A novel N-terminal hydrophobic motif mediates constitutive degradation of serum- and glucocorticoid-induced kinase-1 by the ubiquitin-proteasome pathway
A novel N-terminal hydrophobic motif mediates constitutive degradation of serum- and glucocorticoid-induced kinase-1 by the ubiquitin-proteasome pathway
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DOI：
10.1111/j.1742-4658.2006.05304.x
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发表时间：
2006-07-01
期刊：
影响因子：
5.4
通讯作者：
Conzen, Suzanne D.
中科院分区：
文献类型：
--
作者：
Bogusz, Agata M.;Brickley, Deanna R.;Conzen, Suzanne D.
血清和糖皮质激素诱导的蛋白激酶 1 (SGK-1) 在上皮钠通道 ENaC 的调节中发挥着关键作用。 SGK-1 还与蛋白激酶 B (PKB/AKT-1) 具有显着的催化结构域同源性，并且是抗凋亡磷酸肌醇 3-激酶信号传导的下游效应子。活性 SGK-1 的稳态水平受到快速转录激活和翻译后修饰（包括磷酸化）的严格调节。我们在此表明，内源性 SGK-1 蛋白被多泛素化并被 26S 蛋白酶体快速降解。与其他快速降解的激酶相比，SGK-1 的催化活性和激活位点磷酸化都不需要其泛素修饰和降解。相反，SGK-1 降解需要 N 端结构域内的无赖氨酸六氨基酸（氨基酸 19-24）疏水基序 (GMVAIL)。氨基酸 19-24 的缺失显着增加了 SGK1 的半衰期并阻止其泛素修饰。有趣的是，这个最小区域也是 SGK-1 与内质网结合所必需的。 SGK-1 的泛素修饰和降解越来越受到 GMVAIL 基序周围 6 个 N 末端赖氨酸残基渐进突变的抑制。尽管泛素化显着降低，所有六个赖氨酸突变为精氨酸并没有破坏 SGK-1 的亚细胞定位，这意味着这种修饰本身对于靶向内质网并不需要。这些结果表明，组成型泛素介导的 SGK-1 降解是调节其生物活性的重要机制。
Serum- and glucocorticoid-induced protein kinase-1 (SGK-1) plays a critical role in regulation of the epithelial sodium channel, ENaC. SGK-1 also shares significant catalytic domain homology with protein kinase B (PKB/AKT-1) and is a downstream effector of antiapoptotic phosphoinositide 3-kinase signaling. Steady-state levels of an active SGK-1 are tightly regulated by rapid transcriptional activation and post-translational modification including phosphorylation. We show here that endogenous SGK-1 protein is polyubiquitinated and rapidly degraded by the 26S proteasome. In contrast to other rapidly degraded kinases, neither the catalytic activity of SGK-1 nor activation site phosphorylation was required for its ubiquitin modification and degradation. Instead, SGK-1 degradation required a lysine-less six-amino-acid (amino acids 19-24) hydrophobic motif (GMVAIL) within the N-terminal domain. Deletion of amino acids 19-24 significantly increased the half-life of SGK1 and prevented its ubiquitin modification. Interestingly, this minimal region was also required for the association of SGK-1 with the endoplasmic reticulum. Ubiquitin modification and degradation of SGK-1 were increasingly inhibited by the progressive mutation of six N-terminal lysine residues surrounding the GMVAIL motif. Mutation of all six lysines to arginine did not disrupt the subcellular localization of SGK-1 despite a significant decrease in ubiquitination, implying that this modification per se was not required for targeting to the endoplasmic reticulum. These results suggest that constitutive ubiquitin-mediated degradation of SGK-1 is an important mechanism regulating its biological activity.