The Jak Kinases Differentially Associate with the α and β (Accessory Factor) Chains of the Interferon γ Receptor to Form a Functional Receptor Unit Capable of Activating STAT Transcription Factors

Abstract

Interferon gamma (IFN gamma) induces the expression of early response genes by tyrosine phosphorylation of Jak kinases and transcription factors referred to as STAT proteins. The topology of the IFN gamma receptor is partially understood and the relationship between the alpha chain that binds the ligand and the beta chain that is required for signal transduction is undefined. In a cell line which expresses only the human alpha chain, we show that these cells did not activate Jak kinases or STAT proteins with human IFN gamma, even though Jak1 co-immunoprecipitated with the alpha chain. In cells unexposed to IFN gamma, Jak1 preferentially associated with the alpha chain, while Jak2 associated with the beta chain. There was evidence for Jak1 kinase activity in untreated cells. For Jak2, kinase activity was IFN gamma-dependent. Although the alpha chain was tyrosine-phosphorylated in response to ligand, we found no evidence for tyrosine phosphorylation of the beta chain. These data are consistent with a model of the IFN gamma receptor in which Jak1 associates with the alpha chain, whereas Jak2 associates with the beta chain. IFN gamma clusters at least two receptor units which results in the tyrosine phosphorylation of Jak1 and Jak2, the activation of Jak2 kinase activity, and the recruitment of STAT1 alpha resulting in its activation by tyrosine phosphorylation.

Interferonγ Activation of Raf-1 Is Jak1-dependent and p21ras-independent

“…Both the maximal number of receptors (approximately 1,400/cell) and the concentration at half-saturation (2 ϫ 10 Ϫ10 M) were identical and consistent with the absence of any alteration in the affinity of IFN␥ for the mutant cells. Since binding of IFN␥ measures essentially only the interaction between the ligand and the ␣ chain of the IFN␥ receptor, intact binding parameters may exist, yet there may be no ␤ chain present and thereby no signaling through the receptor (24). Moreover, the presence or absence of the ␤ chain has not been demonstrated in the E1C3 cells.…”

Two Distinct Domains Within the N-Terminal Region of Janus Kinase 1 Interact with Cytokine Receptors

“…Box 1 is defined as a proline-rich motif (4), whereas box 2 is characterized as a cluster of hydrophobic amino acids (2). Most single subunit cytokine receptors, such as erythropoietin, growth hormone, and prolactin, as well as some heterodimeric receptors, such as IFN-␥ and GM-CSF/IL-3 and -5 interact with Jak2 through the highly conserved box 1 motif (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24).…”

“…For example, Tyk2 interacts with a domain of IFN-␣R␣, the ␣-chain of the IFN-␣R (also designated as IFNAR1 or IFN-␣R1), which has only distant homology with the box 1 or box 2 motifs (25,26). Jak1 is activated by multichain receptors such as the IL-6 group of cytokines (IL-6, leukemia inhibitory factor, ciliary neurotrophic factor, oncostatin M), IFN-␣, IFN-␥, IL-10, and those cytokine receptors that belong to the IL-2 family (IL-2, IL-4, IL-7, IL-9, and IL-15) (19,20,(27)(28)(29)(30)(31)(32)(33). The Jak1 binding site on cytokine receptors has been explored in only a few cases.…”

Gamma Irradiation-reduced IFN-γ Expression, STAT1 Signals, and Cell-mediated Immunity

“…The receptor-associated Jak1 and Jak2 are phosphorylated, which then results in kinase activation. The cytoplasmic domain of the receptor is phosphorylated by the activated kinase, which recruits the signal transducer and activator of transcription (STAT) 1 (Greenlund et al, 1994(Greenlund et al, , 1995Sakatsume et al, 1995), as well as its activation by tyrosine phosphorylation, which leads to its dimerization. The dimerized STAT1 translocates to the nucleus, where it activates the transcription of IFN-γ-responsive genes, such as IRF-1 (interferon regulatory factor-1) (Harada et al, 1989).…”