title: Paradoxical instability-activity relationship defines a novel regulatory pathway for retinoblastoma proteins.
abstract: The retinoblastoma (RB) transcriptional corepressor and related family of pocket proteins play central roles in cell cycle control and development, and the regulatory networks governed by these factors are frequently inactivated during tumorigenesis. During normal growth, these proteins are subject to tight control through at least two mechanisms. First, during cell cycle progression, repressor potential is down-regulated by Cdk-dependent phosphorylation, resulting in repressor dissociation from E2F family transcription factors. Second, RB proteins are subject to proteasome-mediated destruction during development. To better understand the mechanism for RB family protein instability, we characterized Rbf1 turnover in Drosophila and the protein motifs required for its destabilization. We show that specific point mutations in a conserved C-terminal instability element strongly stabilize Rbf1, but strikingly, these mutations also cripple repression activity. Rbf1 is destabilized specifically in actively proliferating tissues of the larva, indicating that controlled degradation of Rbf1 is linked to developmental signals. The positive linkage between Rbf1 activity and its destruction indicates that repressor function is governed in a manner similar to that described by the degron theory of transcriptional activation. Analogous mutations in the mammalian RB family member p107 similarly induce abnormal accumulation, indicating substantial conservation of this regulatory pathway.
authors.affiliation: Department of Microbiology and Molecular Genetics, Program in Genetics, Michigan State University, East Lansing, MI 48824-1319, USA.
authors.forename: Pankaj
authors.initials: P
authors.lastname: Acharya
authors.forename: Nitin
authors.initials: N
authors.lastname: Raj
authors.forename: Martin S
authors.initials: MS
authors.lastname: Buckley
authors.forename: Liang
authors.initials: L
authors.lastname: Zhang
authors.forename: Stephanie
authors.initials: S
authors.lastname: Duperon
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Williams
authors.forename: R William
authors.initials: RW
authors.lastname: Henry
authors.forename: David N
authors.initials: DN
authors.lastname: Arnosti
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D030801:Animals, Genetically Modified; D015153:Blotting, Western; D002460:Cell Line; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism*; D004331:Drosophila melanogaster / Q000166:cytology / Q000235:genetics / Q000378:metabolism; D050684:E2F Transcription Factors / Q000378:metabolism; D005123:Eye / Q000254:growth & development / Q000378:metabolism; D005260:Female; D007814:Larva / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D007976:Leupeptins / Q000494:pharmacology; D008297:Male; D008969:Molecular Sequence Data; D009154:Mutation; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding; D055550:Protein Stability; D012045:Regulatory Sequences, Nucleic Acid / Q000235:genetics; D016160:Retinoblastoma Protein / Q000235:genetics / Q000378:metabolism; D050720:Retinoblastoma-Like Protein p107 / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*
chemical_list: D015853:Cysteine Proteinase Inhibitors; D029721:Drosophila Proteins; D050684:E2F Transcription Factors; D007976:Leupeptins; C099965:Rbf protein, Drosophila; D016160:Retinoblastoma Protein; D050720:Retinoblastoma-Like Protein p107; D014157:Transcription Factors; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde