Signaling via the T Cell Antigen Receptor Induces Phosphorylation of Stat1 on Serine 727

The Stat1 transcription factor plays a pivotal role in both, the antiviral and antigrowth actions of interferons. Stat1 acquires the ability to bind DNA by becoming phosphorylated on Tyr701. However, to effectively stimulate gene transcription, it must also be phosphorylated on Ser727. We show that engagement of T cell antigen receptor (TCR)/CD3 complex in either Jurkat cells or peripheral blood lymphocytes stimulates phosphorylation of Ser727 but not Tyr701 of Stat1. This process does not require the expression of tyrosine kinases Lck and Zap-70. Interestingly, pretreatment of T cells with the Src kinase inhibitor PP1 completely abrogated CD3-mediated serine phosphorylation of Stat1, whereas inhibitors to MEK1 and phosphatidylinositol 3-kinase had no effect. Phosphorylation of Ser727 of Stat1 in T cells is not restricted to TCR/CD3 but also results when cells are stimulated via the costimulatory molecule CD28. The combination of CD3 and CD28 did not augment phosphorylation of Stat1 Ser727. Surprisingly, Stat1-mediated transcriptional activity in response to IFN-α was enhanced with CD3 stimulation, whereas CD3 alone had little effect. These findings suggest that Stat1 is a signaling molecule in TCR signaling and may play a role in T cell function. The Stat1 transcription factor plays a pivotal role in both, the antiviral and antigrowth actions of interferons. Stat1 acquires the ability to bind DNA by becoming phosphorylated on Tyr701. However, to effectively stimulate gene transcription, it must also be phosphorylated on Ser727. We show that engagement of T cell antigen receptor (TCR)/CD3 complex in either Jurkat cells or peripheral blood lymphocytes stimulates phosphorylation of Ser727 but not Tyr701 of Stat1. This process does not require the expression of tyrosine kinases Lck and Zap-70. Interestingly, pretreatment of T cells with the Src kinase inhibitor PP1 completely abrogated CD3-mediated serine phosphorylation of Stat1, whereas inhibitors to MEK1 and phosphatidylinositol 3-kinase had no effect. Phosphorylation of Ser727 of Stat1 in T cells is not restricted to TCR/CD3 but also results when cells are stimulated via the costimulatory molecule CD28. The combination of CD3 and CD28 did not augment phosphorylation of Stat1 Ser727. Surprisingly, Stat1-mediated transcriptional activity in response to IFN-α was enhanced with CD3 stimulation, whereas CD3 alone had little effect. These findings suggest that Stat1 is a signaling molecule in TCR signaling and may play a role in T cell function. T cell receptor signal transducers and activators of transcription peripheral blood lymphocyte extracellular signal-regulated kinase mitogen-activated protein kinase MAPK/Erk kinase polyacrylamide gel electrophoresis interferon protein-tyrosine kinase Engagement of the T cell receptor (TCR)/CD31 activates signaling pathways critical for T cell development and function (1.Weiss A. Littman D.R. Cell. 1994; 76: 263-274Abstract Full Text PDF PubMed Scopus (1981) Google Scholar, 2.van Leeuwen J.E.M. Samelson L. Curr. Opin. Immunol. 1999; 11: 242-248Crossref PubMed Scopus (217) Google Scholar). One of the earliest detectable events in this process is the rapid activation of several protein-tyrosine kinases (PTKs) including Fyn, Lck, and Zap-70 and protein-tyrosine phosphatase, CD45. Ultimately, transcription factors become activated and initiate gene expression critical for T cell activation and survival (1.Weiss A. Littman D.R. Cell. 1994; 76: 263-274Abstract Full Text PDF PubMed Scopus (1981) Google Scholar, 2.van Leeuwen J.E.M. Samelson L. Curr. Opin. Immunol. 1999; 11: 242-248Crossref PubMed Scopus (217) Google Scholar). Lck and Fyn are two critical cytoplasmic PTKs of the Src family involved in T cell maturation. Both PTKs can partially substitute for the activity of each other. Mice deficient in Lck show a profound block in T cell development at the early double positive CD4+/CD8+ stage (3.Molina T.J. Kishihara K. Siderovski D.P. van Ewijk W. Narendran A. Timms E. Wakeman A. Paige C.J. Hartmann K.-U. Veillette A. Davidson D. Mak T.W. Nature. 1992; 357: 161-164Crossref PubMed Scopus (906) Google Scholar), whereas mice deficient in Fyn display minor aberration in T cell differentiation, yet their single positive T cells show reduced proliferation in response to mitogens (4.Stein P.L. Lee H.-M. Rich S. Soriano P. Cell. 1992; 70: 741-750Abstract Full Text PDF PubMed Scopus (508) Google Scholar). Interestingly, mice lacking both Lck and Fyn display a complete block in T cell maturation at two stages: CD4−/CD8− to CD4+/CD8+ and CD4+/CD8+to CD4+ and CD8+ (5.van Oers N.S. Lowin-Kropf B. Finlay D. Connolly K. Weiss A. Immunity. 1996; 5: 429-436Abstract Full Text Full Text PDF PubMed Scopus (255) Google Scholar). Another key player in T cell maturation is Zap-70, a member of the Syk/Zap-70 family of cytoplasmic PTKs (6.Chan A.C. van Oers N.S. Tran A. Turka L. Law C.L. Ryan J.C. Clark E.A. Weiss A. J. Immunol. 1994; 152: 4758-4766PubMed Google Scholar). Disruption of the Zap-70 gene in mice leads to a block in T cell differentiation similar to those detected in Lck-deficient mice (7.Negishi I. Motoyama N. Nakayama K.-I. Nakayama K. Senju S. Hatakeyama S. Zhang Q. Chan A.C. Loh D.Y. Nature. 1995; 376: 435-438Crossref PubMed Scopus (484) Google Scholar). A subgroup of patients with severe combined immunodeficiency lack expression of Zap-70, and their T cells are refractory to T cell stimulation (8.Arpaia E. Shahar M. Dadi H. Cohen A. Roifman C.M. Cell. 1994; 76: 947-958Abstract Full Text PDF PubMed Scopus (490) Google Scholar, 9.Elder M.E. Lin D. Clever J. Chan A.C. Hope T.J. Weiss A. Parslow T.G. Science. 1994; 264: 1596-1599Crossref PubMed Scopus (446) Google Scholar). Signal transducers and activators of transcription (Stat) become activated in response to cytokines, growth factors, and osmotic stress (10.Darnell Jr., J.E. Science. 1997; 277: 1630-1635Crossref PubMed Scopus (3448) Google Scholar, 11.Gatsios P. Terstegen L. Schliess F. Haussinger D. Kerr I.M. Heinrich P.C. Graeve L. J. Biol. Chem. 1998; 273: 22962-22968Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar). Upon stimulation, Janus family PTKs (Jaks) become activated and tyrosine phosphorylate Stats. This allows Stats to dimerize through Src homology 2 phosphotyrosine and translocate to the nucleus where they are able to bind DNA and regulate gene transcription (10.Darnell Jr., J.E. Science. 1997; 277: 1630-1635Crossref PubMed Scopus (3448) Google Scholar). Ser727 is conserved in the transactivation domains of Stat1, Stat3, and Stat4. This site contains a mitogen-activated protein kinase (MAPK) recognition motif, PX n(S/T)P (n = 1 or 2) that must be phosphorylated for Stats to gain maximal transcriptional activity (12.Wen Z. Zhong Z. Darnell Jr., J.E. Cell. 1995; 82: 241-250Abstract Full Text PDF PubMed Scopus (1770) Google Scholar). Cytokine-induced tyrosine phosphorylation of Stats can occur independently of serine phosphorylation (13.Zhu X. Wen Z. Xu L.Z. Darnell Jr., J.E. Mol. Cell. Biol. 1997; 17: 6618-6623Crossref PubMed Scopus (141) Google Scholar). The identity of the Stat serine kinase has remained elusive. Both ERK2 and p38 MAPK have been shown to be activated in response to IFNs, and dominant negative forms of these proteins can inhibit transcriptional reporter activity of Stat1 (14.David M. Petricoin III, E.F. Benjamin C. Pine R. Weber M.J. Larner A.C. Science. 1995; 269: 1721-1723Crossref PubMed Scopus (529) Google Scholar,15.Goh K.C. Haque S.J. Williams B.R.G. EMBO J. 1999; 18: 5601-5608Crossref PubMed Scopus (329) Google Scholar). Stats also play a pivotal role in T cell function (16.O'Shea J.J. Immunity. 1997; 7: 1-11Abstract Full Text Full Text PDF PubMed Scopus (347) Google Scholar). T cells in mice deficient in Stat4 and Stat6 are impaired in the development of Th1 and Th2 responses, respectively (17.Thierfelder W.E. van Deursen J.M. Yamamoto K. Tripp R.A. Sarawar S.R. Carson R.T. Sangster M.Y. Vignali D.A.A. Doherty P.C. Grosveld G.C. Ihle J.N. Nature. 1996; 382: 171-174Crossref PubMed Scopus (964) Google Scholar, 18.Kaplan M.H. Sun Y.-L. Hoey T. Grusby M.J. Nature. 1996; 32: 174-177Crossref Scopus (1068) Google Scholar). Stimulation of T lymphoblasts via their TCR receptor has been shown to induce serine but not tyrosine phosphorylation of Stat3 (19.Ng J. Cantrell D. J. Biol. Chem. 1997; 272: 24542-24549Abstract Full Text Full Text PDF PubMed Scopus (130) Google Scholar). This process is mediated by MEK/ERK1/2 signaling pathways (19.Ng J. Cantrell D. J. Biol. Chem. 1997; 272: 24542-24549Abstract Full Text Full Text PDF PubMed Scopus (130) Google Scholar). In contrast, a different study showed that stimulation of an allogen-specific CD4+ human T cell line with anti-CD3 antibody or bacterial superantigen triggered both serine and tyrosine phosphorylation of Stat3 with similar kinetics (20.Gerwien J. Nielsen M. Labuda T. Nissen M.H. Svejgaard A. Geisler C. Ropke C. Odum N. J. Immunol. 1999; 163: 1742-1745PubMed Google Scholar). Furthermore, CD3 stimulation of murine T cells triggers tyrosine phosphorylation of Stat5, a process mediated by the Src kinase Lck, and this stimulation induces transient association of Stat5 with the TCR (21.Welte T. Leitenberg D. Dittel B.N. al-Ramadi B.K. Xie B. Chin Y.E. Janeway Jr., C.A. Bothwell A.L.M. Bottomly K. Fu X.-Y. Science. 1999; 283: 222-225Crossref PubMed Scopus (117) Google Scholar). Here we show that signaling through the TCR/CD3 and CD28 costimulation induces serine but not tyrosine phosphorylation of Stat1 in Jurkat cells, a human T cell leukemia line. This observation can be seen in nontransformed naive, primary human T cells. Phosphorylation of Ser727 of Stat1 can be effectively blocked by the Src kinase inhibitor PP1 in response to TCR/CD3 stimulation and occurs independently of Lck and Zap-70 tyrosine kinases. Moreover, CD3 stimulation augments Stat1-dependent gene transcription in response to IFN-α. These results suggest that Stat1 is another component in T cell signal transduction pathways. The human leukemic T cell line Jurkat, subclone E6, mutant lines derived from this clone, Lck-deficient (J.Cam.1.6), (a gift from A. Weiss, UCSF, San Francisco, CA) and Zap-70-deficient (P116) (a gift from R. Abraham, Duke University, Durham, NC), were cultured in RPMI 1640 medium supplemented with 10% heat-inactivated fetal calf serum (Life Technologies, Inc.), 2 mml-glutamine, penicillin (10 units/ml) and streptomycin (10 μg/ml) at 37 °C and 5% CO2. Peripheral blood lymphocytes (PBLs) were isolated from whole blood by density gradient centrifugation using Ficoll-Hypaque (Amersham Pharmacia Biotech) followed by removal of monocytes by plastic adherence at 37 °C. Antiserum that specifically recognizes the phosphorylated form of serine 727 of Stat1 was used at 1:10000 dilution (a generous gift of D. Frank, Dana-Farber Cancer Institute, Boston, MA). Phosphospecific antibodies against tyrosine 701 of Stat1 and active ERK1/2 were purchased from New England Biolabs Inc. (Beverly, MA) and used at 1:2000 dilution. Monoclonal antibodies to Stat1 and pan-ERK were obtained from Transduction Laboratories (Lexington, KY) and used at 1:1000 dilution. Anti-CD3 monoclonal antibody OKT3 was from Ortho Biotech, Inc. (Raritan, NJ). Anti-CD28 was purchased from Pharmingen (Palo Alto, CA). The inhibitors H7, PD98059, U0126, PP1, SB203580, wortmannin, and LY294002 were purchased from Calbiochem (San Diego, CA). For TCR/CD3 stimulation, cells were left untreated or incubated with 5 μg/ml anti-CD3 OKT3 antibody. For CD28 stimulation, a final dilution of 1:200 dilution of CD28 antibody was used. Cells were incubated for 10 min at 37 °C followed by addition of 10 μg/ml of anti-mouse IgG (Sigma) at 37 °C for the indicated times. In some experiments, cells were treated for 30 min at 37 °C prior to CD3 stimulation with MEK1-specific inhibitor PD98059 (50 μm), p38 MAPK inhibitor SB203580 (10 μm), MEK1 inhibitor U0126 (10 μm), phosphatidylinositol 3-kinase inhibitors LY294002 (10 μm), wortmannin (100 nm), serine/threonine kinase inhibitor H7 (50 μm), or Src kinase inhibitor PPI (10 μm). Cells were then washed once with cold phosphate-buffered saline and lysed in a buffer containing 1% Triton X-100, 50 mm Tris, pH 7.5, 150 mm NaCl, 2 mm EDTA, 1 mm sodium orthovanadate, 1 mm phenylmethylsulfonyl fluoride, 10 mmβ-glycerophosphate. Lysates were vortexed and incubated on ice for 10 min, and insoluble material was cleared by centrifugation at 12,000 rpm for 10 min at 4 °C. Proteins (30 μg of whole cell extract) were separated on 8% SDS-PAGE gels and transferred to polyvinylidene difluoride membrane (Millipore, Bedford, MA). Membranes were immunoblotted with the indicated antibodies using concentrations and conditions recommended by manufacturers. Immunoblots were developed using horseradish peroxidase-conjugated secondary antibodies (Zymed Laboratories Inc., San Francisco, CA) and ECL (Amersham Pharmacia Biotech). Jurkat cells were transiently transfected using Superfect Reagent (Qiagen Inc., Valencia, CA) with 2 μg of 3 × reporter for activity and for μg of was also at 37 cells were left untreated or stimulated with anti-CD3 OKT3 antibody IFN-α or the combination of anti-CD3 IFN-α for were and activity was with a using the reporter to the The of Stat transcription factors in TCR signaling was it was that Stat3 and Stat5 become activated in response to TCR/CD3 stimulation (19.Ng J. Cantrell D. J. Biol. Chem. 1997; 272: 24542-24549Abstract Full Text Full Text PDF PubMed Scopus (130) Google Scholar, T. Leitenberg D. Dittel B.N. al-Ramadi B.K. Xie B. Chin Y.E. Janeway Jr., C.A. Bothwell A.L.M. Bottomly K. Fu X.-Y. Science. 1999; 283: 222-225Crossref PubMed Scopus (117) Google Scholar). We to TCR/CD3 stimulation may also to activation of Stat1 in T cells this transcription factor is critical in the of antiviral and in mice deficient in Stat1 a in T J.E. R. Cell. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar, S.J. J.M. L. D. Jr., Cell. 1998; Full Text Full Text PDF PubMed Scopus Google Scholar). Jurkat cells were stimulated by with an anti-CD3 monoclonal antibody TCR cell were and proteins were separated by The serine phosphorylation of Stat1 was by using an antibody that specifically recognizes Stat1 phosphorylated on Ser727. shown in serine phosphorylated Stat1 was detected early min CD3 stimulation of Jurkat cells, and this form of Stat1 to be activated to 1 the for Stat1 that of the protein are in the 1 the of Jurkat cells a to study TCR signaling these cells are and in the of growth We to to a T we from to Stat1 was serine phosphorylated in response to CD3 were incubated for with and whole cell were for phosphorylation of Ser727 of Stat1 of phosphorylated Stat1 on similar kinetics seen with Jurkat cells was early min, and the signal was to 1 serine and tyrosine phosphorylation of Stat1. We to the of activation of serine phosphorylation of Stat1 CD3 stimulation was similar to that of were stimulated with either CD3 antibody or IFN-α for 30 phosphorylation of Stat1 in whole cell from these cells was by shown in 1 CD3 stimulation phosphorylation of Ser727 of Stat1 with similar to those detected with IFN-α have shown that Stat3 phosphorylated on Ser727 TCR However, are tyrosine phosphorylation of Stat3 occurs in T cells (19.Ng J. Cantrell D. J. Biol. Chem. 1997; 272: 24542-24549Abstract Full Text Full Text PDF PubMed Scopus (130) Google Scholar, J. Nielsen M. Labuda T. Nissen M.H. Svejgaard A. Geisler C. Ropke C. Odum N. J. Immunol. 1999; 163: 1742-1745PubMed Google Scholar). In of this we to CD3 stimulation leads to tyrosine phosphorylation of Stat1 in Jurkat cells by using antibodies that the tyrosine phosphorylated form of these shown in 1 CD3-mediated T cell activation to induce tyrosine phosphorylation of Stat1 a of 1 The lack of Stat1 tyrosine phosphorylation was not to a in Stat activation in these cells of Jurkat cells with IFN-α tyrosine phosphorylation of Stat1 1 results were obtained when were This that activation of the TCR complex stimulates serine phosphorylation of Stat1. CD28 is a molecule on T cells that a signal to T cell activation when to T. J.M. C. S. A. PubMed Scopus Google Scholar). to gene expression and T cells from an or E.A. Immunol. 1998; 18: PubMed Google Scholar). CD28 plays a pivotal role in T cell activation and is with activation through the we to the that stimulation through CD28 may also in the activation of Stat1. Stimulation of Jurkat cells with antibody the phosphorylation of Ser727 Stat1 The of phosphorylated Stat1 similar to those with CD3 in the of serine phosphorylation of Stat1 was by the combination of stimulation 2 The kinetics of activation of Stat1 Ser727 phosphorylation were the with CD3 stimulation not This that the signaling that leads to Stat1 activation in T cells through TCR/CD3 or CD28 receptor stimulation may be show that Stat1 activation also occur in primary T cells, were activated by by CD28 stimulation, or by the combination of both 2 that Jurkat cells, these activators also stimulate phosphorylation of Stat1 Ser727 in peripheral T cells. signaling via both CD3 and CD28 did not phosphorylation of Stat1. to the CD3 CD28 or the combination of CD3 and CD28 stimulation does not tyrosine phosphorylation of Stat1 in Jurkat or in primary T cells phosphorylation of Stat3 was in response to CD28 costimulation with similar kinetics those seen with Stat1 not via the TCR require the expression and activation of Lck and Zap-70 tyrosine are critical in T cell differentiation and function (3.Molina T.J. Kishihara K. Siderovski D.P. van Ewijk W. Narendran A. Timms E. Wakeman A. Paige C.J. Hartmann K.-U. Veillette A. Davidson D. Mak T.W. Nature. 1992; 357: 161-164Crossref PubMed Scopus (906) Google Scholar, 9.Elder M.E. Lin D. Clever J. Chan A.C. Hope T.J. Weiss A. Parslow T.G. Science. 1994; 264: 1596-1599Crossref PubMed Scopus (446) Google Scholar). Lck has been shown to be for activation of Stat5 (21.Welte T. Leitenberg D. Dittel B.N. al-Ramadi B.K. Xie B. Chin Y.E. Janeway Jr., C.A. Bothwell A.L.M. Bottomly K. Fu X.-Y. Science. 1999; 283: 222-225Crossref PubMed Scopus (117) Google Scholar). these signaling were for TCR serine phosphorylation of Stat1, we of Jurkat cell lack expression of either Lck or Zap-70 Weiss A. Cell. 1992; 70: Full Text PDF PubMed Scopus Google Scholar, Zhang W. R. Samelson R. Mol. Cell. Biol. 1998; 18: PubMed Scopus Google Scholar). of either or Zap-70-deficient Jurkat cells with anti-CD3 antibody stimulated phosphorylation of Stat1 Ser727 with similar kinetics to those in cells These findings that phosphorylation of Stat1 Ser727 in response to CD3 stimulation occurs independently of Lck and Zap-70 kinases. The that Lck Zap-70 are for TCR stimulated serine phosphorylation of Stat1 was We to kinases be for this of either Jurkat cells or with inhibitors PD98059 and U0126 did not block the of serine phosphorylation of Stat1 but effectively the activation of ERK1/2 MAPK The H7 serine/threonine kinase inhibitor used at 50 a to effectively block serine phosphorylation of Stats S. J. J. 1997; PubMed Scopus Google Scholar), did not phosphorylation of Stat1 Ser727. The p38 inhibitor SB203580 had no in activation of Stat1 Ser727 in Jurkat cells and phosphatidylinositol 3 is in T cell we also the role of this kinase in Stat1 serine both LY294002 and wortmannin, inhibitors of did not Stat1 serine phosphorylation in Jurkat cells 4 were detected when primary T cells were or when an inhibitor of kinase was used not Surprisingly, the Src kinase inhibitor PP1, completely blocked serine phosphorylation of Stat1 in both Jurkat and primary T cells This was also by of ERK1/2 MAPK activation in both the of CD3-mediated phosphorylation of Ser727 of Stat1 in T cells, we to CD3 stimulation of T cells have an on Stat1-dependent transcriptional by IFN-α. this Jurkat cells were transiently transfected with an and cell were of stimulation with anti-CD3 or the combination of anti-CD3 IFN-α of cells with anti-CD3 a 2 in reporter activity with untreated cells. IFN-α of cells stimulated the reporter using of or in cells incubated with Interestingly, CD3 stimulation of these cells in the of IFN-α enhanced the of transcriptional For with IFN-α units/ml) alone with IFN-α These findings that IFN-α and CD3 to augment gene In this we that of the antigen receptor complex in T cells results in the phosphorylation of serine 727 but not tyrosine 701 of Stat1. This activation in cells, where engagement of the cell receptor induces both serine and tyrosine phosphorylation of Stat1 L. M. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). This that the transcription factor Stat1 is a component in TCR signal The role that phosphorylated Ser727 Stat1 and Stat3 play in T cell to be is that tyrosine phosphorylation of Stats allows these proteins to translocate to the and bind DNA in of Stat phosphorylation is to augment the transcriptional activity of Stats (12.Wen Z. Zhong Z. Darnell Jr., J.E. Cell. 1995; 82: 241-250Abstract Full Text PDF PubMed Scopus (1770) Google Scholar). A of activators and can Stat1 phosphorylation on Ser727 in the of tyrosine phosphorylation P. D. M. T. EMBO J. 1998; 17: PubMed Scopus Google Scholar, P. D. R. A. M. Cohen P. T. S. A. 1999; PubMed Scopus Google Scholar). 727 of Stat1 also to play a critical role in the expression of Stat1 of the family of proteins in the of of A. M. T.W. C.M. Science. 1997; PubMed Scopus Google Scholar). that TCR can with IFN-α to transcriptional A similar observation has been in monocytes where pretreatment with in an enhanced transcriptional activity in response to P. D. M. T. EMBO J. 1998; 17: PubMed Scopus Google Scholar). In a it was that tyrosine phosphorylated Stat1 may be a for a Stat1 serine We can that a similar may also occur with derived from IFN-α and CD3 stimulation in T cells. is that CD3-mediated serine phosphorylation of Stat1 or several of the in T cells. The identity of the Stat1 serine kinase that triggered phosphorylation of Stat1 is not inhibitors PD98059 and SB203580, block the activation of MEK1 and p38 to inhibit serine phosphorylation of Stat1. In the serine/threonine kinase H7, that has been to inhibit serine phosphorylation of Stats is also on CD3-mediated phosphorylation of this This is in to Stat3 phosphorylation on Ser727 in response to CD3 stimulation, can be blocked by the MEK1 PD98059 (19.Ng J. Cantrell D. J. Biol. Chem. 1997; 272: 24542-24549Abstract Full Text Full Text PDF PubMed Scopus (130) Google Scholar). The of phosphatidylinositol 3-kinase and inhibitors not to Stat1 phosphorylation of Ser727 indicated that these proteins also were not involved in Stat1 signaling require the activity of or several tyrosine including Lck and Zap-70. these PTKs were for the serine phosphorylation of Stat1, we Jurkat cell that lack expression of Lck or Zap-70. the TCR complex in these cells did not the of Stat1 serine contrast, the Src kinase inhibitor PP1 effectively block phosphorylation of Stat1 Ser727 and ERK1/2 This that Lck Zap-70 may be involved in Stat1 However, the that the Src kinase inhibitor PP1 did serine phosphorylation of Stat1 that another tyrosine kinase in this family is involved in this One kinase that is to be for TCR function is is that TCR activation of the tyrosine kinase Fyn but Lck Zap-70 D. S. van Oers I. J. Weiss A. J. 1997; PubMed Scopus Google Scholar). has also been to be for serine phosphorylation of Stat1 A. N. T. H. M. P. T. J. T. EMBO J. 1999; 18: PubMed Scopus Google Scholar). One that Fyn activity allows for the activation of followed by activation of a MAPK member for serine phosphorylation of Stat1. are in to this The that Stat1 can be serine phosphorylated through engagement of the TCR another role for this transcription this to T for of the actions of Stat1 through a of tyrosine signaling