression of the N-terminal region of Tyk2 in these cells reconstituted wild-type IFNAR1 level, but did not restore the binding activity of the receptor. Studies of mutant Tyk2 forms deleted at the N terminus indicated that the integrity of the N-terminal region is required to sustain IFNAR1. These studies also showed that the N-terminal region does not directly modulate the basal autophosphorylation activity of Tyk2, but it is required for efficient in vitro IFNAR1 phosphorylation and for rendering the enzyme activatable by IFN-␣. Overall, these results indicate that distinct Tyk2 domains provide different functions to the receptor complex: the N-terminal region sustains IFNAR1 level, whereas the kinase-like domain provides a function toward high affinity ligand binding.The intracellular protein-tyrosine kinases of the Janus kinase (JAK) family play an essential role in cytokine signaling: they interact with receptor components and undergo tyrosine phosphorylation and enzymatic activation upon ligand binding. Their activation is the first step of a cascade of intracellular phosphorylation events ultimately leading to the transcriptional activation of target genes (1, 2). Signaling through the receptor for type I interferons (IFN-␣ and -␤) requires two members of the JAK family, Tyk2 and JAK1 (3, 4). Both enzymes are associated with the receptor, which is composed of IFN-␣ receptor (IFNAR) 1 (5) and IFNAR2-2, the longer splice variant of the IFNAR2 gene (6, 7). Tyk2 was shown to interact with the membrane-proximal region of IFNAR1 (8, 9), and JAK1 with IFNAR2-2 (10, 11). The stoichiometry of the activated receptor͞JAK complex is not known and might differ for the different type I IFNs. Ligand-induced dimerization or oligomerization of the receptor components leads to asymmetric trans-phosphorylation and consequent catalytic activation of Tyk2 and JAK1 (4,12,13).Tyk2 shares with the other members of the JAK family a unique structural framework (see Fig. 1A) comprising seven conserved JAK homology (JH) regions (14). The most Cterminal one (JH1) is a tyrosine kinase (TK) domain, which is flanked by the JH2 or kinase-like (KL) domain of unknown function. The remaining five blocks of homology (JH3 to JH7) extend toward the N terminus of the protein and exhibit variable degrees of conservation among the family members, the most conserved being JH4 with a central core of 18 identical...