title: Ubiquitin modification of serum and glucocorticoid-induced protein kinase-1 (SGK-1).
abstract: The serum and glucocorticoid-induced protein kinase gene (sgk-1) encodes a multifunctional kinase that can be phosphorylated and activated through a phosphatidylinositol 3-kinase-dependent signaling pathway. In many cell types, endogenous SGK-1 steady-state protein levels are very low but can be acutely up-regulated after glucocorticoid receptor-mediated transcriptional activation; in breast epithelial and cancer cell lines, this up-regulation is associated with promotion of cell survival. We and others have noted that ectopically introduced full-length SGK-1 is poorly expressed, although SGK-1 lacking the first 60 amino acids (delta60SGK-1) is expressed at much higher-fold protein levels than wild-type SGK-1 in both human embryonic kidney 293T and MCF10A mammary epithelial cells. In this report, we demonstrate for the first time that the low steady-state expression level of SGK-1 is due to polyubiquitination and subsequent degradation by the 26S proteasome. Deletion of the amino-terminal 60 amino acids of SGK-1 results in a mutant SGK-1 protein that is neither efficiently polyubiquitinated nor degraded by the 26S proteasome, accounting for the higher steady-state levels of the truncated protein. We also demonstrate that a subset of SGK-1 localizes to the plasma membrane and that the polyubiquitin-modified SGK-1 localizes to a membrane-associated fraction of the cell. Taken together, these data suggest that a significant fraction of SGK-1 is membrane-associated and ubiquitinated. These findings are consistent with the recently described role of SGK-1 in phosphorylating the membrane-associated protein Nedd4-2 and the integral membrane Na+/H+ exchanger isoform 3 (NHE3) and suggest a novel mechanism of regulation of SGK-1.
authors.affiliation: Department of Medicine, Section of Hematology/Oncology, University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Deanna R
authors.initials: DR
authors.lastname: Brickley
authors.forename: Christina A
authors.initials: CA
authors.lastname: Mikosz
authors.forename: Christy R
authors.initials: CR
authors.lastname: Hagan
authors.forename: Suzanne D
authors.initials: SD
authors.lastname: Conzen
mesh_terms: D001940:Breast; D001943:Breast Neoplasms / Q000201:enzymology; D002460:Cell Line; D002462:Cell Membrane / Q000201:enzymology; D003546:Cysteine Endopeptidases / Q000378:metabolism; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D004790:Enzyme Induction / Q000187:drug effects; D004847:Epithelial Cells / Q000201:enzymology; D005260:Female; D005938:Glucocorticoids / Q000494:pharmacology*; D006801:Humans; D017874:Immediate-Early Proteins; D007700:Kinetics; D009097:Multienzyme Complexes / Q000378:metabolism; D009687:Nuclear Proteins; D046988:Proteasome Endopeptidase Complex; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000096:biosynthesis* / Q000097:blood*; D011994:Recombinant Proteins / Q000096:biosynthesis; D015533:Transcriptional Activation; D014162:Transfection; D014407:Tumor Cells, Cultured; D014452:Ubiquitins / Q000378:metabolism*; D000099184:Serum-Glucocorticoid Regulated Kinases
chemical_list: D003546:Cysteine Endopeptidases; D015853:Cysteine Proteinase Inhibitors; D005938:Glucocorticoids; D017874:Immediate-Early Proteins; D009097:Multienzyme Complexes; D009687:Nuclear Proteins; D046988:Proteasome Endopeptidase Complex; D017346:Protein Serine-Threonine Kinases; D011994:Recombinant Proteins; D014452:Ubiquitins; D000099184:Serum-Glucocorticoid Regulated Kinases