Paradoxical Instability–Activity Relationship Defines a Novel Regulatory Pathway for Retinoblastoma Proteins

Abstract

Functional overlap of retinoblastoma protein stability and activity reveals a novel conserved regulatory pathway during Drosophila development.

The Evolutionarily Conserved C-terminal Domains in the Mammalian Retinoblastoma Tumor Suppressor Family Serve as Dual Regulators of Protein Stability and Transcriptional Potency

“…Our previous studies in Drosophila further demonstrated that Rbf1 is less stable in its active state (31,42), and we surmised that mammalian RB family proteins likewise become destabilized as they engage target genes in differentiating cells during G 1 phase. To test this hypothesis, the levels of RB family proteins before and after RA treatment were compared with their genomic binding at endogenous target genes.…”

“…Previous studies from our laboratory showed that the Drosophila melanogaster RB homologue Rbf1 is subjected to proteasome-mediated turnover during embryonic development (31,32). We further demonstrated that Rbf1 turnover is influenced by an "instability element" (IE) located within its C-terminal regulatory domain.…”

“…In Drosophila, the Rbf1 homolog harbors an IE within its C-terminal domain that contributes to both repressor activity and destruction (31,33). As indicated by the alignments shown in Fig.…”

“…We further demonstrated that Rbf1 turnover is influenced by an "instability element" (IE) located within its C-terminal regulatory domain. Importantly, the IE region is also critical for full repressor potency for some cell cycle-regulated genes but not for non-canonical targets whose expression is not usually integrated with the cell cycle (31,33). Interestingly, Rbf1 ubiquitination also enhanced specific activity at select cell cycle target genes (33), suggesting that the potency of the repressor at specific genes and overall Rbf1 stability are coordinated.…”

“…Drosophila suggested that Rbf1 turnover and function are linked during embryonic development (31,32). We, therefore, examined the behavior of mammalian RB family members in pluripotent selfrenewing mouse ES cells before and after differentiation.…”