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 γ (IFNγ) 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 IFNγ receptor is partially understood and the relationship between the α chain that binds the ligand and the β chain that is required for signal transduction is undefined. In a cell line which expresses only the human α chain, we show that these cells did not activate Jak kinases or STAT proteins with human IFNγ, even though Jak1 co-immunoprecipitated with the α chain. In cells unexposed to IFNγ, Jak1 preferentially associated with the α chain, while Jak2 associated with the β chain. There was evidence for Jak1 kinase activity in untreated cells. For Jak2, kinase activity was IFNγ-dependent. Although the α chain was tyrosine-phosphorylated in response to ligand, we found no evidence for tyrosine phosphorylation of the β chain. These data are consistent with a model of the IFNγ receptor in which Jak1 associates with the α chain, whereas Jak2 associates with the β chain. IFNγ 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 STAT1α resulting in its activation by tyrosine phosphorylation. Interferon γ (IFNγ) 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 IFNγ receptor is partially understood and the relationship between the α chain that binds the ligand and the β chain that is required for signal transduction is undefined. In a cell line which expresses only the human α chain, we show that these cells did not activate Jak kinases or STAT proteins with human IFNγ, even though Jak1 co-immunoprecipitated with the α chain. In cells unexposed to IFNγ, Jak1 preferentially associated with the α chain, while Jak2 associated with the β chain. There was evidence for Jak1 kinase activity in untreated cells. For Jak2, kinase activity was IFNγ-dependent. Although the α chain was tyrosine-phosphorylated in response to ligand, we found no evidence for tyrosine phosphorylation of the β chain. These data are consistent with a model of the IFNγ receptor in which Jak1 associates with the α chain, whereas Jak2 associates with the β chain. IFNγ 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 STAT1α resulting in its activation by tyrosine phosphorylation. Cytokines such as interferons (IFN),1 1The abbreviations used are: IFNγinterferon γPAGEpolyacrylamide gel electrophoresisILinterleukin. interleukins, and growth factors transduce signals across the cell membrane that result in the expression of early response genes. This process occurs through the activation of transcription factors, STAT proteins, (signal transducers and activators of transcription) and the tyk2/Jak family of protein tyrosine kinases(1Darnell Jr., J.E. Kerr I.M. Stark G.R. Science. 1994; 264: 1415-1421Crossref PubMed Scopus (5157) Google Scholar, 2Larner A.C. Finbloom D.S. Biochim. Biophys. Acta. 1995; (in press)PubMed Google Scholar). Whereas the STAT proteins possess SH2 and SH3 domains(3Fu X.-Y. Cell. 1992; 70: 323-335Abstract Full Text PDF PubMed Scopus (314) Google Scholar), the Tyk/Jak kinases have two putative catalytic domains (of which only the carboxyl one is active) but lack SH2 or SH3 domains(4Wilks A.F. Harpur A.G. Kurban R.R. Ralph S.J. Zurcher G. Ziemiecki A. Mol. Cell. Biol. 1991; 11: 2057-2065Crossref PubMed Scopus (439) Google Scholar). These kinases have been shown to be associated with several cytokine receptors that require dimerization by the ligand to initiate signal transduction. One such ligand is interferon γ (IFNγ), a homodimeric molecule, that bivalently binds to its receptor and activates the Jak/STAT pathway by clustering its receptor. The IFNγ receptor consists of a high affinity ligand binding chain (α) (Ka = 1010M-1) that alone is incapable of transducing a signal across the membrane (5Aguet M. Dembic Z. Merlin G. Cell. 1988; 55: 273-280Abstract Full Text PDF PubMed Scopus (405) Google Scholar). Another transmembrane protein (accessory factor) is required for signal transduction, and through the use of somatic cell hybrid cell lines, its gene has been determined to be located on chromosome 21(6Jung V. Rashidbaigi A. Jones C. Tischfield J.A. Shows T.B. Pestka S. Proc. Natl. Acad. Sci. U. S. A. 1987; 84: 6934-6938Crossref PubMed Scopus (83) Google Scholar). The accessory factor/β chain interacts with the ligand binding component (α chain) in a species specific manner(7Hemmi S. Merlin G. Aguet M. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 2737-2741Crossref PubMed Scopus (37) Google Scholar, 8Gibbs V.C. Williams S.T. Gray P.W. Schreiber R.D. Pennica D. Rice G. Goeddel D.V. Mol. Cell. Biol. 1991; 11: 5860-5866Crossref PubMed Scopus (52) Google Scholar). The gene for the accessory factor (β chain) has been cloned and when co-expressed with the ligand binding α chain reconstitutes a functional receptor unit(9Soh J. Donnelly R.J. Kotenko S. Mariano T.M. Cook J.R. Wang N. Emanuel S. Schwartz B. Miki T. Pestka S. Cell. 1994; 76: 793-802Abstract Full Text PDF PubMed Scopus (233) Google Scholar, 10Hemmi S. Bohni R. Stark G. Di Marco F. Aguet M. Cell. 1994; 76: 803-810Abstract Full Text PDF PubMed Scopus (176) Google Scholar). The cDNA for the β chain encodes a transmembrane protein of 310 amino acids with six putative glycosylation sites and an intracytoplasmic domain of 66 amino acids. interferon γ polyacrylamide gel electrophoresis interleukin. Incubation of cells with IFNγ stimulates an immediate tyrosine phosphorylation of the α chain(11Greenlund A.C. Farrar M.A. Viviano B.L. Schreiber R.D. EMBO J. 1994; 13: 1591-1600Crossref PubMed Scopus (383) Google Scholar, 12Igarashi K. Garotta G. Ozmen L. Ziemiecki A. Wilks A.F. Harpur A.G. Larner A.C. Finbloom D.S. J. Biol. Chem. 1994; 269: 14333-14336Abstract Full Text PDF PubMed Google Scholar). Although several tyrosines within the cytoplasmic domain are phosphorylated, the phosphorylation of tyrosine 440 is necessary for initiation of the signaling cascade as determined by site-directed mutagenesis(13Farrar M.A. Schreiber R.D. Annu. Rev. Immunol. 1993; 11: 571-611Crossref PubMed Scopus (1278) Google Scholar). Tyrosine phosphorylation of the α chain occurs concomitantly with tyrosine phosphorylation of both Jak1 and Jak2(12Igarashi K. Garotta G. Ozmen L. Ziemiecki A. Wilks A.F. Harpur A.G. Larner A.C. Finbloom D.S. J. Biol. Chem. 1994; 269: 14333-14336Abstract Full Text PDF PubMed Google Scholar, 14Muller M. Brinco J. Laxton C. Guschin D. Ziemiecki A. Silvennoinen O. Harpur A.G. Barbieri G. Witthuhn B.A. Schindler C. Pelligrini S. Wilks A.F. Ihle J.N. Stark G.R. Kerr I.M. Nature. 1993; 366: 129-135Crossref PubMed Scopus (658) Google Scholar, 15Watling D. Guschin D. Muller M. Silvennoinen O. Witthuhn B.A. Quelle F.W. Rogers N.C. Schindler C. Ihle J.N. Stark G.R. Kerr I.M. Nature. 1993; 366: 166-170Crossref PubMed Scopus (431) Google Scholar, 16Silvennoinen O. Ihle J.N. Schlessinger J. Levy D.E. Nature. 1993; 366: 583-585Crossref PubMed Scopus (298) Google Scholar). Both kinases form a multimolecular complex with the IFNγ receptor immediately after ligand binding(12Igarashi K. Garotta G. Ozmen L. Ziemiecki A. Wilks A.F. Harpur A.G. Larner A.C. Finbloom D.S. J. Biol. Chem. 1994; 269: 14333-14336Abstract Full Text PDF PubMed Google Scholar). STAT1α has been shown to bind to a peptide of the IFNγ receptor containing the phosphorylated tyrosine 440(11Greenlund A.C. Farrar M.A. Viviano B.L. Schreiber R.D. EMBO J. 1994; 13: 1591-1600Crossref PubMed Scopus (383) Google Scholar). The contribution of each chain of the IFNγ receptor (IFNγR) to the formation of this complex and the relationship of kinase activity to ligand binding still needs to be defined. We have carried out a series of experiments designed to assess the relationship between the α and β chains of the receptor and the Jak kinases in both cell lines, primary monocytes, and in somatic cell hybrids that express either the human α chain or both the human α and β chains of the receptor. HeLa cells were maintained in stationary flasks or adapted for growth in spinner flasks as described previously(12Igarashi K. Garotta G. Ozmen L. Ziemiecki A. Wilks A.F. Harpur A.G. Larner A.C. Finbloom D.S. J. Biol. Chem. 1994; 269: 14333-14336Abstract Full Text PDF PubMed Google Scholar). Human peripheral blood monocytes were obtained from volunteers by leukapheresis. The monocytes were purified from the mononuclear cells by Ficoll-Hypaque (LSM, Organon Teknika, Durham, NC) sedimentation followed by countercurrent centrifugal elutriation, were >95% pure, and used without further culturing. The T41 cell line is a mouse L929 fibroblast cell line stably transfected with the human IFNγ receptor α chain. The 3-1-12 cell line is a mouse-man somatic cell hybrid (WavR4d-F9-4a) retaining only chromosome 21 (17Tan Y.H. Tischfield J. Ruddle F.H. J. Exp. Med. 1973; 137: 317-330Crossref PubMed Scopus (218) Google Scholar) and has been similarly transfected with the human IFNγ receptor α chain. Chromosome 21 also encodes for the IFNα receptor, and the responsiveness of the 3-1-12 cell line to IFNα can be used to assess the ability of the cells to retain chromosome 21(18Slate D. Shulman L. Lawrence J. Revel M. Ruddle F. J. Virol. 1978; 25: 319-325Crossref PubMed Google Scholar). The ability of the T41 and 3-1-12 cell lines to respond to IFNα or IFNγ was monitored by the enhancement of major histocompatibility class I antigen expression. Cells were treated with IFN for 24 h and then incubated with antibodies specific for major histocompatibility class I antigens followed by incubation with fluorescein isothiocyanate-conjugated secondary antibody. The cells were then analyzed by flow cytometric analysis on a FACScan. Recombinant human IFNγ (Genentech, Inc., S. San Francisco, CA) was radiolabeled using Bolton-Hunter reagent (Amersham Corp.) to high specific activity as described(19Finbloom D.S. Hoover D.L. Wahl L.M. J. Immunol. Google Scholar). of were incubated at for h with and a of IFNγ was used in to for IFNγ was from IFNγ by a as described D.S. Hoover D.L. Wahl L.M. J. Immunol. Google Scholar). cell were from the specific activity of the radiolabeled with IFNγ, cells were with and with cell and proteins were determined by as described Finbloom D.S. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: PubMed Scopus Google Scholar). of protein were incubated with the of of the of the gene in binding R. Proc. Natl. Acad. Sci. U. S. A. 1991; PubMed Scopus Google Scholar). The was then to a polyacrylamide gel in to from to were for and for at This was with for the amino acids containing of and and the cells in incubation for The cells were then and the to as described with IFNγ, cells were in in a of The was then with a of protein for h at of the protein from the by the was incubated with either Inc., or K. M. Larner A.C. Finbloom D.S. Mol. Cell. Biol. 1993; 13: PubMed Scopus Google Scholar) for h at antibodies were One is a purified α chain of the G. L. M. Dembic Z. D. J. Biol. Chem. Full Text PDF PubMed Google Scholar), and the is a a peptide of the cytoplasmic domain of the accessory factor/β chain acids This was affinity purified on an was then for h at to the The protein were then in and in For designed to assess cells were in the as that was for and the were in containing The were then analyzed using followed by to as K. Garotta G. Ozmen L. Ziemiecki A. Wilks A.F. Harpur A.G. Larner A.C. Finbloom D.S. J. Biol. Chem. 1994; 269: 14333-14336Abstract Full Text PDF PubMed Google Scholar). untreated or treated with IFNγ, were and the to as described The were then in kinase this was either and of when was or when was The was incubated for at and the several with to the of The were then to analysis followed by either or with antibodies and The ability of the somatic cell hybrid 3-1-12 cell line to retain chromosome 21 was using For the T41 cells was no in human major histocompatibility class I expression as by in untreated cells for IFNα This was consistent with the that the L929 cells are in and not respond to human IFNα cells respond to In the 3-1-12 cell line to human IFNα by from to These data that chromosome 21 was within this cell In to that the transfected chain was both the T41 and the 3-1-12 cell line were for the expression of by the binding of The α chain alone is to an high affinity binding Both cell lines of binding the T41 cells whereas the 3-1-12 cells The at binding for both cell lines was which was consistent with a high affinity binding The affinity and of cell for these cell lines was to for D.S. Hoover D.L. Wahl L.M. J. Immunol. Google Scholar). The ability of the two cell lines to signal was by the formation of the binding In T41 cells only IFNγ the formation of In both and human IFNγ in of the complex in 3-1-12 cells. the of the signal was for human IFNγ with IFNγ in the 3-1-12 cell line was the result of expression of the β component with the α component of the both the Jak1 and Jak2 tyrosine kinases have been shown to be in the signal transduction pathway for IFNγ, we the α component alone was for the tyrosine phosphorylation of these tyrosine Incubation of the 3-1-12 cell line with IFNγ in the tyrosine phosphorylation of both Jak1 and Jak2 In in cells only the α component of the receptor, no tyrosine phosphorylation of Jak1 or Jak2 was This that the α chain alone in of of binding ligand with high affinity not ligand tyrosine phosphorylation of the Jak these cells were for the tyrosine phosphorylation of STAT1α in response to IFNγ, the results were to that for the Jak kinases in 3-1-12 cells a functional receptor was human tyrosine phosphorylation of the STAT1α T41 cells that were treated with IFNγ tyrosine phosphorylation of Jak1 and Jak2 not and STAT1α There was no STAT1α tyrosine phosphorylation in response to human IFNγ in the T41 cell line of protein Jak2, and were each of Jak kinases and STAT1α in response to IFNγ in 3-1-12 and T41 cells. Cells were to either or human IFNγ for at and then in the were incubated with and analyzed by The the membrane with and then using The the membrane for Jak1 protein and using the were incubated with and analyzed as described in A. the were to and analyzed as described in A. untreated cells. human We have shown that the Jak1 kinase associated with the and that ligand binding then in its tyrosine K. Garotta G. Ozmen L. Ziemiecki A. Wilks A.F. Harpur A.G. Larner A.C. Finbloom D.S. J. Biol. Chem. 1994; 269: 14333-14336Abstract Full Text PDF PubMed Google Scholar). We the α chain or the β chain was required for this Both the T41 and 3-1-12 cell lines were and to with chain The were with antibodies Both the T41 and the 3-1-12 cells Jak1 associated with the The protein was in both and in T41 cells only the α chain is these data that Jak1 preferentially associated with the α a was and with Jak2, no was in the T41 or the 3-1-12 cells even when these cells were treated with IFNγ not Jak2 preferentially associates with the β chain, the expression of this gene for to the in the 3-1-12 cell The lack of in the T41 cells that Jak2 not with the α chain. Jak1 associated with the IFNγ receptor in a untreated we this kinase catalytic activity as by an in kinase with and were for kinase activity using either or followed by by In cells untreated with IFNγ was a of Jak1 kinase activity as by of Jak1 There was no activity after with IFNγ In was no Jak2 kinase activity in cells untreated with with IFNγ was Jak2 kinase activity the Jak1 was to of the was on tyrosine not of the Jak kinases were in each the was using in with analysis by results were obtained Jak1 tyrosine kinase activity with no further kinase activity after IFNγ the Jak2 and tyrosine kinase activity only after cells been with no in the only the tyrosine phosphorylation is in response to IFNγ and kinase of Jak1 and Jak2 in HeLa cells using Cells were treated as described in the to were to an in kinase in the or of in kinase in the of The the with The the membrane with the and using in kinase in the of The and are as described in A. The of tyrosine-phosphorylated Jak1 and Jak2 in the tyrosine phosphorylation in cells. untreated cells. the chain was tyrosine-phosphorylated ligand we determined the kinase that was for this was with the IFNγ receptor. Cells were treated with or without IFNγ and then in to The were then with antibodies the chain. The was then analyzed by an in kinase In untreated cells and was tyrosine phosphorylation of the chain in the of For the tyrosine phosphorylation of the and Jak kinases was in response to IFNγ the from untreated cells were with to the in kinase the ability to the was This that the tyrosine kinase that was for tyrosine phosphorylation of the chain was with the in a Jak1 was shown to with the α chain and be in untreated cells by an in kinase Jak1 was a for the tyrosine phosphorylation of the α chain of receptor. the chain were for the of was a of Jak1 in cells with and and even though protein was the In to assess the of the chain, we antibodies a peptide of the β chain of amino acids of the intracytoplasmic The of this was on human peripheral blood The chain and associated chains have been in these cells by both and followed by D.S. Wahl L.M. J. Biol. Chem. 1991; Full Text PDF PubMed Google Scholar). monocytes were and the to with the chain was a at that was not with antibodies or antibodies the chain was in cells in not The of the chain as a protein is consistent with the amino of 310 amino acids with of such as the the antibodies the β chain the α chain, but antibodies the α chain only the α chain and associated Jak kinases and The ability of the to the α chain of the receptor was by with with and the membrane with these the chain with the β chain in cells untreated or treated with chain is not tyrosine-phosphorylated in response to HeLa cells were either to IFNγ and or untreated and and in The were then incubated with either and chain and or chain and The were then analyzed by with antibodies In to or not the chain the α chain tyrosine phosphorylation in response to the binding of IFNγ to the we treated cells with IFNγ and then in chain was that the α chain of the receptor was tyrosine-phosphorylated in response to IFNγ In a the chain at the tyrosine phosphorylated This has been by the with kinase antibodies not In when the chain antibodies were was no tyrosine phosphorylation of the β chain was that the was the receptor complex the tyrosine-phosphorylated α chain at and the Jak kinases at were in the In to Jak2 preferentially associated with the chain, HeLa cells were either untreated or treated with IFNγ and then in with were by and then with Jak2 associated with the β chain in a IFNγ signaling was a consistent in the of Jak2 that be co-immunoprecipitated with the β chain, that recruitment of Jak2 signal transduction or that the affinity of Jak2 for the antibodies Jak1 were to no of Jak1 was not either that was no between Jak1 and the chain or that the that have activation by IFNγ is tyrosine phosphorylation of In to STAT1α be within the we cells to IFNγ at and then in The were incubated with antibodies and the with In cells treated with IFNγ were two tyrosine-phosphorylated that to the chain and the kinases both of which can also be shown to be with the Interferon γ activation of STAT1α both Jak1 and Jak2 as as tyrosine phosphorylation of the α chain of the IFNγ receptor. Although has been that component of the receptor is necessary for signal transduction to only has the gene for this accessory factor (β chain) been cloned and shown to for a transmembrane protein of 310 amino acids with a intracytoplasmic J. Donnelly R.J. Kotenko S. Mariano T.M. Cook J.R. Wang N. Emanuel S. Schwartz B. Miki T. Pestka S. Cell. 1994; 76: 793-802Abstract Full Text PDF PubMed Scopus (233) Google Scholar, 10Hemmi S. Bohni R. Stark G. Di Marco F. Aguet M. Cell. 1994; 76: 803-810Abstract Full Text PDF PubMed Scopus (176) Google Scholar). is the of this protein in the formation of the ligand In this we show that both the α and β are required to activate STAT proteins. In cells only the α chain, we show that Jak1 associates with this component but not tyrosine-phosphorylated after with IFNγ in of high affinity binding of the ligand to the α chain. in cells both the α and β component are Jak1 and Jak2 tyrosine-phosphorylated in response to IFNγ Jak1 with the α component and no with the receptor after IFNγ In Jak2 is with the β but also is either to the receptor after ligand binding or its affinity for the β chain after ligand The data from the somatic cell hybrid cells that the of Jak2 with the receptor β chain is an for activation of STAT1α by tyrosine phosphorylation. Although the α chain is tyrosine-phosphorylated in we have been to tyrosine phosphorylation of the β chain. is that this component not with the STAT proteins as has been to for the α chain of the IFNγ receptor A.C. Farrar M.A. Viviano B.L. Schreiber R.D. EMBO J. 1994; 13: 1591-1600Crossref PubMed Scopus (383) Google Scholar) and the ligand binding component of the J. Schindler U. M. Science. 1994; PubMed Scopus Google Scholar). We have also shown that in the α and β can be co-immunoprecipitated in untreated and is that dimerization of at least two signal transduction. In untreated cells Jak1 is with the α chain by in kinase is as by the chain is and to in kinase the α chain with the Jak kinases are tyrosine-phosphorylated in cells unexposed to This that the kinase for the tyrosine phosphorylation of the α chain in is with the receptor and be of the in results in of the activity and of We have shown that in untreated cells no Jak2 is with the chain K. Garotta G. Ozmen L. Ziemiecki A. Wilks A.F. Harpur A.G. Larner A.C. Finbloom D.S. J. Biol. Chem. 1994; 269: 14333-14336Abstract Full Text PDF PubMed Google Scholar). This is to that with the factor receptor β chain or of the receptor. Both Jak1 and Jak2 are with the β of the and factor receptor, and of the receptors in with to in tyrosine phosphorylation of the β component is after a N. T. S. Witthuhn B.A. Quelle F.W. Silvennoinen O. Barbieri G. Pelligrini S. Ihle J.N. Science. 1994; PubMed Scopus Google Scholar, C. J. J. Schindler C. Ziemiecki A. Harpur A.G. Wilks A.F. K. T. T. Barbieri G. Pelligrini S. M. F. Science. 1994; PubMed Scopus Google Scholar). The of activation of the Jak kinase in cells unexposed to ligand the cells In the factor was but in kinase activity for Jak1 but not Jak2 in untreated cells N. T. S. Witthuhn B.A. Quelle F.W. Silvennoinen O. Barbieri G. Pelligrini S. Ihle J.N. Science. 1994; PubMed Scopus Google Scholar). In cells and was kinase activity of Jak1 in untreated cells that was after T. A. Silvennoinen O. Witthuhn B.A. Ihle J.N. T. Science. 1994; PubMed Scopus Google Scholar, J.A. M. M. M. M. Witthuhn B.A. Silvennoinen O. Ihle J.N. Science. 1994; PubMed Scopus Google Scholar). In untreated was in tyrosine of Jak1 that did not after J.A. M. M. M. M. Witthuhn B.A. Silvennoinen O. Ihle J.N. Science. 1994; PubMed Scopus Google Scholar). of in kinase activity for Jak1 in untreated cells that or not be after ligand For receptors Jak1 and Jak2 or that Jak1 to in kinase activity in cells untreated with These data are consistent with the that the after clustering of receptors by ligand Jak1 to Jak2 activation and in is to the receptor or binds to the K. Garotta G. Ozmen L. Ziemiecki A. Wilks A.F. Harpur A.G. Larner A.C. Finbloom D.S. J. Biol. Chem. 1994; 269: 14333-14336Abstract Full Text PDF PubMed Google Scholar). is at this that STAT1α can be shown to of the of the kinases have been shown to with membrane domains of the cytoplasmic of receptors that are in cytokine R. C. S. EMBO J. 1991; PubMed Scopus Google Scholar, M. M. M. K. M. T. T. Proc. Natl. Acad. Sci. U. S. A. 1991; PubMed Scopus Google Scholar). is that is binding of kinases to these Jak1 associates with the α chain and Jak2 with the β chain of the IFNγ receptor. For the receptor, Jak1 associates with the membrane of the whereas associated with the of the γ a in have cell T. A. Silvennoinen O. Witthuhn B.A. Ihle J.N. T. Science. 1994; PubMed Scopus Google Scholar, J.A. M. M. M. M. Witthuhn B.A. Silvennoinen O. Ihle J.N. Science. 1994; PubMed Scopus Google Scholar). In cell lines Jak Jak2 did not with the receptor. The to receptor be a for of signal transduction through the The of Jak2 with the β chain of the to be a for IFNγ This was also for the T. A. Silvennoinen O. Witthuhn B.A. Ihle J.N. T. Science. 1994; PubMed Scopus Google Scholar, J.A. M. M. M. M. Witthuhn B.A. Silvennoinen O. Ihle J.N. Science. 1994; PubMed Scopus Google Scholar). and the data in this that Jak2 is required for the tyrosine phosphorylation of whereas Jak1 is with the tyrosine phosphorylation of the chain. in receptor that Tyk/Jak one of the kinases with the receptor the tyrosine phosphorylation of one or chains of the receptor, whereas the kinase be for the activation of STAT proteins.