RFX1, a transactivator of hepatitis B virus enhancer I, belongs to a novel family of homodimeric and heterodimeric DNA-binding proteins.

Abstract

RFX1 is a transactivator of human hepatitis B virus enhancer I. We show here that RFX1 belongs to a previously unidentified family of DNA-binding proteins ofwhich we have cloned three members, RFX1, RFX2, and RFX3, from humans and mice. Members of the RFX family constitute the nuclear complexes that have been referred to previously as enhancer factor C, EP, methylation-dependent DNA-binding protein, or rpL30a. RFX proteins share five strongly conserved regions which include the two domains required for DNA binding and dimerization. RFX1 is a transcription factor that was initially cloned by virtue of its affinity for the X-box motif (31, 32), a conserved cis-acting regulatory element present in the promoters of all major histocompatibility complex (MHC) class II genes from all species examined (for reviews, see references 3, 13, and 41). RFX1 was subsequently also found to bind with high affinity to inverted repeats known as enhanced factor C (EF-C) sites (also called EP or methylation-dependent DNAbinding protein [MDBP] sites), which are cis-acting regulatory elements present in the enhancers of several unrelated viruses, including hepatitis B virus (HBV), polyomavirus, and cytomegalovirus (CMV) (7,10,15,28,29,42,(50)(51)(52). Among these EF-C sites, the functional importance of the site in the HBV enhancer (EnhI) has been most clearly demonstrated (7,11,15,29,37,42 . Second, there is a C-terminal dimerization domain which does not display obvious sequence similarity to known dimerization motifs (32). Moreover, in contrast to the majority of dimeric DNAbinding proteins, the DNA-binding and dimerization domains of RFX1 are not closely linked in the protein and are functionally independent, such that RFX1 can bind in vitro as either a monomer or a dimer (32, 37). Finally, identity between RFX1 and EF-C (EP or MDBP) (37) has allowed us to document a unique feature not previously described for other cloned eukaryotic transcription factors. Namely, in addition to its classical site-specific DNA-binding activity, RFX1 (MDBP or EF-C) also shows a peculiar site-specific affinity for certain methylated sequences (10,22,37,45,50,52). These methylation-dependent RFX1 (MDBP or EF-C) binding sites are recognized specifically only when they contain 5-methylcytosine at CpG dinucleotides.Depending on the cell line examined, EF-C (EP or MDBP) sites are bound by one to three different nuclear complexes having identical target site specificity (10,29,37,50), and only one of these complexes represents an RFX1 homodimer (37). The rpL30(x and MHC class II X-box sites are also bound by several closely related complexes (17,24,35). This suggested the existence of a family of RFX proteins. Considering the novel features of RFX1, its crucial role in activating the enhancer of HBV, and its suspected role for transcription of the MHC class II and rpL30 genes, it was of interest to identify these additional RFX factors. We have now cloned and characterized two other highly conserved RFX genes, RFX2 and RFX3, from both humans and mice, the...

EMR4, a Novel Epidermal Growth Factor (EGF)-TM7 Molecule Up-regulated in Activated Mouse Macrophages, Binds to a Putative Cellular Ligand on B Lymphoma Cell Line A20

“…Moreover, members of other gene families were also found closely linked with the EGF-TM7 genes within these two loci. For example, transcription factors RFX1 and RFX2, two closely related sitespecific DNA-binding proteins important for immune system functions (66,67), are tightly linked with CD97 and EMR1, respectively, both in human and mouse genomes (11,46,48). These findings have given rise to the notion that both sets of gene families were derived from an ancient gene duplication event encompassing their predecessors and subsequently separated during evolution (46).…”

Breast Cancer Cells Proliferation Is Regulated by Tyrosine Phosphatase SHP1 through c-jun N-Terminal Kinase and Cooperative Induction of RFX-1 and AP-4 Transcription Factors

“…RFX-1 is a member of a unique winged helix family of related transcription factors that form homo-and heterodimers binding to the conserved X box in MHC class II antigen promoter region (45,46). Partially palindromic sequences to which RFX-1 binds were found in the enhancers of hepatitis B, polyoma, cytomegalovirus, and EpsteinBarr viruses (47).…”

“…The target genes to which RFX-1 binds include genes encoding the IL-5 receptor and ribosomal rpL30 proteins (48)(49)(50). RFX-1 forms heterodimeric complexes with other family members such as RFX-2 and RFX-3 (46). Binding of RFX-1 to genes encoding cMyc, proliferating cell nuclear antigen, and collagen a2 has a negative effect on gene expression (50).…”

c-Abl: activation and nuclear targets

“…The hepatitis B virus (HBV) enhancer contains the EP element, which generates a nucleo-protein complex containing cAbl 35,36 and RFX1. 37 An EP-like box is found in the enhancers of other viruses, such as polyoma and the lentivirus EIAV. 38 Interestingly, the EP associated c-Abl is catalytically active in tyrosine phosphorylation.…”