Biochemical and cellular insights into the Baz2B protein, a non-catalytic subunit of the chromatin remodeling complex.

Baz2B is a regulatory subunit of the ATP-dependent chromatin remodeling complexes BRF1 and BRF5, which control access to DNA during DNA-templated processes. Baz2B has been implicated in several diseases and also in unhealthy ageing, however limited information is available on the domains and cellular roles of Baz2B. To gain more insight into the Baz2B function, we biochemically characterized the TAM (Tip5/ARBP/MBD) domain with the auxiliary AT-hook motifs and the bromodomain (BRD). We observed alterations in histone code recognition in bromodomains carrying cancer-associated point mutations, suggesting their potential involvement in disease. Furthermore, the depletion of Baz2B in the Hap1 cell line resulted in altered cell morphology, reduced colony formation and perturbed transcriptional profiles. Despite that, super-resolution microscopy images revealed no changes in the overall chromatin structure in the absence of Baz2B. These findings provide insights into the biological function of Baz2B.

Enhanced nucleosome assembly at CpG sites containing an extended 5-methylcytosine analogue.

Methylation of cytosine to 5-methylcytosine (mC) at CpG sites is a prevalent reversible epigenetic mark in vertebrates established by DNA methyltransferases (MTases); the attached methyl groups can alter local structure of DNA and chromatin as well as binding of dedicated proteins. Nucleosome assembly on methylated DNA has been studied extensively, however little is known how the chromatin structure is affected by larger chemical variations in the major groove of DNA. Here, we studied the nucleosome formation in vitro on DNA containing an extended 5mC analog, 5-(6-azidohex-2-ynyl)cytosine (ahyC) installed at biological relevant CpG sites. We found that multiple ahyC residues on 80-Widom and Hsp70 promoter DNA fragments proved compatible with nucleosome assembly. Moreover, unlike mC, ahyC increases the affinity of histones to the DNA, partially altering nucleosome positioning, stability, and the action of chromatin remodelers. Based on molecular dynamics calculations, we suggest that these new features are due to increased DNA flexibility at ahyC-modified sites. Our findings provide new insights into the biophysical behavior of modified DNA and open new ways for directed design of synthetic nucleosomes.

Regulation and rate enhancement during transcription-coupled DNA repair.

Transcription-coupled DNA repair (TCR) is a subpathway of nucleotide excision repair (NER) that is triggered when RNA polymerase is stalled by DNA damage. Lesions targeted by TCR are repaired more quickly than lesions repaired by the transcription-independent "global" NER pathway, but the mechanism underlying this rate enhancement is not understood. Damage recognition during bacterial NER depends upon UvrA, which binds to the damage and loads UvrB onto the DNA. Bacterial TCR additionally requires the Mfd protein, a DNA translocase that removes the stalled transcription complexes. We have determined the properties of Mfd, UvrA, and UvrB that are required for the elevated rate of repair observed during TCR. We show that TCR and global NER differ in their requirements for damage recognition by UvrA, indicating that Mfd acts at the very earliest stage of the repair process and extending the functional similarities between TCR in bacteria and eukaryotes.

Chromatin targeting signals, nucleosome positioning mechanism and non-coding RNA-mediated regulation of the chromatin remodeling complex NoRC.

Active and repressed ribosomal RNA (rRNA) genes are characterised by specific epigenetic marks and differentially positioned nucleosomes at their promoters. Repression of the rRNA genes requires a non-coding RNA (pRNA) and the presence of the nucleolar remodeling complex (NoRC). ATP-dependent chromatin remodeling enzymes are essential regulators of DNA-dependent processes, and this regulation occurs via the modulation of DNA accessibility in chromatin. We have studied the targeting of NoRC to the rRNA gene promoter; its mechanism of nucleosome positioning, in which a nucleosome is placed over the transcription initiation site; and the functional role of the pRNA. We demonstrate that NoRC is capable of recognising and binding to the nucleosomal rRNA gene promoter on its own and binds with higher affinity the nucleosomes positioned at non-repressive positions. NoRC recognises the promoter nucleosome within a chromatin array and positions the nucleosomes, as observed in vivo. NoRC uses the release mechanism of positioning, which is characterised by a reduced affinity for the remodeled substrate. The pRNA specifically binds to NoRC and regulates the enzyme by switching off its ATPase activity. Given the known role of pRNA in tethering NoRC to the rDNA, we propose that pRNA is a key factor that links the chromatin modification activity and scaffolding function of NoRC.

Using stable MutS dimers and tetramers to quantitatively analyze DNA mismatch recognition and sliding clamp formation.

The process of DNA mismatch repair is initiated when MutS recognizes mismatched DNA bases and starts the repair cascade. The Escherichia coli MutS protein exists in an equilibrium between dimers and tetramers, which has compromised biophysical analysis. To uncouple these states, we have generated stable dimers and tetramers, respectively. These proteins allowed kinetic analysis of DNA recognition and structural analysis of the full-length protein by X-ray crystallography and small angle X-ray scattering. Our structural data reveal that the tetramerization domains are flexible with respect to the body of the protein, resulting in mostly extended structures. Tetrameric MutS has a slow dissociation from DNA, which can be due to occasional bending over and binding DNA in its two binding sites. In contrast, the dimer dissociation is faster, primarily dependent on a combination of the type of mismatch and the flanking sequence. In the presence of ATP, we could distinguish two kinetic groups: DNA sequences where MutS forms sliding clamps and those where sliding clamps are not formed efficiently. Interestingly, this inability to undergo a conformational change rather than mismatch affinity is correlated with mismatch repair.

An N-terminal clamp restrains the motor domains of the bacterial transcription-repair coupling factor Mfd.

Motor proteins that translocate on nucleic acids are key players in gene expression and maintenance. While the function of these proteins is diverse, they are driven by highly conserved core motor domains. In transcription-coupled DNA repair, motor activity serves to remove RNA polymerase stalled on damaged DNA, making the lesion accessible for repair. Structural and biochemical data on the bacterial transcription-repair coupling factor Mfd suggest that this enzyme undergoes large conformational changes from a dormant state to an active state upon substrate binding. Mfd can be functionally dissected into an N-terminal part instrumental in recruiting DNA repair proteins (domains 1-3, MfdN), and a C-terminal part harboring motor activity (domains 4-7, MfdC). We show that isolated MfdC has elevated ATPase and motor activities compared to the full length protein. While MfdN has large effects on MfdC activity and thermostability in cis, these effects are not observed in trans. The structure of MfdN is independent of interactions with MfdC, implying that MfdN acts as a clamp that restrains motions of the motor domains in the dormant state. We conclude that releasing MfdN:MfdC interactions serves as a central molecular switch that upregulates Mfd functions during transcription-coupled DNA repair.

Structural and functional analysis of the MutS C-terminal tetramerization domain.

The Escherichia coli DNA mismatch repair (MMR) protein MutS is essential for the correction of DNA replication errors. In vitro, MutS exists in a dimer/tetramer equilibrium that is converted into a monomer/dimer equilibrium upon deletion of the C-terminal 53 amino acids. In vivo and in vitro data have shown that this C-terminal domain (CTD, residues 801-853) is critical for tetramerization and the function of MutS in MMR and anti-recombination. We report the expression, purification and analysis of the E.coli MutS-CTD. Secondary structure prediction and circular dichroism suggest that the CTD is folded, with an alpha-helical content of 30%. Based on sedimentation equilibrium and velocity analyses, MutS-CTD forms a tetramer of asymmetric shape. A single point mutation (D835R) abolishes tetramerization but not dimerization of both MutS-CTD and full-length MutS. Interestingly, the in vivo and in vitro MMR activity of MutS(CF/D835R) is diminished to a similar extent as a truncated MutS variant (MutS800, residues 1-800), which lacks the CTD. Moreover, the dimer-forming MutS(CF/D835R) has comparable DNA binding affinity with the tetramer-forming MutS, but is impaired in mismatch-dependent activation of MutH. Our data support the hypothesis that tetramerization of MutS is important but not essential for MutS function in MMR.

Identifying an interaction site between MutH and the C-terminal domain of MutL by crosslinking, affinity purification, chemical coding and mass spectrometry.

To investigate protein-protein interaction sites in the DNA mismatch repair system we developed a crosslinking/mass spectrometry technique employing a commercially available trifunctional crosslinker with a thiol-specific methanethiosulfonate group, a photoactivatable benzophenone moiety and a biotin affinity tag. The XACM approach combines photocrosslinking (X), in-solution digestion of the crosslinked mixtures, affinity purification via the biotin handle (A), chemical coding of the crosslinked products (C) followed by MALDI-TOF mass spectrometry (M). We illustrate the feasibility of the method using a single-cysteine variant of the homodimeric DNA mismatch repair protein MutL. Moreover, we successfully applied this method to identify the photocrosslink formed between the single-cysteine MutH variant A223C, labeled with the trifunctional crosslinker in the C-terminal helix and its activator protein MutL. The identified crosslinked MutL-peptide maps to a conserved surface patch of the MutL C-terminal dimerization domain. These observations are substantiated by additional mutational and chemical crosslinking studies. Our results shed light on the potential structures of the MutL holoenzyme and the MutH-MutL-DNA complex.

Use of Single-Cysteine Variants for Trapping Transient States in DNA Mismatch Repair.

DNA mismatch repair (MMR) is necessary to prevent incorporation of polymerase errors into the newly synthesized DNA strand, as they would be mutagenic. In humans, errors in MMR cause a predisposition to cancer, called Lynch syndrome. The MMR process is performed by a set of ATPases that transmit, validate, and couple information to identify which DNA strand requires repair. To understand the individual steps in the repair process, it is useful to be able to study these large molecular machines structurally and functionally. However, the steps and states are highly transient; therefore, the methods to capture and enrich them are essential. Here, we describe how single-cysteine variants can be used for specific cross-linking and labeling approaches that allow trapping of relevant transient states. Analysis of these defined states in functional and structural studies is instrumental to elucidate the molecular mechanism of this important DNA MMR process.

Chromatin Remodelers: From Function to Dysfunction.

Chromatin remodelers are key players in the regulation of chromatin accessibility and nucleosome positioning on the eukaryotic DNA, thereby essential for all DNA dependent biological processes. Thus, it is not surprising that upon of deregulation of those molecular machines healthy cells can turn into cancerous cells. Even though the remodeling enzymes are very abundant and a multitude of different enzymes and chromatin remodeling complexes exist in the cell, the particular remodeling complex with its specific nucleosome positioning features must be at the right place at the right time in order to ensure the proper regulation of the DNA dependent processes. To achieve this, chromatin remodeling complexes harbor protein domains that specifically read chromatin targeting signals, such as histone modifications, DNA sequence/structure, non-coding RNAs, histone variants or DNA bound interacting proteins. Recent studies reveal the interaction between non-coding RNAs and chromatin remodeling complexes showing importance of RNA in remodeling enzyme targeting, scaffolding and regulation. In this review, we summarize current understanding of chromatin remodeling enzyme targeting to chromatin and their role in cancer development.

The unstructured C-terminal extension of UvrD interacts with UvrB, but is dispensable for nucleotide excision repair.

During nucleotide excision repair (NER) in bacteria the UvrC nuclease and the short oligonucleotide that contains the DNA lesion are removed from the post-incision complex by UvrD, a superfamily 1A helicase. Helicases are frequently regulated by interactions with partner proteins, and immunoprecipitation experiments have previously indicated that UvrD interacts with UvrB, a component of the post-incision complex. We examined this interaction using 2-hybrid analysis and surface plasmon resonance spectroscopy, and found that the N-terminal domain and the unstructured region at the C-terminus of UvrD interact with UvrB. We analysed the properties of a truncated UvrD protein that lacked the unstructured C-terminal region and found that it showed a diminished affinity for single-stranded DNA, but retained the ability to displace both UvrC and the lesion-containing oligonucleotide from a post-incision nucleotide excision repair complex. The interaction of the C-terminal region of UvrD with UvrB is therefore not an essential feature of the mechanism by which UvrD disassembles the post-incision complex during NER. In further experiments we showed that PcrA helicase from Bacillus stearothermophilus can also displace UvrC and the excised oligonucleotide from a post-incision NER complex, which supports the idea that PcrA performs a UvrD-like function during NER in gram-positive organisms.

Solubility engineering of the HhaI methyltransferase.

DNA methylation is involved in epigenetic control of numerous cellular processes in eukaryotes, however, many mechanistic aspects of this phenomenon are not yet understood. A bacterial prototype cytosine-C5 methyltransferase, M.HhaI, serves as a paradigm system for structural and mechanistic studies of biological DNA methylation, but further analysis of the 37 kDa protein is hampered by its insufficient solubility (0.15 mM). To overcome this problem, three hydrophobic patches on the surface of M.HhaI that are not involved in substrate interactions were subjected to site-specific mutagenesis. Residues M51 or V213 were substituted by polar amino acids of a similar size, and/or the C-terminal tetrapeptide FKPY was replaced by a single glycine residue (Delta324G). Two out of six mutants, delta324G and V213S/delta324G, showed improved solubility in initial analyses and were purified to homogeneity using a newly developed procedure. Biochemical studies of the engineered methyltransferases showed that the deletion mutant delta324G retained identical DNA binding, base flipping and catalytic properties as the wild-type enzyme. In contrast, the engineered enzyme showed (i) a significantly increased solubility (>0.35 mM), (ii) high-quality 2D-[(15)N,(1)H] TROSY NMR spectra, and (iii) (15)N spin relaxation times evidencing the presence of a monomeric well-folded protein in solution.

Mapping protein-protein interactions between MutL and MutH by cross-linking.

Strand discrimination in Escherichia coli DNA mismatch repair requires the activation of the endonuclease MutH by MutL. There is evidence that MutH binds to the N-terminal domain of MutL in an ATP-dependent manner; however, the interaction sites and the molecular mechanism of MutH activation have not yet been determined. We used a combination of site-directed mutagenesis and site-specific cross-linking to identify protein interaction sites between the proteins MutH and MutL. Unique cysteine residues were introduced in cysteine-free variants of MutH and MutL. The introduced cysteines were modified with the cross-linking reagent 4-maleimidobenzophenone. Photoactivation resulted in cross-links verified by mass spectrometry of some of the single cysteine variants to their respective Cys-free partner proteins. Moreover, we mapped the site of interaction by cross-linking different combinations of single cysteine MutH and MutL variants with thiol-specific homobifunctional cross-linkers of varying length. These results were used to model the MutH.MutL complex and to explain the ATP dependence of this interaction.