Notch signaling pathway plays a fundamental role in the regulation of various cellular and developmental processes. Ligand binding releases the intracellular domain of Notch receptor (NICD), which interacts with transcription factor CSL to activate transcription of target genes. In the absence of NICD binding, CSL represses gene expression via the direct recruitment of corepressor proteins including SMRT and KyoT2. Structural and functional analyses revealed the molecular basis of these interactions, in which NICD and corepressor proteins competitively bind to β-trefoil domain (BTD) of CSL using a conserved ΦWΦP motif (Φ: any hydrophobic residues). We previously reported that a ΦWΦP motif (1834IWRP1837) of SMRT directly interacts with CSL-BTD. Here, we performed systematic analyses of the binding surface of CSL to SMRT, KyoT2, and NICD using diverse CSL-BTD mutants defective in SMRT binding. Among CSL mutants, the F235, A258 and P260 residues are generally required for these interactions by providing a non-polar binding surface for ΦWΦP motifs. In contrast, L225F, E231G, and E233G mutants evidenced different binding profiles depending on BTD-binders, suggesting these residues as the specific requirements for their interactions. These results demonstrate that residues within and/or outside the core ΦWΦP motifs of coregulators significantly affect high-affinity binding to CSL and that binding interfaces between CSL-BTD and its binders are rather flexible.
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