Role of Brahma related gene 1 in the transcription of the major histocompatibility complex class II genes.

摘要

Major histocompatability complex class H (MHC II) molecules play a central role in immune responses and their expression is regulated primarily at the level of transcription. The RFX heterotrimeric complex binds to the X1 box present in the MHC II gene promoter and is expressed ubiquitously. However, the cell type specific expression is regulated by the Class II transactivator (CIITA), which is also considered as the master switch of MHC II genes. Defects in the genes encoding the RFX complex or CIITA abrogate MHC II gene transcription. Understanding the structure function relationships of the RFX complex and CIITA protein will enable us to manipulate immune responses. Additionally, MHC II gene transcription serves as an excellent model to study transcription.; One of the factors that inhibits transcription is chromatin structure by making DNA inaccessible to factors that bind DNA and promote transcription. In past few years, the activity of two major classes of chromatin modifying complexes, ATP dependent chromatin remodeling and histone modifying enzymes (like histone acetyltransferases and histone deacetylases) have began to emerge as key regulators of chromatin structure which in turn control gene transcription. In the first chapter we present evidence that identifies the role of human SWI/SNF complex, an ATP dependent chromatin remodeling complex in MHC II gene transcription. We found that CIITA requires functional Brahma related gene 1 (Brg1), an ATPase containing subunit of the human SWI/SNF complex to activate transcription. We also show that CIITA can bind to Brg1.; In the chapter II we found that both CIITA and RFXAP (subunit of RFX complex) are capable of binding to Brg1 and mapped the interacting regions. We conclude through our observations that RFX complex and or CIITA (depending on cell type) may promote the recruitment of the SWI/SNF and histone modifying complexes to the promoter and the latter two complexes cooperatively interact to alter chromatin structure such that DNA becomes accessible to facilitate transcription. However, chromatin remodeling was not sufficient to activate transcription in the absence of CIITA. This suggests that CIITA may be required for other activities that are essential for transcription presumably by promoting transcriptional elongation.; In chapter IV we have identified the molecular basis of loss of function due to single amino acid substitution (from phenylalanine to serine at 962 position) of CIITA protein to be due to loss of nuclear localization. Finally in chapter V, we investigated the mechanism of Tat transactivator protein from HIV-1, mediated inhibition of CIITA function in mouse cells and conclude that unlike human cells, Tat mediated inhibition of CIITA was not due to cyclin T1 in mouse cells.