The WD Motif-containing Protein Receptor for Activated Protein Kinase C (RACK1) Is Required for Recruitment and Activation of Signal Transducer and Activator of Transcription 1 through the Type I Interferon Receptor

Abstract

An obligatory step in the activation ofSignal Transducers and Activators of Transcription (STATs) by cytokines is their docking to specific receptors via phosphotyrosines. However, this model does not address whether STATs pre-associate with their corresponding receptor or exist free in the cytoplasm before receptor activation. In this report, we demonstrate that pre-association of STAT1 with the receptor is required for type I interferon (IFN) signaling. Interestingly, the interaction between the human type I IFN receptor and STAT1 is not direct but mediated by the adapter protein receptor for activated protein kinase C (RACK1). Disruption of the IFNα receptor-RACK1 interaction abolishes not only IFNα-induced tyrosine phosphorylation of STAT1 but also activation of STAT2, indicating that RACK1 plays a central role in early signaling through the Jak-STAT pathway. These findings demonstrate the involvement of RACK1 in STAT1 activation and raise the possibility that other STATs may pre-associate with cytokine receptors through similar adapter-STAT-mediated interactions. An obligatory step in the activation ofSignal Transducers and Activators of Transcription (STATs) by cytokines is their docking to specific receptors via phosphotyrosines. However, this model does not address whether STATs pre-associate with their corresponding receptor or exist free in the cytoplasm before receptor activation. In this report, we demonstrate that pre-association of STAT1 with the receptor is required for type I interferon (IFN) signaling. Interestingly, the interaction between the human type I IFN receptor and STAT1 is not direct but mediated by the adapter protein receptor for activated protein kinase C (RACK1). Disruption of the IFNα receptor-RACK1 interaction abolishes not only IFNα-induced tyrosine phosphorylation of STAT1 but also activation of STAT2, indicating that RACK1 plays a central role in early signaling through the Jak-STAT pathway. These findings demonstrate the involvement of RACK1 in STAT1 activation and raise the possibility that other STATs may pre-associate with cytokine receptors through similar adapter-STAT-mediated interactions. interferon Signal Transducer andActivator of Transcription interleukin IFNα receptor receptor for activated protein kinase C long form of the β subunit of the IFNαR glutathione S-transferase human STAT3-Interacting Protein Cytokines and interferons (IFNs)1 bind to receptors of the cytokine receptor superfamily (1Ihle J.N. Kerr I.M. Trends Genet. 1995; 12: 69-73Abstract Full Text PDF Scopus (821) Google Scholar, 2Kishimoto T. Taga T. Akira S. Cell. 1994; 76: 253-262Abstract Full Text PDF PubMed Scopus (1248) Google Scholar, 3Leonard W.J. O'Shea J. Annu. Rev. Immunol. 1998; 16: 293-322Crossref PubMed Scopus (1480) Google Scholar), resulting in the activation of kinases of the Jak family and transcription factors designated STATs or Signal Transducers andActivators of Transcription (4Darnell J.E.J. Science. 1997; 277: 1630-1635Crossref PubMed Scopus (3383) Google Scholar, 5Schindler S. Darnell J.J.E. Annu. Rev. Biochem. 1995; 64: 621-651Crossref PubMed Scopus (1651) Google Scholar, 6Stahl N. Farruggella T.J. Boulton T.J. Zhong Z. Darnell J.J.E. Yancopoulos G.D. Science. 1995; 267: 1349-1353Crossref PubMed Scopus (866) Google Scholar, 7Schindler C. Exp. Cell Res. 1999; 253: 7-14Crossref PubMed Scopus (115) Google Scholar). The Jak-STAT pathway has evolved as the paradigm of cytokine and IFN signaling (4Darnell J.E.J. Science. 1997; 277: 1630-1635Crossref PubMed Scopus (3383) Google Scholar, 5Schindler S. Darnell J.J.E. Annu. Rev. Biochem. 1995; 64: 621-651Crossref PubMed Scopus (1651) Google Scholar, 6Stahl N. Farruggella T.J. Boulton T.J. Zhong Z. Darnell J.J.E. Yancopoulos G.D. Science. 1995; 267: 1349-1353Crossref PubMed Scopus (866) Google Scholar, 7Schindler C. Exp. Cell Res. 1999; 253: 7-14Crossref PubMed Scopus (115) Google Scholar). Although STAT can be activated by different cytokines (i.e. STAT1 is activated by IFNα, IFNγ, IL6, leukemia inhibitory factor, IL10, etc.), studies with knockout mice clearly indicated that their function is well restricted to precise systems. For example, STAT1 is only required for the physiological functions of IFNα and IFNγ (reviewed in Ref. 7Schindler C. Exp. Cell Res. 1999; 253: 7-14Crossref PubMed Scopus (115) Google Scholar). STATs are recruited to distinct phosphotyrosines within the receptor complex and then are phosphorylated, probably by Jaks, on the highly conserved C-terminal tyrosines (i.e. tyrosine 701 of Stat1), allowing the SH2 domain of one STAT to interact with the phosphorylated tyrosine on another STAT to form homo- or heterodimers. STAT dimers translocate to the nucleus, where they bind specific DNA elements to activate or inhibit transcription of specific genes (reviewed in Refs. 5Schindler S. Darnell J.J.E. Annu. Rev. Biochem. 1995; 64: 621-651Crossref PubMed Scopus (1651) Google Scholar and 8Ihle J.E. Cell. 1996; 84: 331-334Abstract Full Text Full Text PDF PubMed Scopus (1266) Google Scholar). One distinctive feature in the type I IFN system is that STAT2 is pre-associated with IFNαRβL chain (9Nadeau O.W. Domanski P. Usacheva A. Uddin S. Platanias L.C. Pitha P. Raz R. Levy D. Majchrzak B. Fish E. Colamonici O.R. J. Biol. Chem. 1999; 274: 4045-4052Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar, 10Li X. Leung S. Kerr I.M. Stark G.S. Mol. Cell. Biol. 1997; 17: 2048-2056Crossref PubMed Scopus (165) Google Scholar). Activation of STAT2 in response to type I IFNs (IFNα, β, or ω) requires the presence of this constitutive site and one or more of the five proximal tyrosines of the βL chain (9Nadeau O.W. Domanski P. Usacheva A. Uddin S. Platanias L.C. Pitha P. Raz R. Levy D. Majchrzak B. Fish E. Colamonici O.R. J. Biol. Chem. 1999; 274: 4045-4052Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar). However, the mechanism for STAT1 activation by type I IFNs has not been elucidated. It is known that activation of STAT1 requires the previous activation of STAT2 (11Leung S. Qureshi S.A. Kerr I.M. Darnell J.E.J. Stark G.R. Mol. Cell. Biol. 1995; 15: 1312-1317Crossref PubMed Google Scholar), but it has not been determined whether receptor tyrosines are also required for activation. This is in clear contrast to the activation of STAT1 by IFNγ, which requires docking of STAT1 to a phosphorylated tyrosine on the α chain of the receptor (12Greenlund A.C. Farrar M.A. Viviano B.L. Schreiber R.D. EMBO J. 1994; 13: 1591-1600Crossref PubMed Scopus (376) Google Scholar). We have recently reported (13Croze E. Usacheva A. Asarnow D. Minshall R.D. Perez H.D. Colamonici O.R. J. Immunol. 2000; 165: 5127-5132Crossref PubMed Scopus (47) Google Scholar) that RACK1, originally described as aReceptor for Activated CKinase β (14Mochly-Rosen D. Smith B.L. Chen C.H. Disatnik M.H. Ron D. Biochem. Soc Trans. 1995; 23: 596-600Crossref PubMed Scopus (91) Google Scholar, 15Mochly-Rosen D. Khaner H. Lopez J. Smith B.L. J. Biol. Chem. 1991; 266: 14866-14868Abstract Full Text PDF PubMed Google Scholar, 16Ron D. Chen C.H. Caldwell J. Jamieson L. Orr E. Mochly-Rosen D. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 839-843Crossref PubMed Scopus (646) Google Scholar), constitutively interacts with the β long subunit of the type I IFN receptor (IFNαRβL/IFNAR2). RACK1 has a molecular mass of 36,000 daltons and is composed of 7 WD repeats that resemble the structure of the β subunit of G proteins (Gβ) (17Garcia-Higuera I. Fenoglio J. Li Y. Lewis C. Panchenko M.P. Reiner O. Smith T.F. Neer E.J. Biochemistry. 1996; 35: 13985-13994Crossref PubMed Scopus (164) Google Scholar, 18Neer E.J. Smith T.F. Cell. 1996; 84: 175-178Abstract Full Text Full Text PDF PubMed Scopus (186) Google Scholar). RACK1 also interacts with protein kinase C β, src (19Chang B.Y. Conroy K.B. Machleder E.M. Cartwright C.A. Mol. Cell. Biol. 1998; 18: 3245-3256Crossref PubMed Google Scholar), β integrins (20Liliental J. Chang D.D. J. Biol. Chem. 1998; 273: 2379-2383Abstract Full Text Full Text PDF PubMed Scopus (302) Google Scholar), PDE4D5 (21Yarwood S.J. Steele M.R. Scotland G. Houslay M.D. Bolger G.B. J. Biol. Chem. 1999; 274: 14909-14917Abstract Full Text Full Text PDF PubMed Scopus (261) Google Scholar), and the β common subunit of the granulocyte/macrophage colony-stimulating factor/IL3/IL5 receptors (22Geijsen N. Spaargaren M. Raaijmakers J.A. Lammers J.W. Koenderman L. Coffer P.J. Oncogene. 1999; 18: 5126-5130Crossref PubMed Scopus (79) Google Scholar). We report here that RACK1 constitutively interacts with non-phosphorylated STAT1 and functions as an adaptor between this factor and the long form of the β subunit of the IFNαR (IFNαRβL). No interaction between RACK1 and other STAT factors was detected. The interaction between IFNαRβL and RACK1 is critical for normal STAT activation and IFN signaling. This is supported by the finding that mutations in the RACK1 binding site of IFNαRβL, which includes the Box 2 motif, impaired IFNα-induced tyrosine phosphorylation of STAT1 and STAT2 and the development of the antiviral state. U-266 and Daudi cells were grown in RPMI (Life Technologies, Inc.) supplemented with 10% (v/v) fetal bovine serum. Human IFNα2 (specific activity, 2.2 × 108 units/mg) was a gift of Ronald Bordens (Schering-Plough). The anti-phosphotyrosine antibody 4G10 was purchased from Upstate Biotechnologies Inc., and the anti-RACK-1, anti-STAT1, -STAT2, -STAT5, and -Jak1 monoclonal antibodies were purchased from Transduction Laboratories, Inc. Polyclonal antibodies against STAT3, STAT4, STAT5, and STAT6 were kindly provided by Drs. Evan Parganas and James Ihle (St. Jude Children's Research Hospital, Memphis, TN). The anti-Stat1 and -Stat2 sera were kindly provided by Dr. A. Larner (Cleveland Clinic, Cleveland, OH). Antiviral assays were performed as previously described (23Cheung S.C. Chattopadhyay S.K. Hartley J.W. Morse H.C. Pitha P.M. J. Immunol. 1991; 146: 121-127PubMed Google Scholar, 24Ghislain J. Sussman G. Goelz S. Ling L.E. Fish E.N. J. Biol. Chem. 1995; 270: 21785-21792Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar). U-266 or Daudi cells (1 × 107 cells) were treated as indicated and then lysed in lysis buffer (20 mm Tris-HCl, pH 6.6 containing 1% Nonidet P-40, 50 mm NaCl, 1 mm EDTA, 2.5% glycerol (v/v), 1.0 mm sodium fluoride, 1.0 mmsodium orthovanadate, 1.0 mm phenylmethylsulfonyl fluoride, 0.5 μg/ml leupeptin, and 5.0 μg/ml trypsin inhibitor) for 30 min at 4 °C. Immunoprecipitations were performed as previously described (9Nadeau O.W. Domanski P. Usacheva A. Uddin S. Platanias L.C. Pitha P. Raz R. Levy D. Majchrzak B. Fish E. Colamonici O.R. J. Biol. Chem. 1999; 274: 4045-4052Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar). Proteins were transferred to polyvinylidene difluoride membranes, immunoblotted with the indicated antibodies, and developed using a chemiluminescent detection method (Pierce). The different GST fusion proteins encoding different regions of the cytoplasmic domain of IFNαRβL have been described previously (25Domanski P. Nadeau O.W. Fish E. Kellum M. Platanias L.C. Pitha P. Colamonici O.R. J. Biol. Chem. 1997; 272: 26388-26393Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar). For mapping of the RACK1 binding site of IFNαRβL, a combination of two or three alanine mutations per construct was introduced in the GSTβL300–375 that contains the minimum region that binds RACK1. GST fusion expression constructs with mutations of the RACK1 site were made by polymerase chain reaction using the Quickchange kit (Stratagene). All mutations were confirmed by sequencing. A GST fusion protein encoding the full-length RACK1 (GST-RACK1) was produced by polymerase chain reaction and subcloned into the pGEX-KG vector. GST fusion proteins were produced in BL-21 cells as described previously (25Domanski P. Nadeau O.W. Fish E. Kellum M. Platanias L.C. Pitha P. Colamonici O.R. J. Biol. Chem. 1997; 272: 26388-26393Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar). Pull-down experiments and immunoblotting were performed using the same procedure described above for immunoprecipitations. STAT1 and RACK1 were produced by a T7 wheat germ in vitro transcription/translation kit (Promega) following the manufacturer's procedure. [35S]methionine-labeled STAT1 and RACK1 proteins alone or in combination were incubated with GST-βL overnight, washed, and analyzed by SDS-polyacrylamide gel electrophoresis as described for immunoprecipitations. Mammalian expression constructs with mutations of the RACK1 site of IFNαRβL (amino acids 302, and were made by polymerase chain reaction using the Quickchange kit (Stratagene). All mutations were confirmed by sequencing. were subcloned into the and into the and were to cells cells the human α chain of the Ref. P. Nadeau O.W. Fish E. Kellum M. Platanias L.C. Pitha P. Colamonici O.R. J. Biol. Chem. 1997; 272: 26388-26393Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar). were in containing and and were by using the monoclonal antibody O.R. Domanski P. J. Biol. Chem. Full Text PDF PubMed Google Scholar) that It has been that proteins containing WD repeats may as or adaptor proteins T.F. C. Neer E.J. Biochem. Sci. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). RACK1 interacts with a region of IFNαRβL (amino acids (13Croze E. Usacheva A. Asarnow D. Minshall R.D. Perez H.D. Colamonici O.R. J. Immunol. 2000; 165: 5127-5132Crossref PubMed Scopus (47) Google Scholar) that is required for activation of STATs and the antiviral response (25Domanski P. Nadeau O.W. Fish E. Kellum M. Platanias L.C. Pitha P. Colamonici O.R. J. Biol. Chem. 1997; 272: 26388-26393Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar), we that RACK1 STAT1 to the receptor We performed experiments using an monoclonal antibody to for an interaction between RACK1 and 1 A that the antibody STAT1 but not or STAT2 not in This interaction is it be by as 1 The and antibodies also and as previously reported M. C. Larner A.C. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). However, the for in 1 may in to of the STAT1 This that STAT1 interacts with RACK1. It be that we have not been to RACK1 using One is that antibodies to the C-terminal of the where the RACK1 binding site may be and the the a GST fusion protein that the full-length RACK1 protein was to whether other STAT proteins bind RACK1. that but not GST binds to However, to bind STAT2, or that the interaction between RACK1 and STAT1 is Interestingly, GST-βL also STAT1 in This from previous using STAT1 produced in wheat germ in vitro in which interaction between IFNαRβL and STAT1 was (9Nadeau O.W. Domanski P. Usacheva A. Uddin S. Platanias L.C. Pitha P. Raz R. Levy D. Majchrzak B. Fish E. Colamonici O.R. J. Biol. Chem. 1999; 274: 4045-4052Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar). One is that a protein as RACK1 in but in the wheat germ in vitro may as an adapter between IFNαRβL and STAT1 We that RACK1 STAT1 to IFNαRβL, or mutations in IFNαRβL that RACK1 binding also the of STAT1 with the this we performed experiments using GST fusion proteins containing different regions of the cytoplasmic domain of 2 that GST fusion proteins encoding the cytoplasmic domain and proteins at acids and but not at acids were also to interact with a GST fusion protein encoding acids 2 and the acids of the cytoplasmic domain also interacts with The same GST-βL fusion proteins that STAT1 also with RACK1 2 This that the interaction domain for STAT1 to acids of IFNαRβL and is to the RACK1 binding site 2 and E. Usacheva A. Asarnow D. Minshall R.D. Perez H.D. Colamonici O.R. J. Immunol. 2000; 165: 5127-5132Crossref PubMed Scopus (47) Google Scholar). We performed an alanine of this region to the RACK1 and STAT1 binding Although RACK1 or STAT1 binding to mutations produced a in binding of STAT1 to IFNαRβL that the in binding of RACK1 to this receptor chain The in binding was acids within the region and of IFNαRβL 2 2 and were to The in RACK1 and STAT1 binding the that RACK1 functions as an adaptor between IFNαRβL and Although the interaction between IFNαRβL and STAT1 is in 1 and 2 the between proteins is not STAT1 is produced in wheat germ (9Nadeau O.W. Domanski P. Usacheva A. Uddin S. Platanias L.C. Pitha P. Raz R. Levy D. Majchrzak B. Fish E. Colamonici O.R. J. Biol. Chem. 1999; 274: 4045-4052Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar). A for this is that RACK1 functions as an adaptor between IFNαRβL and STAT1 and that the wheat germ of RACK1 to interact with IFNαRβL, or we the of GST-βL to bind STAT1 produced alone or with RACK1 using a wheat germ in vitro 2 that GST-βL interacts with RACK1 but not STAT1 proteins are produced 2 2 and However, RACK1 and STAT1 are in vitro GST-βL STAT1 IFNαRβL and STAT1 interact only RACK1 is that RACK1 functions as an adaptor between IFNαRβL and Although the experiments above demonstrate that the of IFNαRβL, RACK1, and STAT1 form a they not address whether RACK1 interacts with the phosphorylated form of This is an STAT1 is phosphorylated it from the receptor to form a complex in with STAT2 and address this STAT1 phosphorylation was by U-266 cells with IFNα for we the of to with as determined by immunoblotting with the anti-phosphotyrosine antibody Immunoprecipitations with an or GST alone were as and The were in in and immunoblotted using anti-phosphotyrosine or A that GST-βL can the of STAT1 IFNα and but bind non-phosphorylated STAT1 in cells 2 as well as the non-phosphorylated IFNα the antibody the phosphorylated and non-phosphorylated of STAT1 4 and were in similar experiments in which the same was immunoblotted with anti-phosphotyrosine and then antibodies and the not These demonstrate that RACK1 interacts only with the non-phosphorylated form of STAT1 and the that STAT1 from the complex phosphorylated to form a This was by the finding that min IFNα STAT1 was to the RACK1 and in the cytoplasm In RACK1 is in the as previously reported (13Croze E. Usacheva A. Asarnow D. Minshall R.D. Perez H.D. Colamonici O.R. J. Immunol. 2000; 165: 5127-5132Crossref PubMed Scopus (47) Google Scholar), STAT1 was in cytoplasm and and The of the procedure is by the of normal and were as for STAT1 and RACK1 a and the of the interaction between IFNαRβL, RACK1, and STAT1 in IFNα we the human IFNαRβL chain with mutations of the RACK1 binding site in the IFNαRβL chain were by We whether of the interaction between the receptor and RACK1 the activation of Human IFNα2 of STAT1 phosphorylation in cells mutations of the RACK1 binding site of IFNαRβL Interestingly, tyrosine phosphorylation of STAT2 was also by the of the RACK1 site and tyrosine phosphorylation of STAT1 and STAT2 was cells were treated with that the STAT pathway was activated through the The in tyrosine phosphorylation of STAT1 and STAT2 in response to was not to a in kinase tyrosine phosphorylation of was normal These that the interaction between IFNαRβL and RACK1 is not only for normal activation of STAT1 but also for STAT2 tyrosine phosphorylation through a mechanism that to be of the RACK1 site on IFN STAT activation and the of an antiviral state. cells with IFNαRβL type or with mutations of the RACK1 binding site and were treated with IFNα2 or or at for Cell were with and or sera was performed with the anti-phosphotyrosine antibody 4G10 by and with the antibodies STAT2, and the of IFN to cells against the of was determined using a antiviral were with of IFNs from in containing fetal bovine for The was and with containing a of that of the cells in Cell was determined using We whether was to an antiviral response in cells a of the RACK1 binding site of IFNαRβL and that of against which the in two mutations of the RACK1 The of was also that by in cells the type These demonstrate that of RACK1 to the IFNαR complex is critical for the activation of STAT1 and STAT2 and for the of an antiviral state. The in this demonstrate that the adaptor protein RACK1 STAT1 to the human type I IFN The interaction between the receptor and RACK1 is required for activation of STAT1 and the of an antiviral by The interaction between STAT1 and IFNαRβL and the direct of STAT2 with the of the same chain (9Nadeau O.W. Domanski P. Usacheva A. Uddin S. Platanias L.C. Pitha P. Raz R. Levy D. Majchrzak B. Fish E. Colamonici O.R. J. Biol. Chem. 1999; 274: 4045-4052Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar, 10Li X. Leung S. Kerr I.M. Stark G.S. Mol. Cell. Biol. 1997; 17: 2048-2056Crossref PubMed Scopus (165) Google Scholar) demonstrate that activation of the STAT pathway by type I IFNs requires the pre-association of STAT factors with the These findings raise the whether a model in which STATs pre-associate with cytokine receptors through adaptor proteins containing a WD also to other cytokine systems. It has been recently reported that the WD protein interacts with STATs and S. G. C. Proc. Natl. Acad. Sci. U. S. A. 2000; PubMed Scopus Google Scholar). However, are between RACK1 and interacts with more one STAT and that it is not STAT or cytokine and it has not been whether with cytokine it is to that and RACK1 are of a family of proteins in Jak-STAT signaling. The that other may exist is also supported by indicating that activation of by in the presence of receptors of tyrosines Larner A.C. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar). The to a model in which STATs are pre-associated with cytokine receptors is that the only system that requires pre-association is the type I IFN pathway. In this the of STAT1 through RACK1 may a more of STAT activation to the of type I IFNs on or may the for phosphorylation of a specific tyrosine on the receptor for STAT1 activation. we have not been to address the activation of STAT1 is on the previous activation of STAT2 (11Leung S. Qureshi S.A. Kerr I.M. Darnell J.E.J. Stark G.R. Mol. Cell. Biol. 1995; 15: 1312-1317Crossref PubMed Google Scholar). However, of tyrosines to IFNαRβL constructs in which tyrosines been for to STAT1 phosphorylation of STAT2 This the possibility the IFNγ where phosphorylation of tyrosine of the α chain is critical for STAT1 phosphorylation (12Greenlund A.C. Farrar M.A. Viviano B.L. Schreiber R.D. EMBO J. 1994; 13: 1591-1600Crossref PubMed Scopus (376) Google Scholar, A.C. Viviano B.L. H. J. Schreiber R.D. 1995; Full Text PDF PubMed Scopus Google Scholar), tyrosine phosphorylation of the IFNαR is not critical for STAT1 activation. also that STAT1 is it can from RACK1 and the receptor This is supported by the finding that RACK1 interacts only with the form of STAT1 and that type I IFN STAT1 is in the nucleus, RACK1 in the These that RACK1 not be of the or factor complex and does not translocate to the This and previous finding that RACK1 is not (13Croze E. Usacheva A. Asarnow D. Minshall R.D. Perez H.D. Colamonici O.R. J. Immunol. 2000; 165: 5127-5132Crossref PubMed Scopus (47) Google Scholar) the that RACK1 is an adaptor or protein in specific signaling as STAT1 to the for their activation. It be that the region of IFNαRβL that interacts with RACK1 to at in with the Box 2 It has been that this a role in the activation of by the chain J.A. W.J. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar). that the interaction between RACK1 and the Box 2 be for the of specific signaling proteins as STAT1 for the receptor that to activate signaling as the of RACK1 it to in which the interaction between RACK1 and STAT1 has been experiments are to whether the impaired tyrosine phosphorylation of STAT1 and STAT2 RACK1 interact with the receptor is only to the to STAT1 or to the that RACK1 may also other signaling mechanism may the finding that RACK1 is also required for phosphorylation of the of RACK1 the of This is also by the finding that the Box 2 is in signaling by cytokine receptors in which STAT1 is not required for it be that RACK1, as well as the of binds the SH2 domain of and protein kinase C β, the possibility that or other proteins with similar may be recruited by RACK1 to cytokine We are the possibility that RACK1 proteins activated by type I IFNs as receptor and S. D. Y. S. Colamonici O.R. Fish E.N. 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