Structural Insights of Transcriptionally Active, Full-Length Androgen Receptor Coactivator Complexes

Abstract

Steroid hormone receptors activate gene transcription by binding specific DNA sequences and recruiting coactivators to initiate transcription of their target genes. For most nuclear hormone receptors (NRs), the ligand-dependent activation function domain-2 (AF-2), residing in the C-terminal ligand binding domain (LBD), is a primary contributor to the NR transcriptional activity. In contrast to other steroid receptors such as estrogen receptor-α (ERα), the transcriptional activation function of androgen receptor (AR) is thought to be largely dependent on its ligand-independent activation function domain-1 (AF-1) located in its N-terminal domain (NTD). It remains unclear why AR utilizes a different activation function domain from other steroid receptors despite the fact that NRs share similar domain organizations and have a highly conserved DNA binding domain (DBD) and LBD. Here we present cryo-electron microscopic structures of DNA-bound full-length AR and its functional complex structure with its key coactivators, steroid receptor coactivator-3 (SRC-3) and the histone acetyltransferase p300. The structures reveal that androgen-bound full-length AR dimerization follows a unique head-to-head and tail-to-tail manner with all of the domains involved. The DBD and LBD are located at the center of the dimer interface. The NTDs wrap around the LBDs through their unique intra- and inter-molecular N- and C-terminal interactions and connect to each other. We observe that unlike ERα, AR binds a single SRC-3 molecule along with a single p300 molecule. The AR NTD appears to be the primary site for recruitment of both coactivators. The N-terminal region of SRC-3, rather than its receptor interaction domain, is important for this interaction. The structures presented here highlight the importance of the AR NTD for its transcriptional functions and provide a structural basis for understanding the assembly of the AR:coactivator complex and its domain contributions to transcriptional regulation.