The interaction of TRβ1-N terminus with steroid receptor coactivator-1 (SRC-1) serves a full transcriptional activation function of SRC-1

Abstract

Steroid receptor coactivator-1 (SRC-1) plays a crucial role in nuclear receptor-mediated transcription including thyroid hormone receptor (TR)-dependent gene expression. Interaction of the TR-ligand binding domain and SRC-1 through LXXLL motifs is required for this action. However, potential interactions between the TRβ1-N terminus (N) and SRC-1 have not been explored and thus are examined in this manuscript. Far-Western studies showed that protein construct containing TRβ1-N + DNA binding domain (DBD) bound to nuclear receptor binding domain (NBD)-1 (amino acid residue, aa 595-780) of SRC-1 without ligand. Mammalian two-hybrid studies showed that NBD-1, as well as SRC-1 (aa 595-1440), bound to TRβ1-N+DBD in the absence of ligand in CV-1 cells. However, NBD-2 (aa 1237-1440) did not bind to this protein. Glutathione-S-transferase pull-down studies showed that TRβ1-N (aa 1-105) bound to the broad region of SRC-1-C terminus. Expression vectors encoding a series of truncations and/or point mutations of TRβ1 were used in transient transfection-based reporter assays in CV-1 cells. N-terminal truncated TRβ1 (δN-TRβ1) showed lower activity than that of wild-type in both artificial F2-thyroid hormone response element and native malic enzyme response element. These results suggest that there is the interaction between N terminus of TRβ1 and SRC-1, which may serve a full activation of SRC-1, together with activation function-2 on TRβ1-mediated transcription. Copyright © 2006 by The Endocrine Society.