CCAAT/Enhancer-binding Protein β DNA Binding Is Auto-inhibited by Multiple Elements That Also Mediate Association with p300/CREB-binding Protein (CBP)

摘要

Signaling through Ras GTPases controls the activity of many transcription factors including CCAAT/enhancer-binding protein (C/EBPβ), which regulates oncogenic H-Ras^V12-induced senescence and growth arrest. Here we report that C/EBPβ (LAP) DNA binding is inhibited by N-terminal sequences and derepressed by oncogenic Ras signaling. Sequence and mutational analyses showed that auto-repression involves two LXXLF (φXXφφ)-like motifs (LX1 and LX2) and a third element, auto-inhibitory domain (AID), located within conserved region CR5. LX1 is a critical component of the transactivation domain and has been shown to mediate C/EBPβ binding to the TAZ2 region of p300/CREB-binding protein coactivators. C/EBPβ auto-repression also involves a C-terminal regulatory domain (CRD) adjacent to the leucine zipper. CRD contains a third φXXφφ motif (LX3) and a short sequence, KQL, which has similarity to a region in the protein-binding site of TAZ2. The C/EBPβ N- and C-terminal domains physically associate in a manner that requires the basic region and CRD. We propose a model in which the regulatory sequences form a hydrophobic core that reciprocally inhibits DNA binding and transactivation. We also suggest a mechanism for C/EBPβ derepression involving several recently identified modifications within AID and CRD. Finally, we show that association of activated C/EBPβ with p300/CREB-binding protein requires the LX2 and AID auto-inhibitory elements. Thus, the N-terminal regulatory elements have dual roles in auto-inhibition and coactivator binding.