An extended LXXLL motif sequence determines the nuclear receptor binding specificity of TRAP220.

The interaction of coactivators with the ligand-binding domain of nuclear receptors (NRs) is mediated by amphipathic alpha-helices containing the signature motif LXXLL. TRAP220 contains two LXXLL motifs (LXM1 and LXM2) that are required for its interaction with NRs. Here we show that the nuclear receptor interaction domain (NID) of TRAP220 interacts weakly with Class I NRs. In contrast, SRC1 NID binds strongly to both Class I and Class II NRs. Interaction assays using nine amino acid LXXLL core motifs derived from SRC1 and TRAP220 revealed no discriminatory NR binding preferences. However, an extended LXM1 sequence containing amino acids -4 to +9, (where the first conserved leucine is +1) showed selective binding to thyroid hormone receptor and reduced binding to estrogen receptor. Replacement of either TRAP220 LXXLL motif with the corresponding 13 amino acids of SRC1 LXM2 strongly enhanced the interaction of the TRAP220 NID with the estrogen receptor. Mutational analysis revealed combinatorial effects of the LXM1 core and flanking sequences in the determination of the NR binding specificity of the TRAP220 NID. In contrast, a mutation that increased the spacing between TRAP220 LXM1 and LXM2 had little effect on the binding properties of this domain. Thus, a 13-amino acid sequence comprising an extended LXXLL motif acts as the key determinant of the NR binding specificity of TRAP220. Finally, we show that the NR binding specificity of full-length TRAP220 can be altered by swapping extended LXM sequences.