Cloning, expression and purification of the DNA binding domain of RFX protein

The RFX DNA binding domain (DBD) is a novel highly conserved motif belonging to a large number of dimer DNA binding proteins which have diverse regulatory functions in eukaryotic organisms. To characterize this novel motif, a 78mer polypeptide corresponding to the DBD of human hRFX (hrfX1/DBD), a prototypical member of the RFX family has been cloned and overproduced in Escherichia coli. A purification procedure using cation exchange chromatography has also been developed.     

A consensus motif in the RFX DNA binding domain and binding domain mutants with altered specificity.

The RFX DNA binding domain is a novel motif that has been conserved in a growing number of dimeric DNA-binding proteins, having diverse regulatory functions, in eukaryotic organisms ranging from yeasts to humans. To characterize this novel motif, we have performed a detailed dissection of the site-specific DNA binding activity of RFX1, a prototypical member of the RFX family. First, we have performed a site selection procedure to define the consensus binding site of RFX1. Second, we have developed a new mutagenesis-selection procedure to derive a precise consensus motif, and to test the accuracy of a secondary structure prediction, for the RFX domain. Third, a modification of this procedure has allowed us to isolate altered-specificity RFX1 mutants. These results should facilitate the identification both of additional candidate genes controlled by RFX1 and of new members of the RFX family. Moreover, the altered-specificity RFX1 mutants represent valuable tools that will permit the function of RFX1 to be analyzed in vivo without interference from the ubiquitously expressed endogenous protein. Finally, the simplicity, efficiency, and versatility of the selection procedure we have developed make it of general value for the determination of consensus motifs, and for the isolation of mutants exhibiting altered functional properties, for large protein domains involved in protein-DNA as well as protein-protein interactions.     

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

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 of which 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 rpL30 alpha. RFX proteins share five strongly conserved regions which include the two domains required for DNA binding and dimerization. They have very similar DNA-binding specificities and heterodimerize both in vitro and in vivo. mRNA levels for all three genes, particularly RFX2, are elevated in testis. In other cell lines and tissues, RFX mRNA levels are variable, particularly for RFX2 and RFX3. RFX proteins share several novel features, including new DNA-binding and dimerization motifs and a peculiar dependence on methylated CpG dinucleotides at certain sites.     

Role of conserved residues of the WRKY domain in the DNA-binding of tobacco WRKY family proteins.

Four cDNA clones of tobacco that could code for polypeptides with two WRKY domains were isolated. Among four NtWRKYs and other WRKY family proteins, sequence similarity was basically limited to the two WRKY domains. Glutathione S-transferase fusion proteins with the C-terminal WRKY domain of four NtWRKYs bound specifically to the W-box (TTGACC), and the N-terminal WRKY domain showed weaker binding activity with the W-box compared to the C-terminal domain. The DNA-binding activity of the WRKY domain was abolished by o-phenanthroline and this inhibition was recovered specifically by Zn2+. Substitution of the conserved cysteine and histidine residues of the plant-specific C2H2-type zinc finger-like motif in the WRKY domain abolished the DNA binding. In addition, mutations in the invariable WRKYGQK sequence at the N-terminal side of the zinc finger-like motif also significantly reduced the DNA-binding activity, suggesting that these residues are required for proper folding of the DNA-binding zinc finger.     

Nuclear import and DNA-binding activity of RFX1. Evidence for an autoinhibitory mechanism.

RFX1 binds and regulates the enhancers of a number of viruses and cellular genes. RFX1 belongs to the evolutionarily conserved RFX protein family that shares a DNA-binding domain and a conserved C-terminal region. In RFX1 this conserved region mediates dimerization, and is followed by a unique C-terminal tail, containing a highly acidic stretch. In HL-60 cells nuclear translocation of RFX1 is regulated by protein kinase C with unknown mechanisms. By confocal fluorescence microscopy, we have identified a nonclassical nuclear localization signal (NLS) at the extreme C-terminus. The adjacent 'acidic region', which showed no independent NLS activity, potentiated the function of the NLS. Subcellular fractionation showed that the tight association of RFX1 with the nucleus is mediated by its DNA-binding domain and enhanced by the dimerization domain. In contrast, the acidic region inhibited nuclear association, by down-regulating the DNA-binding activity of RFX1. These data suggest an autoinhibitory interaction, which may regulate the function of RFX1 at the level of DNA binding. The C-terminal tail thus constitutes a composite localization domain, which on the one hand mediates nuclear import of RFX1, and on the other hand inhibits its association with the nucleus and binding to DNA. The participation of the acidic region in both activities suggests a mechanism by which the nuclear import and DNA-binding activity of RFX1 may be coordinately regulated by phosphorylation by kinases such as PKC.     

Vertebrate DM domain proteins bind similar DNA sequences and can heterodimerize on DNA

Background:  

The DM domain is a zinc finger-like DNA binding motif first identified in the sexual regulatory proteins Doublesex (DSX) and MAB-3, and is widely conserved among metazoans. DM domain proteins regulate sexual differentiation in at least three phyla and also control other aspects of development, including vertebrate segmentation. Most DM domain proteins share little similarity outside the DM domain. DSX and MAB-3 bind partially overlapping DNA sequences, and DSX has been shown to interact with DNA via the minor groove without inducing DNA bending. DSX and MAB-3 exhibit unusually high DNA sequence specificity relative to other minor groove binding proteins. No detailed analysis of DNA binding by the seven vertebrate DM domain proteins, DMRT1-DMRT7 has been reported, and thus it is unknown whether they recognize similar or diverse DNA sequences.     

The four human muscle regulatory helix-loop-helix proteins Myf3-Myf6 exhibit similar hetero-dimerization and DNA binding properties.

The muscle regulatory proteins Myf3, Myf4, Myf5, and Myf6 share a highly conserved DNA binding and dimerization domain consisting of a cluster of basic amino acids and a potential helix-loop-helix structure. Here we demonstrate that the four human muscle-specific HLH proteins have similar DNA binding and dimerization properties. The members of this family form protein complexes of comparable stability with the ubiquitously expressed HLH proteins E12, E2-2, and E2-5 and bind to the conserved DNA sequence CANNTG designated as E-box with similar efficiency in vitro. The binding affinities of the various complexes are greatly influenced by the variable internal and flanking nucleotides of the consensus motif. Combinations of Myf proteins with one another and with lyl-1, and HLH protein from human T cells, do not bind to DNA in vitro. Our results suggest that combinatorial associations of the various tissue-specific and more widely expressed HLH factors do not result in differential recognition of DNA sequences by Myf proteins.     

Engrailed and Hox homeodomain proteins contain a related Pbx interaction motif that recognizes a common structure present in Pbx.

Hox gene products have the ability to interact with either extradenticle or pbx gene products to bind cooperatively to DNA. The region in Hox proteins that is required for this interaction is located N-terminal of the homeodomain and contains a highly conserved hexapeptide. We now show that the engrailed gene products also contain a Pbx interaction motif positioned within a previously conserved region, the EH2 domain. The EH2 domain is located N-terminal of the homeodomain. Two tryptophan residues present in the Drosophila and murine Engrailed EH2 domain are required for cooperativity with extradenticle and Pbx, respectively. A second conserved domain, EH3, is required as well for cooperativity with Pbx, since deletions or an insertion in this region reduce cooperative DNA binding. Peptides containing the Pbx interaction motif of either Engrailed or Hox are capable of destabilizing Engrailed-Pbx and Hox-Pbx cooperative DNA binding. These data indicate that the Pbx interaction motifs present in Hox and engrailed gene products recognize a common structure present in the Pbx family of homeodomain proteins.     

DNA binding properties of a chemically synthesized DNA binding domain of hRFX1.

The RFX DNA binding domain (DBD) is a novel highly conserved motif belonging to a large number of dimeric DNA binding proteins which have diverse regulatory functions in eukaryotic organisms, ranging from yeasts to human. To characterize this novel motif, solid phase synthesis of a 76mer polypeptide corresponding to the DBD of human hRFX1 (hRFX1/DBD), a prototypical member of the RFX family, has been optimized to yield large quantities (approximately 90 mg) of pure compound. Preliminary two-dimensional1H NMR experiments suggested the presence of helical regions in this sequence in agreement with previously reported secondary structure predictions. In gel mobility shift assays, this synthetic peptide was shown to bind in a cooperative manner the 23mer duplex oligodeoxynucleotide corresponding to the binding site of hRFX1, with a 2:1 stoichoimetry due to an inverse repeat present in the 23mer. The stoichiometry of this complex was reduced to 1:1 by decreasing the length of the DNA sequence to a 13mer oligonucleotide containing a single half-site. Surface plasmon resonance measurements were achieved using this 5'-biotylinated 13mer oligonucleotide immobilized on an avidin-coated sensor chip. Using this method an association constant (K a = 4 x 10(5)/M/s), a dissociation constant (K d = 6 x 10(-2)/s) and an equilibrium dissociation constant (K D = 153 nM) were determined for binding of hRFX1/DBD to the double-stranded 13mer oligonucleotide. In the presence of hRFX1/DBD the melting temperature of the 13mer DNA was increased by 16 degreesC, illustrating stabilization of the double-stranded conformation induced by the peptide.     

The Drosophila retained/dead ringer gene and ARID gene family function during development

The recently discovered ARID family of proteins interact with DNA through a phylogenetically conserved sequence termed the A/T Interaction Domain (ARID). The retained/dead ringer (retn/dri) gene of Drosophila melanogaster is a founding member of the ARID gene family, and of the eARID subfamily. This subfamily exhibits an extended region of sequence similarity beyond the core ARID motif and a separate conserved domain termed the REKLES domain. retn/dri is involved in a range of developmental processes, including axis patterning and muscle development. The retn/dri ARID motif has been shown by in vitro studies to exhibit sequence-specific DNA binding activity. Here we demonstrate that the ARID domain is essential for the in vivo function of retn/dri during embryonic development by showing that a mutant form of RETN/DRI, deleted for part of the ARID domain and unable to bind DNA in vitro, cannot rescue the retn/dri mutant phenotype. In the presence of wild-type RETN/DRI this construct acts as a dominant negative, providing additional support for the proposal that RETN/DRI acts in a multiprotein complex. In contrast, we are yet to find an in vivo role for the REKLES domain, despite its clear evolutionary conservation. Finally, we have used germline clone analysis to reveal a requirement for retn/dri in the Drosophila preblastoderm syncytial mitoses.     

Characterization of functional domains of human EB1 family proteins

EB1 family proteins are evolutionarily conserved proteins that bind microtubule plus-ends and centrosomes and regulate the dynamics and organization of microtubules. Human EB1 family proteins, which include EB1, EBF3, and RP1, also associate with the tumor suppressor protein adenomatous polyposis coli (APC) and p150glued, a component of the dynactin complex. The structural basis for interaction between human EB1 family proteins and their associated proteins has not been defined in detail. EB1 family proteins have a calponin homology (CH) domain at their N terminus and an EB1-like C-terminal motif at their C terminus; the functional importance of these domains has not been determined. To better understand functions of human EB1 family proteins and to reveal functional similarities and differences among these proteins, we performed detailed characterizations of interactions between human EB1 family proteins and their associated proteins. We show that amino acids 1-133 of EB1 and EBF3 and the corresponding region of RP1, which contain a CH domain, are necessary and sufficient for binding microtubules, thus demonstrating for the first time that a CH domain contributes to binding microtubules. EB1 family proteins use overlapping but different regions that contain the EB1-like C-terminal motif to associate with APC and p150glued. Neither APC nor p150glued binding domain is necessary for EB1 or EBF3 to induce microtubule bundling, which requires amino acids 1-181 and 1-185 of EB1 and EBF3, respectively. We also determined that the EB1 family protein-binding regions are amino acids 2781-2820 and 18-111 of APC and p150glued, respectively.     

The Fanconi anemia associated protein FAAP24 uses two substrate specific binding surfaces for DNA recognition

To maintain the integrity of the genome, multiple DNA repair systems exist to repair damaged DNA. Recognition of altered DNA, including bulky adducts, pyrimidine dimers and interstrand crosslinks (ICL), partially depends on proteins containing helix-hairpin-helix (HhH) domains. To understand how ICL is specifically recognized by the Fanconi anemia proteins FANCM and FAAP24, we determined the structure of the HhH domain of FAAP24. Although it resembles other HhH domains, the FAAP24 domain contains a canonical hairpin motif followed by distorted motif. The HhH domain can bind various DNA substrates; using nuclear magnetic resonance titration experiments, we demonstrate that the canonical HhH motif is required for double-stranded DNA (dsDNA) binding, whereas the unstructured N-terminus can interact with single-stranded DNA. Both DNA binding surfaces are used for binding to ICL-like single/double-strand junction-containing DNA substrates. A structural model for FAAP24 bound to dsDNA has been made based on homology with the translesion polymerase iota. Site-directed mutagenesis, sequence conservation and charge distribution support the dsDNA-binding model. Analogous to other HhH domain-containing proteins, we suggest that multiple FAAP24 regions together contribute to binding to single/double-strand junction, which could contribute to specificity in ICL DNA recognition.     

The Leu-Arg-Glu (LRE) adhesion motif in proteins of the neuromuscular junction with special reference to proteins of the carboxylesterase/cholinesterase family

Short linear motifs confer evolutionary flexibility on proteins as they can be added with relative ease allowing the acquisition of new functions. Such motifs may mediate a variety of signalling functions. The adhesion-mediating Leu-Arg-Glu (LRE) motif is enriched in laminin beta 2, and has been observed in other proteins, including members of the carboxylesterase/cholinesterase family. It acts as a stop signal for growing axons in the developing neuromuscular junction, binding to the voltage-gated calcium channel. In this bioinformatic analysis, we have investigated the presence of the motif in proteins of the neuromuscular junction, and have also examined its structural position and potential for ligand interaction, as well as phylogenetic conservation, in the carboxylesterase/cholinesterase family. The motif was observed to occur with a significantly higher frequency than expected in the UniProt/Swiss-Prot database, as well as in four individual species (human, mouse, Caenorhabditis elegans and Drosophila melanogaster). Examination of its presence in neuromuscular junction proteins showed it to be enriched in certain proteins of the synaptic basement membrane, including laminin, agrin, acetylcholinesterase and tenascin. A highly significant enrichment was observed in cytoskeletal proteins, particularly intermediate filament proteins and members of the spectrin family. In the carboxylesterase/cholinesterase family, the motif was observed in four conserved positions in the protein structure. It is present in the majority of mammalian acetylcholinesterases, as well as acetylcholinesterases from electric fish and a number of invertebrates. In insects, it is present in the ace-2, rather than in the synaptic ace-1, enzyme. It is also observed in the cholinesterase-like adhesion molecules (neuroligins, neurotactin and glutactin). It is never seen in butyrylcholinesterases, which do not mediate cell adhesion. In conclusion, the significant enrichment of the motif in certain classes of protein, as well as its conserved presence and structural positioning in one protein family, suggests that it has specific functions both in cell adhesion in the neuromuscular junction and in maintaining the structural integrity of the cytoskeleton.