Functional interactions between the retinoblastoma (Rb) protein and Sp-family members: superactivation by Rb requires amino acids necessary for growth suppression.
The transient expression of the retinoblastoma protein (Rb) regulates the transcription of a variety of growth-control genes, including c-fos, c-myc, and the gene for transforming growth factor beta 1 via discrete promoter sequences termed retinoblastoma control elements (RCE). Previous analyses have shown that Sp1 is one of three RCE-binding proteins identified in nuclear extracts and that Rb functionally interacts with Sp1 in vivo, resulting in the "superactivation" of Sp1-mediated transcription. By immunochemical and biochemical criteria, we report that an Sp1-related transcription factor, Sp3, is a second RCE-binding protein. Furthermore, in transient cotransfection assays, we report that Rb "superactivates" Sp3-mediated RCE-dependent transcription in vivo and that levels of superactivation are dependent on the trans-activator (Sp1 or Sp3) studied. Using expression vectors carrying mutated Rb cDNAs, we have identified two portions of Rb required for superactivation: (i) a portion of the Rb "pocket" (amino acids 614-839) previously determined to be required for physical interactions between Rb and transcription factors such as E2F-1 and (ii) a novel amino-terminal region (amino acids 140-202). Since both of these regions of Rb are targets of mutation in human tumors, our data suggest that superactivation of Sp1/Sp3 may play a role in Rb-mediated growth suppression and/or the induction of differentiation.
Udvadia
A J
AJ
Department of Molecular Cancer Biology, Duke University Medical Center, Durham, NC 27710, USA.
Templeton
D J
DJ
Horowitz
J M
JM
D000596:Amino Acids / Q000378:metabolism*; D000818:Animals; D002455:Cell Division; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004331:Drosophila melanogaster; D005260:Female; D005786:Gene Expression Regulation; D016762:Genes, fos; D016259:Genes, myc; D005822:Genetic Vectors; D006801:Humans; D009687:Nuclear Proteins / Q000096:biosynthesis / Q000378:metabolism; D015345:Oligonucleotide Probes; D010957:Plasmids; D011401:Promoter Regions, Genetic; D016160:Retinoblastoma Protein / Q000378:metabolism*; D016329:Sp1 Transcription Factor / Q000096:biosynthesis / Q000378:metabolism*; D051705:Sp3 Transcription Factor; D014157:Transcription Factors / Q000378:metabolism*; D014158:Transcription, Genetic; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000096:biosynthesis / Q000235:genetics; D014407:Tumor Cells, Cultured; D002583:Uterine Cervical Neoplasms
D000596:Amino Acids; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D015345:Oligonucleotide Probes; D016160:Retinoblastoma Protein; C497121:SP3 protein, human; D016329:Sp1 Transcription Factor; D014157:Transcription Factors; D016212:Transforming Growth Factor beta; D051705:Sp3 Transcription Factor