SRC-3 Coactivator Functional Lifetime Is Regulated by a Phospho-Dependent Ubiquitin Time Clock

SRC-3/AIB1 is an important growth coactivator whose activity should be tightly regulated since excess activation results in oncogenesis. Herein, we provide evidence that coordinated phosphorylation-dependent ubiquitination regulates SRC-3 coactivator activation and transcriptional specificity. We discovered a critical “actron/degron” element in SRC-3 that is required for this phosphorylation-dependent ubiquitination event and identified GSK3 and SCFFbw7α as the respective responsible kinase and E3 ubiquitin ligase. Interestingly, despite that SCFFbw7α enhances ubiquitination and promotes eventual transcription-coupled degradation of SRC-3 in a phosphorylation- and Fbw7α dosage-dependent manner, our results also uncovered a nonproteolytic “activation” code for SRC-3 ubiquitination induced by Fbw7α. We propose that ubiquitination of SRC-3 is a phospho-mediated biphasic event and that a transition from multi-(mono)ubiquitination (SRC-3 activation) to long-chain polyubiquitination (SRC-3 degradation) is processive during the transcriptional coactivation of select transcription factors and can serve as a “transcriptional time clock” to control both the activation and the functional lifetime of coactivators.