Retinoid-dependent transcription: the RAR/RXR-TBP-EIA/EIA-LA connection

Retinoids play a fundamental role in regulating normal cell proliferation and differentiation. The most spectacular effects of retinoids in vitro can be observed with embryonal carcinoma (EC) cells that can be induced to differentiate into endodermal, mesodermal and neuroectodermal lineages. An early and essential step in the differentiation process is the activation of the retinoic acid receptor-beta 2 (RAR beta 2) promoter that requires a co-operation between RAR, the EC-cell specific adenovirus early gene product 1A (E1A)-like activity and the TATA-binding protein (TBP). In differentiated cells, this signalling pathway can be mimicked by ectopic expression of the adenoviral E1A protein. Here we show that E1A13S but not E1A12S augments the level of transcription. Analysis of the binding kinetics of E1A13S to TBP by the surface plasmon resonance (SPR) technique reveals that the affinity of TBP for a consensus TATA-box sequence is significantly and specifically increased by E1A13S only. Intriguingly, a specific interaction can only be obtained with crude TBP overexpressed in HeLa cells via vaccinia virus as opposed to bacterially expressed TBP, suggesting a cofactor requirement for the interaction. Co-immunoprecipitation experiments show that E1A13S is an integral component of the RNA polymerase II-specific TBP-containing complex in adenovirus transformed embryonal kidney 293 cells. Taken together the results suggest that E1A13S mediates transcriptional activation by providing a physical bridge between TBP/transcription factor IID (TFIID) and retinoic acid receptor.