Direct demonstration of rapid degradation of nuclear sterol regulatory element-binding proteins by the ubiquitin-proteasome pathway.
Sterol regulatory element-binding proteins (SREBPs) are synthesized as membrane-bound precursors and processed to generate transcriptionally active forms. The active SREBPs translocate to the nucleus, induce the expression of responsive genes, and are degraded very rapidly. Treatment with proteasome inhibitors elevates the amount of the endogenous nuclear SREBPs, but not the precursors, in HeLa cells. Nuclear forms of human SREBP-1a (amino acids 1-487) and SREBP-2 (amino acids 1-481), which are transiently expressed in stable Chinese hamster ovary cell lines (CHO-487 and -481), are also stabilized by proteasome inhibitors, suggesting that the nuclear SREBPs are likely to be substrates for the proteasome-dependent proteolysis. The stabilized nuclear SREBPs actively induce the expression of responsive genes including hydroxymethylglutaryl (HMG)-CoA synthase, fatty acid synthase, and the low density lipoprotein receptor. The rapid turnover of nuclear SREBP-1a is not affected by the intracellular sterol levels, and the half-life is estimated to be approximately 3 h. The nuclear SREBPs are found conjugated with a polyubiquitin chain. When this conjugation is inhibited by overexpression of mutant ubiquitin that is defective in polyubiquitination, the nuclear SREBPs are partly stabilized and induce the expression of the responsive gene, suggesting that the ubiquitin-conjugated SREBPs are substrates for the proteasome. Taken together, these results demonstrate that the ubiquitin-proteasome system degrades SREBPs and that this system controls the expression of SREBP-responsive genes.
Hirano
Y
Y
Department of Applied Biological Chemistry and Biotechnology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan.
Yoshida
M
M
Shimizu
M
M
Sato
R
R
D000818:Animals; D001704:Biopolymers / Q000378:metabolism; D022762:CCAAT-Enhancer-Binding Proteins / Q000378:metabolism*; D016466:CHO Cells; D019556:COS Cells; D002467:Cell Nucleus / Q000378:metabolism; D006224:Cricetinae; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D049452:Green Fluorescent Proteins; D006367:HeLa Cells; D006801:Humans; D008164:Luminescent Proteins / Q000378:metabolism; D009097:Multienzyme Complexes / Q000378:metabolism*; D010957:Plasmids / Q000378:metabolism; D025821:Polyubiquitin; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D051780:Sterol Regulatory Element Binding Protein 1; D051782:Sterol Regulatory Element Binding Protein 2; D013997:Time Factors; D014157:Transcription Factors / Q000378:metabolism*; D014452:Ubiquitins / Q000378:metabolism*
D001704:Biopolymers; D022762:CCAAT-Enhancer-Binding Proteins; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D008164:Luminescent Proteins; D009097:Multienzyme Complexes; C497387:SREBF1 protein, human; C497390:SREBF2 protein, human; D051780:Sterol Regulatory Element Binding Protein 1; D051782:Sterol Regulatory Element Binding Protein 2; D014157:Transcription Factors; D014452:Ubiquitins; D025821:Polyubiquitin; D049452:Green Fluorescent Proteins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex