Protein modification with ubiquitin (Ub) confers proteasome-mediated and non-degradative functions. In particular, protein monoubiquitination (monoUbq) is a non-proteolytic signal, which is decoded by specific non-covalent Ub-binding surfaces often located within monoubiquitinated proteins. In this way, protein monoUbq creates a network of signalling interactions that regulates several physiological processes including cell proliferation.

Among other pleiotropic effects, 17beta-estradiol (E2) binds to the estrogen receptor alpha (ERa) and regulates cell proliferation through nuclear (gene transcription) and extranuclear membrane-initiated (activation of signalling cascades) effects. MonoUbq of the ERa exists and seems to contribute to the ERa nuclear effects. Nonetheless, the monoUbq-dependent regulation of the ERa extranuclear activities that lead to the activation of cell proliferation as well as the ability of E2 to modulate receptor monoUbq are not known. Thus, we sought to determine the impact of monoUbq in the regulation of the E2:ERa-dependent cell proliferation.

Here, we show that ERa monoUbq endogenously occurs and is negatively modulated by E2. Furthermore, mutation of the ERa monoUbq sites prevents the E2:ERa-mediated activation of signalling pathways to cell proliferation. In addition, a previously unrecognized Ub-binding surface has been found within the ERa and could contribute to the E2 proliferation effects. Altogether, these data indicate that the ERa belongs to the Ub-based signalling network and that monoUbq regulates ERa activities in a non-proteolytic manner, thus demonstrating the physiological function of ERa monoUbq in the regulation of the E2-induced cell proliferation.