P300, AND P300-ASSOCIATED PROTEINS, ARE COMPONENTS OF TATA-BINDING PROTEIN (TBP) COMPLEXES

The transforming proteins encoded by the adenovirus E1A gene bind to a 300-kDa cellular product, p300, via the N-terminal E1A sequences. Res idues important for p300 binding are required for the transformation f unction of E1A and for other E1A-mediated gene-regulating functions, i ncluding activation of cell cycle-regulated products and repression of tissue-specific enhancer activity. Recent evidence indicates that p30 0 is a DNA-binding protein with specific affinity for known enhancer m otifs, suggesting that p300 may be a component of transcription factor complexes. The possibility that upstream element-binding factors migh t interact with basal transcription factors led us to investigate whet her p300 interacts, directly or indirectly, with the TATA-binding prot ein (TBP). We report here that TBP-specific immunoprecipitations show a 300-kDa protein co-precipitating with TBP. This protein is lost from the precipitated material if the lysates are boiled in sodium dodecyl sulfate prior to immunoprecipitation, implying that its presence does not result from non-specific antibody cross-reactivity, but is depend ent on specific association with TBP. The TBP-associated 300-kDa prote in and p300 originally defined by EIA association show indistinguishab le partial proteolytic digest patterns, indicating that these are iden tical or closely related species. Moreover, p300-specific complexes an d TBP-specific complexes include at least two additional common polype ptide species, phosphoproteins of 64 and 59 kDa. These results suggest that p300 interacts with TBP, possibly through intermediate protein-p rotein associations. They thus provide additional biochemical evidence for postulated protein-protein interactions between upstream regulato ry factors and the basal transcriptional machinery.