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

FEBS J. 2006 Jul;273(13):2913-28. doi: 10.1111/j.1742-4658.2006.05304.x.

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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Amino Acid Motifs
  • Amino Acid Sequence
  • Animals
  • Apoptosis
  • COS Cells
  • Catalytic Domain
  • Chlorocebus aethiops
  • Endoplasmic Reticulum / metabolism
  • Humans
  • Immediate-Early Proteins / chemistry
  • Immediate-Early Proteins / physiology*
  • Molecular Sequence Data
  • Phosphatidylinositol 3-Kinases / metabolism
  • Proteasome Endopeptidase Complex / chemistry
  • Proteasome Endopeptidase Complex / metabolism*
  • Protein-Serine-Threonine Kinases / chemistry
  • Protein-Serine-Threonine Kinases / physiology*
  • Ubiquitin / chemistry*
  • Ubiquitin / metabolism

Substances

  • Immediate-Early Proteins
  • Ubiquitin
  • Protein-Serine-Threonine Kinases
  • serum-glucocorticoid regulated kinase
  • Proteasome Endopeptidase Complex
  • ATP dependent 26S protease