Core LXXLL motif sequences in CREB-binding protein, SRC1, and RIP140 define affinity and selectivity for steroid and retinoid receptors

An alpha -helical motif containing the sequence LXXLL is required for the l igand-dependent binding of transcriptional co-activators to nuclear recepto rs. By using a peptide inhibition assay, we have defined the minimal "core" LXXLL motif as an 8-amino acid sequence spanning positions -2 to +6 relati ve to the primary conserved leucine residue. In yeast two-hybrid assays, co re LXXLL motif sequences derived from steroid receptor co-activator (SRC1), the 140-kDa receptor interacting protein (RIP140), and CREB-binding protei n (CBP) displayed differences in selectivity and affinity for nuclear recep tor ligand binding domains. Although core LXXLL motifs from SRC1 and RIP140 mediated strong interactions with steroid and retinoid receptors, three LX XLL motifs present in the global co-activator CBP were found to have very w eak affinity for these proteins. Core motifs with high affinity for steroid and retinoid receptors were generally found to contain a hydrophobic resid ue at position -1 relative to the first conserved leucine and a nonhydropho bic residue at position +2. Our results indicate that variant residues in L XXLL core motifs influence the affinity and selectivity of co-activators fo r nuclear receptors.