Wang D, Moriggl R, Stravopodis D, Carpino N, Marine J C, Teglund S, Feng J, Ihle J N
Department of Biochemistry, St Jude Children's Research Hospital, Memphis, TN 38105, USA.
EMBO J. 2000 Feb 1;19(3):392-9. doi: 10.1093/emboj/19.3.392.
Cytokines induce the tyrosine phosphorylation and associated activation of signal transducers and activators of transcription (Stat). The mechanisms by which this response is terminated are largely unknown. Among a variety of inhibitors examined, the proteasome inhibitors MG132 and lactacystin affected Stat4, Stat5 and Stat6 turnover by significantly stabilizing the tyrosine-phosphorylated form. However, these proteasome inhibitors did not affect downregulation of the tyrosine-phosphorylated Stat1, Stat2 and Stat3. With Stat5 isoforms, we have observed that tyrosine-phosphorylated carboxyl-truncated forms of Stat5 proteins were considerably more stable than phosphorylated wild-type forms of the protein. Also, the C-terminal region of Stat5 could confer proteasome-dependent downregulation to Stat1. With a series of C-terminal deletion mutants, we have defined a relatively small, potentially amphipathic alpha-helical region that is required for the rapid turnover of the phosphorylated Stat5 proteins. The region is also required for transcriptional activation, suggesting that the functions are linked. The results are consistent with a model in which the transcriptional activation domain of activated Stat5 is required for its transcriptional activity and downregulation through a proteasome-dependent pathway.
细胞因子可诱导信号转导子和转录激活子（Stat）的酪氨酸磷酸化及相关激活。这种反应终止的机制在很大程度上尚不清楚。在所检测的多种抑制剂中，蛋白酶体抑制剂MG132和乳胞素通过显著稳定酪氨酸磷酸化形式来影响Stat4、Stat5和Stat6的周转。然而，这些蛋白酶体抑制剂并不影响酪氨酸磷酸化的Stat1、Stat2和Stat3的下调。对于Stat5亚型，我们观察到酪氨酸磷酸化的羧基截短形式的Stat5蛋白比磷酸化的野生型蛋白稳定得多。此外，Stat5的C末端区域可赋予Stat1蛋白酶体依赖性下调。通过一系列C末端缺失突变体，我们确定了一个相对较小的、潜在的两亲性α螺旋区域，该区域是磷酸化Stat5蛋白快速周转所必需的。该区域也是转录激活所必需的，这表明这些功能是相关联的。这些结果与一个模型一致，即活化的Stat5的转录激活结构域对于其转录活性以及通过蛋白酶体依赖性途径的下调是必需的。
