Activation Domains from Both Monomers Contribute to Transcriptional Stimulation by Sterol Regulatory Element-binding Protein Dimers

Sterol regulatory element-binding proteins (SREBPs) are basic helix-loop-helix leucine zipper proteins that act as dimers to activate genes in lipid metabolism. Three SREBP isoforms, 1a, 1c, and 2, are expressed at varying levels in different tissues. Thus, homo- and heterodimers probably contribute to overall SREBP activity. No studies have directly evaluated the formation or activation properties of SREBP homo- and heterodimers. Studies with overexpressed SREBP monomers are inconclusive regarding the function of a particular SREBP dimer because of potential dimerization with endogenous proteins. To assess activation by a particular SREBP dimer, we fused DNA encoding individual monomers together via a predicted flexible polypeptide tether. Tethered SREBP dimers bound DNA equivalently to the monomeric proteins and were resistant to dominant negative SREBP-1 inhibition, confirming preferential formation of intramolecular dimers. Tethered SREBP-1a and -2 homodimers, similar to the monomeric forms, activated target genes more robustly than tethered SREBP-1c homodimers. A forced SREBP-1a/2 heterodimer had similar activity to the respective homodimers. However, SREBP-1c in a heterodimer with either SREBP-1a or -2 attenuated the activity relative to the SREBP-1a or -2 homodimers. These experiments provide some of the first data showing that the integrity of both activation domains in a dimeric transcription factor is required for maximal activity. In addition, the results support a model where changes in SREBP-1c protein expression that occur in response to insulin signaling and liver X receptor signaling would be predicted to increase or decrease overall SREBP activity in a tissue-specific fashion depending on the initial fractional contribution of SREBP-1c to total cellular levels of SREBP.