Signaling within a Coactivator Complex: Methylation of SRC-3/AIB1 Is a Molecular Switch for Complex Disassembly

Abstract

Recent studies indicate that steroid receptor-mediated transcriptional initiation is a cyclical process involving multiple rounds of coactivator assembly and disassembly. Steroid receptor coactivator 3 (SRC-3) coactivator phosphorylation has been shown to regulate coactivator complex assembly, but the mechanisms by which coactivator disassembly is triggered are not well understood. In this study, we provide in vitro and in vivo evidence that members of the SRC coactivator family serve as substrates for the enzymatic coactivator coactivator-associated arginine methyltransferase 1 (CARM1). Methylation of SRC-3 was localized to an arginine in its CARM1 binding region and correlated with decreased estrogen receptor alpha-mediated transcription, as seen with both cell-based and in vitro transcription assays. Consistent with this finding, we demonstrated that methylation promotes dissociation of the SRC-3/CARM1 coactivator complex. Methylation of SRC-3 is regulated by estrogen signaling in MCF7 cells and serves as a molecular switch for disassembly of the SRC-3 transcriptional coactivator complex. We propose that CARM1 is a dual-function coactivator, as it not only activates transcription by modifying core histone tails but also terminates hormone signaling by disassembly of the coactivator complex.

We thank Charles Foulds and Bin He for critical reading of the manuscript. We also thank Mark Bedford for CARM1 wild-type and knockout MEFs, Michael R. Stallcup for the pSG5-HA-CARM1 plasmid, Dean Edwards for ER alpha and PR-A proteins, Steven Nordeen for SRC-1 recombinant protein, W. Lee Kraus for ERE-E4 plasmid and p300 baculovirus, and Sophia Tsai and Ming-Jer Tsai for helpful discussion.

This work was supported by grants from NIH/NICHD, Welch Foundation, and NIDDK-NURSA (B.W.O.).