Integrated stability and activity control of the Drosophila Rbf1 retinoblastoma protein.
The retinoblastoma (RB) family transcriptional corepressors regulate diverse cellular events including cell cycle, senescence, and differentiation. The activity and stability of these proteins are mediated by post-translational modifications; however, we lack a general understanding of how distinct modifications coordinately impact both of these properties. Previously, we showed that protein turnover and activity are tightly linked through an evolutionarily conserved C-terminal instability element (IE) in the Drosophila RB-related protein Rbf1; surprisingly, mutant proteins with enhanced stability were less, not more active. To better understand how activity and turnover are controlled in this model RB protein, we assessed the impact of Cyclin-Cdk kinase regulation on Rbf1. An evolutionarily conserved N-terminal threonine residue is required for Cyclin-Cdk response and showed a dominant impact on turnover and activity; however, specific residues in the C-terminal IE differentially impacted Rbf1 activity and turnover, indicating an additional level of regulation. Strikingly, specific IE mutations that impaired turnover but not activity induced dramatic developmental phenotypes in the Drosophila eye. Mutation of the highly conserved Lys-774 residue induced hypermorphic phenotypes that mimicked the loss of phosphorylation control; mutation of the corresponding codon of the human RBL2 gene has been reported in lung tumors. Our data support a model in which closely intermingled residues within the conserved IE govern protein turnover, presumably through interactions with E3 ligases, and protein activity via contacts with E2F transcription partners. Such functional relationships are likely to similarly impact mammalian RB family proteins, with important implications for development and disease.
['Zhang|Liang|L|', 'Wei|Yiliang|Y|', 'Pushel|Irina|I|', 'Heinze|Karolin|K|', 'Elenbaas|Jared|J|', 'Henry|R William|RW|', 'Arnosti|David N|DN|']
D000818:Animals D030801:Animals, Genetically Modified D001665:Binding Sites D015153:Blotting, Western D002460:Cell Line D018844:Cyclin-Dependent Kinases D016213:Cyclins D029721:Drosophila Proteins D005123:Eye D006801:Humans D008239:Lysine D008854:Microscopy, Electron D009154:Mutation D010766:Phosphorylation D055550:Protein Stability D016160:Retinoblastoma Protein D012694:Serine D013912:Threonine D014157:Transcription Factors D014921:Wings, Animal
Drosophila Phosphorylation Retinoblastoma Protein (pRb, RB) Transcription Repressor Tumor Suppressor Gene