Studies of the Retinoblastoma tumor suppressor protein (RB) have been at the forefront of cancer research as its loss of function has been implicated in a diverse profile of deadly human cancers. In this study, we describe research that has contributed to our understanding of the regulation of the RB network by the ubiquitin-proteasome system. Through studies on the Drosophila melanogaster RB family homolog proteins, Rbf1 and Rbf2, we uncovered a novel regulatory pathway that governs their function as transcriptional repressors of diverse gene sets. First, we showed that a C-terminal autonomous degron, which we termed the Instability Element (IE), directs Rbf1 ubiquitination and mediates its gene-specific repression functions. The Rbf1 degron-mediated ubiquitination was paradoxically found to be a critical component for enhancing proteasome-mediated Rbf1 degradation as well as potentiating Rbf1-mediated repression of a subset of its target genes. Thus, this study uncovered a direct role for Rbf1 ubiquitination in Rbf1 transcriptional repression and adds a key piece to the long unresolved puzzle as to how RB proteins simultaneously regulate mutually exclusive cellular processes such as cell-cycle progression and apoptosis. Interestingly, previous studies have linked gene activation to protein degradation via the promoter-associated proteasome. Hence, our findings suggest that Rbf repression similarly involves the proteasome and this intricate instability-activity relationship potentially provides regulatory responsiveness to changes in environmental conditions. These finding are also relevant to the mammalian RB family proteins as the Rbf1 degron was found to be evolutionarily conserved both in terms of its structure and roles in protein turnover and repression. Second, we show that in case of Rbf2, the evolutionarily conserved pocket domain enhances its ubiquitin-proteasome-mediated degradation. Additionally, unlike Rbf1, the Rbf2 N-terminus which harbors a conserved `Domain of unknown function (DUF)', as well as the pocket domain are required for Rbf2-mediated repression of cell-cycle promoters. Thus, the two Drosophila RB family proteins utilize distinct protein domains to enact their roles in the regulation of cell cycle.Furthermore, we show evidence that mutation of the Rbf1 degron, unexpectedly, leads to an enhanced rate of cellular DNA replication due to its unique ability to stabilize but not inhibit the activator protein, dE2F1. Thus, through a gain of function, the Rbf1 degron dysfunction has the potential to convert the tumor suppressor into an oncoprotein. This important observation is especially relevant in the context of cancer cells where certain mutations in RB family degron may provide a selective growth advantage. In such cancers, an additional therapeutic intervention may be required to counteract the effects of the rogue RB family alleles.
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