A novel N-terminal hydrophobic motif mediates constitutive degradation of serum- and glucocorticoidinduced kinase-1 by the ubiquitin-proteasome pathway

Agata M. Bogusz, Deanna R. Brickley, Travis Pew, Suzanne D. Conzen

Research output: Contribution to journalArticle

46 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)2913-2928
Number of pages16
JournalFEBS Journal
Volume273
Issue number13
DOIs
StatePublished - Jul 2006

Keywords

  • Endoplasmic reticulum
  • PtdIns-3K
  • SGK-1
  • Stress signalling
  • Ubiquitination

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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