MHC class II enhanceosome: how is the class II transactivator recruited to DNA-bound activators?

MHC class II (MHCII) determinants play a crucial role in the immune response by presenting antigenic peptides to T cells. Their expression is controlled from compact promoters at the transcriptional level. Pre-assembled regulatory factor X (RFX) and nuclear factor Y (NFY) complexes form a platform on DNA. The class II transactivator (CIITA) can then be recruited through multiple protein±protein interactions. In this report, we de®ned domains of CIITA that are responsible for its interactions with these DNA-bound factors. Furthermore, using DNA-af®nity precipitation, we demonstrated that although CIITA binds at least ®ve activators, RFX5, RFXAP, RFXANK/B, NFYB and NFYC, its assembly on the promoter requires the addition of nuclear extracts. We conclude that not only does the platform bind DNA via multiple, spatially constrained nteractions, but that it can recruit only modi®ed and/or complexed CIITA to MHCII promoters.