Phosphorylation and ubiquitination of the transcription factor sterol regulatory element-binding protein-1 in response to DNA binding

Members of the sterol regulatory element-binding protein (SREBP) family of transcription factors control cholesterol and lipid metabolism and play critical roles during adipocyte differentiation. The transcription factor SREBP1 is degraded by the ubiquitin-proteasome system following phosphorylation of Thr⁴²⁶ and Ser⁴³⁰ in its phosphodegron. We now demonstrate that the glycogen synthase kinase (GSK)-3β-dependent phosphorylation of these residues in SREBP1 is enhanced in response to specific DNA binding. DNA binding enhances the direct interaction between the C-terminal domain of SREBP1 and GSK-3β. Accordingly, we demonstrate that GSK-3β is recruited to the promoters of SREBP target genes in vivo. As a result of the phosphorylation of Thr⁴²⁶ and Ser⁴³⁰, the ubiquitin ligase Fbw7 is recruited to SREBP molecules associated with target promoters. Using a reconstituted ubiquitination system, we demonstrate that Fbw7-mediated ubiquitination of SREBP1 is dependent on its DNA binding activity. Thus, DNA binding could provide a mechanistic link between the phosphorylation, ubiquitination, and degradation of active transcription factors.