Proteasome-mediated CCAAT/enhancer-binding protein delta (C/EBPdelta) degradation is ubiquitin-independent.
C/EBPdelta (CCAAT/enhancer-binding protein delta) is a member of the C/EBP family of nuclear proteins that function in the control of cell growth, survival, differentiation and apoptosis. We previously demonstrated that C/EBPdelta gene transcription is highly induced in G(0) growth-arrested mammary epithelial cells but the C/EBPdelta protein exhibits a t(1/2) of only approximately 120 min. The goal of the present study was to investigate the role of C/EBPdelta modification by ubiquitin and C/EBPdelta proteasome-mediated degradation. Structural and mutational analyses demonstrate that an intact leucine zipper is required for C/EBPdelta ubiquitination; however, the leucine zipper does not provide lysine residues for ubiquitin conjugation. C/EBPdelta ubiquitination is not required for proteasome-mediated C/EBPdelta degradation and the presence of ubiquitin does not increase C/EBPdelta degradation by the proteasome. Instead, the leucine zipper stabilizes the C/EBPdelta protein by forming homodimers that are poor substrates for proteasome degradation. To investigate the cellular conditions associated with C/EBPdelta ubiquitination we treated G(0) growth-arrested mammary epithelial cells with DNA-damage- and oxidative-stress-inducing agents and found that C/EBPdelta ubiquitination is induced in response to H2O2. However, C/EBPdelta protein stability is not influenced by H2O2 treatment. In conclusion, our results demonstrate that proteasome-mediated protein degradation of C/EBPdelta is ubiquitin-independent.
Zhou
Shanggen
S
The Ohio State Biochemistry Program, Department of Veterinary Biosciences, The Ohio State University, 1925 Coffey Road, Columbus, OH 43210-1093, USA.
Dewille
James W
JW
D000818:Animals; D051743:CCAAT-Enhancer-Binding Protein-delta / Q000378:metabolism*; D002478:Cells, Cultured; D004847:Epithelial Cells; D006367:HeLa Cells; D006801:Humans; D006861:Hydrogen Peroxide / Q000494:pharmacology; D008239:Lysine / Q000235:genetics; D051379:Mice; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D025801:Ubiquitin / Q000502:physiology*
D025801:Ubiquitin; D051743:CCAAT-Enhancer-Binding Protein-delta; D006861:Hydrogen Peroxide; D046988:Proteasome Endopeptidase Complex; D008239:Lysine