The p160 Coactivator PAS-B Motif Stabilizes Nuclear Receptor Binding and Contributes to Isoform-specific Regulation by Thyroid Hormone Receptors

Abstract

Thyroid hormone receptors (TRs) are hormone-regulated transcription factors that play multiple roles in vertebrate endocrinology and development. TRs are expressed as a series of distinct receptor isoforms that mediate different biological functions. The TR␤2 isoform is expressed primarily in the hypothalamus, pituitary, cochlea, and retina, and displays an enhanced response to hormone agonist relative to the other TR isoforms. We report here that the unusual transcriptional properties of TR␤2 parallel the ability of this isoform to bind p160 coactivators cooperatively through multiple contact surfaces; the more broadly expressed TR␤1 isoform, in contrast, utilizes a single contact mechanism. Intriguingly, the PAS-B domain in the p160 N terminus plays a previously unanticipated role in permitting TR␤2 to recruit coactivator at limiting triiodothyronine concentrations. The PAS-B sequences also play an important role in coactivator binding by estrogen receptor-␣. We propose that the PAS-B domain of the p160 coactivators is an important modulator of coactivator recruitment for a specific subset of nuclear receptors, permitting stronger transcriptional activation at lower hormone concentrations than would otherwise occur, and allowing isoform-specific mRNA splicing to customize the hormone response in different tissues.

A Mechanism for Pituitary-Resistance to Thyroid Hormone (PRTH) Syndrome: a Loss in Cooperative Coactivator Contacts by Thyroid Hormone Receptor (TR)β2

“…6A, black lines, and Table 1). Analogous results were observed for the two other p160 coactivators, GR-interacting protein (GRIP)1 and activator of thyroid and retinoic acid receptor (ACTR), whereas both TR␤ isoforms bound equally to the p220 Mediator coactivator (29). All three RTH mutations tested (R338L, R429Q, and P453S) impaired the EC 50 of both TR␤1 and TR␤2 in this assay (Fig.…”

“…(SRC1) (1-1441) compared with wtTR␤1 in a glutathione-S-transferase (GST)-pull-down assay (29), and this was observed both as a higher maximum binding (B max ) and a lower EC 50 (Fig. 6A, black lines, and Table 1).…”

“…In contrast, wtTR␤2 displays an enhanced ability to bind to certain coactivators in vitro relative to wtTR␤1 (28,29,31,32,34). We therefore investigated the relationship of coactivator binding to the transcriptional properties of TR␤2 in detail, focusing on the contributions of the charge clusters to this phenomenon.…”

“…TR␤2, but not TR␤1, binding to the p160 LXXLL motifs is also stabilized by a PAS-B domain within these coactivators that can operate independently or in conjunction with the Q-rich region (29). This stabilization requires the integrity of the TR␤2 N-terminal domain and can be observed in both the absence and presence of T 3 .…”

Estrogen Receptors Recruit SMRT and N-CoR Corepressors through Newly Recognized Contacts between the Corepressor N Terminus and the Receptor DNA Binding Domain

“…The eluted proteins were resolved by SDS-PAGE and were visualized and quantified by using a Storm PhosphorImager system (GE Healthcare Bio-Sciences Corp., Piscataway, NJ). Coimmunoprecipitation assays were performed as described previously (50).…”