Synergistic, p160 coactivator-dependent enhancement of estrogen receptor function by CARM1 and p300

Members of the p160 coactivator family (steroid receptor coactivator-l (SRC -1), glucocorticoid receptor interacting protein 1 (GRIP1), and activator o f thyroid and retinoic acid receptors (ACTR)) mediate transcriptional activ ation by nuclear receptors, After being recruited to the promoter by nuclea r receptors, the p160 coactivator transmits the activating signal via two C -terminal activation domains, AD1 and AD2, AD1 is a binding site for the re lated coactivators cAMP-response element binding protein binding protein (C BP) and p300, whereas AD2 binds to another coactivator, coactivator-associa ted arginine methyltransferase 1 (CARM1), a protein-arginine methyltransfer ase. The current study explored the cooperative functional and mechanistic relationships among GRIP1, CARM1, and p300 in transient transfection assays , where they enhanced the ability of the estrogen receptor (ER) to activate transcription of a reporter gene. The coactivator functions of p300 and CA RM1 depended on the co-expression of GRIP1. Simultaneous co-expression of a ll three coactivators caused a synergistic enhancement of ER function. Dele tion of the AD1 domain of GRIP1 abolished the ability of p300 to potentiate ER activity but had no effect on CARM1-mediated stimulation. In contrast, when the AD2 domain of GRIP1 was deleted, p300 still stimulated ER function through the mutant GRIP1, but CARM1 failed to do so. Thus, both binding of p300 to AD1 and binding of CARM1 to AD2 are required for their respective coactivator functions and for their synergy. Furthermore, CARM1 and p300 fu nction independently through different activating domains of GRIP1, and the ir synergy suggests that they enhance transcription by different, complemen tary. mechanisms.