CIITA leucine-rich repeats control nuclear localization, in vivo recruitment to the major histocompatibility complex (MHC) class II enhanceosome, andMHC class II gene transactivation

The major histocompatibility complex (MHC) class II transactivator CIITA pl ays a pivotal role in the control of the cellular immune response through t he quantitative regulation of MHC class II expression. We have analyzed a r egion of CIITA with similarity to Leucine-rich repeats (LRRs). CIITA LRR al anine mutations abolish both the transactivation capacity of full-length CI ITA and the dominant-negative phenotype of CIITA mutants with N-terminal de letions. We demonstrate direct interaction of CIITA with the MHC class II p romoter binding protein RFX5 and could also detect novel interactions with RFXANK, NF-YB, and -YC. However, none of these interactions is influenced b y CIITA LRR mutagenesis. On the other hand, chromatin immunoprecipitation s hows that in vivo binding of CIITA to the MHC class II promoter is dependen t on LRR integrity. LRR mutations lead to an impaired nuclear localization of CIITA, indicating that a major function of the CIITA LRRs is in nucleocy toplasmic translocation. There is, however, evidence that the CIITA LRRs ar e also involved more directly in MHC class II gene transactivation. CIITA i nteracts with a novel protein of 33 kDa in a manner sensitive to LRR mutage nesis. CIITA is therefore imported into the nucleus by an LRR-dependent mec hanism, where it activates transcription through multiple protein-protein i nteractions with the MHC class II promoter binding complex.