REGULATORY FACTOR-X, A BARE LYMPHOCYTE SYNDROME TRANSCRIPTION FACTOR,IS A MULTIMERIC PHOSPHOPROTEIN COMPLEX

Regulatory factor X (RFX) is a transcription factor that binds the con served X1 box of MHC class II promoters and is essential for transcrip tion of class II genes, The subunit structure of the native RFX comple x was examined by coimmunoprecipitation using polyclonal antisera to t he 75-kDa subunit of RFX, RFX5. Two polypeptides with apparent masses of 41 and 36 kDa coimmunoprecipitated with RFX5 and appear to be subun its of the native RFX complex, Metabolic labeling of wild-type and mut ant B cells with [P-32]orthophosphate demonstrated that each of the RF X subunits was phosphorylated in vivo and that the phosphorylation of the RFX subunits was independent of the essential MHC class II regulat ory factor, CIITA. The trimeric RFX complex was also present in fibrob last cells with or without IFN-gamma treatment, Both the p41 and p36 s ubunits were absent in immunoprecipitations of RFX5 from lysates of in dependently established B cell lines from bare lymphocyte syndrome com plementation groups B and D. Together, these results suggest that RFX complex assembly is required for class II expression and that the muta tions in bare lymphocyte syndrome complementation groups B and D resul t in an inability to assemble the RFX complex.