Three distinct domains of SSI-1/SOCS-1/JAB protein are required for its suppression of interleukin 6 signaling.
Cytokine-inducible protein SSI-1 [signal transducers and activators of transcription (STAT)-induced STAT inhibitor 1, also referred to as SOCS-1 (suppressor of cytokine signaling 1) or JAB (Janus kinase-binding protein)] negatively regulates cytokine receptor signaling by inhibition of JAK kinases. The SSI family of proteins includes eight members that are structurally characterized by an SH2 domain and a C-terminal conserved region that we have called the SC-motif. In this study, we investigated the roles of these domains in the function of SSI-1. Results of reporter assays demonstrated that the pre-SH2 domain (24 aa in front of the SH2 domain) and the SH2 domain of SSI-1 were required for the suppression by SSI-1 of interleukin 6 signaling. Coexpression studies of COS7 cells revealed that these domains also were required for inhibition of three JAKs (JAK1, JAK2, and TYK2). Furthermore, deletion of the SH2 domain, but not the pre-SH2 domain, resulted in loss of association of SSI-1 with TYK2. Thus, SSI-1 associates with JAK family kinase via its SH2 domain, and the pre-SH2 domain is required for the function of SSI-1. Deletion of the SC-motif markedly reduced expression of SSI-1 protein in M1 cells, and this reduction was reversed by treatment with proteasome inhibitors, suggesting that this motif is required to protect the SSI-1 molecule from proteolytic degradation. Based on these findings, we concluded that three distinct domains of SSI-1 (the pre-SH2 domain, the SH2 domain, and the SC-motif) cooperate in the suppression of interleukin 6 signaling.
Narazaki
M
M
Department of Medicine III, Osaka University Medical School, 2-2, Yamada-oka, Suita, Osaka 565-0871, Japan.
Fujimoto
M
M
Matsumoto
T
T
Morita
Y
Y
Saito
H
H
Kajita
T
T
Yoshizaki
K
K
Naka
T
T
Kishimoto
T
T
D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D017124:Conserved Sequence; D006801:Humans; D015850:Interleukin-6 / Q000502:physiology*; D047908:Intracellular Signaling Peptides and Proteins; D008156:Luciferases / Q000235:genetics; D051379:Mice; D008807:Mice, Inbred BALB C; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010446:Peptide Fragments / Q000737:chemistry / Q000276:immunology; D017354:Point Mutation; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors*; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012097:Repressor Proteins; D016415:Sequence Alignment; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D000071222:Suppressor of Cytokine Signaling 1 Protein; D050826:Suppressor of Cytokine Signaling Proteins; D014162:Transfection; D018909:src Homology Domains; D000093242:Interleukin-6 Inhibitors
D002352:Carrier Proteins; D015850:Interleukin-6; D047908:Intracellular Signaling Peptides and Proteins; D008156:Luciferases; D010446:Peptide Fragments; D011505:Protein-Tyrosine Kinases; D011993:Recombinant Fusion Proteins; D011994:Recombinant Proteins; D012097:Repressor Proteins; D000071222:Suppressor of Cytokine Signaling 1 Protein; D050826:Suppressor of Cytokine Signaling Proteins; D000093242:Interleukin-6 Inhibitors; C108944:SOCS1 protein, human; C491590:Socs1 protein, mouse