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.