Identification of an estrogen receptor a non covalent ubiquitin-binding surface: role in 17b-estradiol- induced transcriptional activity

Ubiquitin (Ub)-binding domains (UBDs) located in Ub receptors decode the ubiquitination signal by non-covalently engaging the Ub modification on their binding partners and transduce the Ub signalling through Ub-based molecular interactions. In this way, inducible protein ubiquitination regulates diverse biological processes. The estrogen receptor alpha (ERa) is a ligand-activated transcription factor that mediates the pleiotropic effects of the sex hormone 17b-estradiol (E2). Fine regulation of E2 pleiotropic actions depends on E2dependent ERa association with a plethora of binding partners and/or on the E2 modulation of receptor ubiquitination. Indeed, E2induced ERa polyubiquitination triggers receptor degradation and transcriptional activity, and E2-dependent reduction in ERa monoubiquitination is crucial for E2 signalling. Monoubiquitinated proteins often contain UBDs, but whether non-covalent Ub–ERa binding could occur and play a role in E2–ERa signalling is unknown. Here, we report an Ub-binding surface within the ERa ligand binding domain that directs in vitro the receptor interaction with both ubiquitinated proteins and recombinant Ub chains. Mutational analysis reveals that ERa residues leucine 429 and alanine 430 are involved in Ub binding. Moreover, impairment of ERa association to ubiquitinated species strongly affects E2-induced ERa transcriptional activity. Considering the importance of UBDs in the Ub-based signalling network and the central role of different ERa binding partners in the modulation of E2-dependent effects, our discoveries provide novel insights into ERa activity that could also be relevant for ERa-dependent diseases.