Differences between the silencing-related properties of the extreme carboxyl-terminal regions of thyroid hormone receptors alpha 1 and beta 1

Human thyroid hormone receptor (TR) is encoded by two distinct genes, TR al pha and TR beta. TR heterodimerizes with retinoid X receptor (RXR) and bind s efficiently to the thyroid hormone (T-3) response element (TRE) of target genes. In the absence of T-3, unliganded TR suppresses the basal promoter activity of positively regulated genes (silencing). Silencing mediator for retinoid and thyroid hormone receptors (SMRT) and nuclear receptor co-repre ssor (N-CoR) interact with unliganded TR and function as corepressor protei ns. Previously, we found beta F451X with carboxyl (C)-terminal 11-amino aci d deletion had stronger silencing potency than wild-type TR beta1 and beta E449X with C-terminal 13-amino acid deletion on a subset of TREs. In the pr esent study, to assess the isoform-specific effects of the C-terminal trunc ations on TR silencing, we constructed two mutant TR alpha 1s (alpha F397X and alpha E395X) with the same respective C-terminal truncations as beta F4 51X and beta E449X and analysed their silencing activities. Unlike beta F45 1X and beta E449X, alpha F397X and alpha E395X showed similarly stronger si lencing potency than wild-type TR alpha1. We further studied the abilities of wild-type and the mutant TR beta 1s and als on RXR and co-repressor bind ing by a two-hybrid interference assay. beta F451X had significantly strong er abilities to bind to RXR and SMRT than did wild-type TR beta1 and beta E 449X. In contrast, wild-type TR alpha1, alpha F397X and alpha E395X showed similar abilities to bind to RXR and SMRT. beta E449X and alpha E395X, whic h have identical C-terminal truncation, showed less ability to bind to N-Co R than did wild-type TR beta1 and beta F451X and wild-type TR alpha1 and al pha F397X respectively. These results indicate that an identical C-terminal truncation gives rise to different effects on TR beta1 and alpha1 with res pect to silencing potency, RXR binding and SMRT binding. The difference in the silencing potency among wild-type TR beta1, beta F451X and beta E449X c orrelated well with the difference in the ability to bind co-repressor SMRT .