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A novel N‐terminal hydrophobic motif mediates constitutive degradation of serum‐ and glucocorticoid‐induced kinase‐1 by the ubiquitin–proteasome pathway
Author(s) -
Bogusz Agata M.,
Brickley Deanna R.,
Pew Travis,
Conzen Suzanne D.
Publication year - 2006
Publication title -
the febs journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.981
H-Index - 204
eISSN - 1742-4658
pISSN - 1742-464X
DOI - 10.1111/j.1742-4658.2006.05304.x
Subject(s) - ubiquitin , proteasome , glucocorticoid , microbiology and biotechnology , chemistry , degradation (telecommunications) , biology , biochemistry , endocrinology , gene , computer science , telecommunications
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 26 S 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.