Narazaki M, Fujimoto M, Matsumoto T, Morita Y, Saito H, Kajita T, Yoshizaki K, Naka T, Kishimoto T
Department of Medicine III, Osaka University Medical School, 2-2, Yamada-oka, Suita, Osaka 565-0871, Japan.
Proc Natl Acad Sci U S A. 1998 Oct 27;95(22):13130-4. doi: 10.1073/pnas.95.22.13130.
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.
细胞因子诱导蛋白SSI-1[信号转导子和转录激活子(STAT)诱导的STAT抑制剂1，也称为细胞因子信号转导抑制因子1(SOCS-1)或Janus激酶结合蛋白(JAB)]通过抑制JAK激酶对细胞因子受体信号传导起负调节作用。SSI蛋白家族包括8个成员，其结构特征为一个SH2结构域和一个我们称为SC基序的C末端保守区域。在本研究中，我们研究了这些结构域在SSI-1功能中的作用。报告基因检测结果表明，SSI-1的SH2结构域前的结构域(24个氨基酸)和SH2结构域是SSI-1抑制白细胞介素6信号传导所必需的。COS7细胞的共表达研究表明，这些结构域也是抑制三种JAKs(JAK1、JAK2和TYK2)所必需的。此外，SH2结构域的缺失而非SH2结构域前的结构域的缺失导致SSI-1与TYK2的结合丧失。因此，SSI-1通过其SH2结构域与JAK家族激酶结合，而SH2结构域前的结构域是SSI-1功能所必需的。SC基序的缺失显著降低了M1细胞中SSI-1蛋白的表达，而蛋白酶体抑制剂处理可逆转这种降低，这表明该基序是保护SSI-1分子免受蛋白水解降解所必需的。基于这些发现，我们得出结论，SSI-1的三个不同结构域(SH2结构域前的结构域、SH2结构域和SC基序)协同抑制白细胞介素6信号传导。
