RFXAP: Proteins and Enzymes

The identification of transacting factors controlling major histocompatibility complex (MHC) class II gene transcription via the proximal enhancer of the promoter has been greatly facilitated by a genetic approach, namely, the analysis of cell lines that are characterized by regulatory defects abolishing transcription of these genes (Mach et al., 1996). Most of these MHC class II regulatory mutants are cell lines derived from patients suffering from MHC class II deficiency, also referred to as the bare lymphocyte syndrome type II, a rare autosomal recessive disorder characterized by the lack of constitutive and inducible MHC class II expression in all cell types and tissues. The genetic lesions responsible for this lack of expression lie not in MHC class II genes themselves, but in transacting regulatory genes required for their transcription (Mach et al., 1996). Durand et al. (1997) noted that at least 3 different complementation groups (groups A, B, and C) have been identified by means of somatic cell fusion experiments. the defects in complementation groups B, C, and D all lead to a deficiency in RFX, a nuclear protein complex that binds to the X box of MHC class II promoters Durand et al. (1997) isolated a novel gene that encodes the 36-kD subunit of RFX. They called this subunit RFX-associated protein (RFXAP) because it is a subunit of the RFX complex and interacts with RFX5, yet it does not contain the characteristic RFX DNA-binding motif.