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Purified X2 binding protein (X2BP) cooperatively binds the class II MHC X box region in the presence of purified RFX, the X box factor deficient in the bare lymphocyte syndrome

Moreno, C. S.
Emery, P.
West, J. E.
Boss, J. M.
Published in Journal of Immunology. 1995, vol. 155, no. 9, p. 4313-4321
Abstract The conserved X2 box sequence of MHC class II promoters is homologous to TRE/CRE elements, and is required for B cell expression and IFN-gamma induction of MHC class II genes. The X2 binding protein (X2BP) was initially identified as a DNA-binding activity that specifically interacts with the conserved X2 box sequence in both the MHC HLA-DRA and HLA-DRB promoters. To begin to demonstrate that X2BP is the X2 box factor responsible for class II expression in B cells, we have purified X2BP to homogeneity from B cell nuclear extracts using DNA-affinity chromatography. X-box DNA-affinity purification indicates that X2BP is most likely composed of two polypeptides of 120 kDa and 46 kDa. The 120-kDa protein was specifically cross-linked to an X-box probe by exposure to UV irradiation. The 46-kDa subunit of X2BP cross-reacted with anti-rat CREB polyclonal Abs but not to anti-human CREB Abs in Western analysis and supershift assays, indicating that it may be a novel member of the ATF/CREB family. Purified X2BP interacted with purified RFX, a factor that binds to the adjacent X1 box and is absent in some cell lines that are mutant for MHC class II transcription. This interaction increases the DNA-binding half-life of RFX from 5 to at least 60 min, suggesting that X2BP functions in class II MHC gene expression by forming a stable complex with RFX.
Keywords Base SequenceCross ReactionsDNA-Binding Proteins/genetics/ isolation & purificationGenes, MHC Class IIHalf-LifeHeparinHumansLymphoma, B-CellMolecular Sequence DataMolecular WeightNuclear Proteins/genetics/ isolation & purificationPeptides/chemistry/isolation & purificationProtein BindingTranscription Factors/genetics/ isolation & purificationTumor Cells, Cultured
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Other version: http://www.jimmunol.org/cgi/reprint/155/9/4313.pdf
PMID: 7594590
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