DNA totally flipped-out by methylase

Opening of DNA double helix at room temperature: Application of alpha-cyclodextrin self-aggregates

Self-aggregation of alpha-cyclodextrin (alpha-CD) can induce DNA opening at room temperature (25 degrees C) owing to the hydroxyl groups on the surface of the spherical aggregates of alpha-CD, which promote hydrogen bonding with the flipped-out bases in DNA duplex prohibiting them from reverting back.

Structural evidence of a passive base-flipping mechanism for AGT, an unusual GT-B glycosyltransferase

The Escherichia coli T4 bacteriophage uses two glycosyltransferases to glucosylate and thus protect its DNA: the retaining alpha-glucosyltransferase (AGT) and the inverting beta-glucosyltransferase (BGT). They glucosylate 5-hydroxymethyl cytosine (5-HMC) bases of duplex DNA using UDP-glucose as the sugar donor to form an alpha-glucosidic linkage and a beta-glucosidic linkage, respectively. Five structures of AGT have been determined: a binary complex with the UDP product and four ternary complexes with UDP or UDP-glucose and oligonucleotides containing an A:G, HMU:G (hydroxymethyl uracyl) or AP:G (apurinic/apyrimidinic) mismatch at the target base-pair. AGT adopts the GT-B fold, one of the two folds known for GTs. However, while the sugar donor binding mode is classical for a GT-B enzyme, the sugar acceptor binding mode is unexpected and breaks the established consensus: AGT is the first GT-B enzyme that predominantly binds both the sugar donor and acceptor to the C-terminal domain. Its active site pocket is highly similar to four retaining GT-B glycosyltransferases (trehalose-6-phosphate synthase, glycogen synthase, glycogen and maltodextrin phosphorylases) strongly suggesting a common evolutionary origin and catalytic mechanism for these enzymes. Structure-guided mutagenesis and kinetic analysis do not permit identification of a nucleophile residue responsible for a glycosyl-enzyme intermediate for the classical double displacement mechanism. Interestingly, the DNA structures reveal partially flipped-out bases. They provide evidence for a passive role of AGT in the base-flipping mechanism and for its specific recognition of the acceptor base.

Atomistic view of base flipping in DNA

Base flipping is essential for the enzyme-catalysed methylation of DNA. In our previous studies, the flipping of bases out of duplex DNA in DNA alone and when bound to the (cytosine-C5)-methyltransferase from HhaI (M.HhaI) were investigated via potential of mean force calculations. Insights into various experimental observations were obtained. In the present paper we present an overview of previous computational studies of base flipping along with new detailed structural and energetic analysis on atomic events that contribute to the free energy surfaces. The contributions from different intrinsic and environmental effects to the base-flipping process are explored, and experimental data derived from a variety of methods are reconciled. A detailed protein-facilitated base-flipping mechanism is proposed. Ground-state destabilization is achieved via disruption of the target base Watson-Crick interactions by substitution with favourable DNA-protein interactions. In addition, specific DNA-protein interactions and favourable solvation effects further promote target base flipping along the major groove through the protein matrix, and maximal interactions occur between the DNA and the protein upon reaching the fully flipped state. Other DNA binding proteins that involve distortion of DNA's conformation may use a similar mechanism to that by which M.HhaI facilitates base flipping.

Structural evidence of a passive base-flipping mechanism for beta-glucosyltransferase

Beta-glucosyltransferase (BGT) is a DNA-modifying enzyme and a glycosyltransferase. This inverting enzyme transfers glucose from UDP-glucose to the 5-hydroxymethyl cytosine bases of T4 phage DNA. From previous structural analyses we showed that Asp-100 and Asn-70 were, respectively, the catalytic base and the key residue for specific DNA recognition (Larivière, L., Gueguen-Chaignon, V., and Moréra, S. (2003) J. Mol. Biol. 330, 1077-1086). Here, we supply biochemical evidence supporting their essential roles in catalysis. We have also shown previously that BGT uses a base-flipping mechanism to access 5-hydroxymethyl cytosine (Larivière, L., and Moréra, S. (2002) J. Mol. Biol. 324, 483-490). Whether it is an active or a passive process remains unclear, as is the case for all DNA cleaving and modifying enzymes. Here, we report two crystal structures: (i) BGT in complex with a 13-mer DNA containing an A:G mismatch and (ii) BGT in a ternary complex with UDP and an oligonucleotide containing a single central G:C base pair. The binary structure reveals a specific complex with the flipped-out, mismatched adenine exposed to the active site. Unexpectedly, the other structure shows the non-productive binding of an intermediate flipped-out base. Our structural analysis provides clear evidence for a passive process.

Backbone dynamics of DNA containing 8-oxoguanine: importance for substrate recognition by base excision repair glycosylases

Except for the functional groups sited within the major or minor grooves, the bases of B-DNA are quite protected from the external environment. Enzymes that modify the bases often "flip out" the target into an extrahelical position before the chemistry step is carried out. Examples of this mechanism are the base excision repair glycosylases and the restriction enzyme methylases. The question arises about the mechanism of substrate recognition for these enzymes and how closely it is linked to the base flipping step. Molecular dynamics simulations (AMBER, PME electrostatics) of fully solvated, cation neutralized, DNA sequences containing 8-oxoguanine (8OG) and of appropriate normal (control) DNAs have been carried out. The dynamics trajectories were analyzed to identify those properties of the DNA structure in the vicinity of the altered base, or its dynamics, that could contribute to molecular discrimination between substrate and non-substrate DNA sites. The results predict that the FPG enzyme should flip out the cytosine base paired with the scissile 8OG, not the target base itself.

Spontaneous base flipping in DNA and its possible role in methyltransferase binding

Recent crystallographic studies showed that HhaI and other methyltransferases flip their target DNA base completely out of a DNA helix. This base flipping is also a key feature in a number of other enzyme-catalyzed processes involving DNA. The mechanism of base flipping by these enzymes remains elusive. Based on a full atomic level description of bond rotational motions we have studied the energetics of flipping a base in a B-DNA duplex in the absence of the enzyme. We have also investigated the effect of the restraints from enzyme-distorted DNA backbone on the movement of a flipped base in several methytransferase bound DNA crystal structures. Our study on crystal B-DNA helices showed that a base could be flipped at an energy cost close to the enthalpy observed for base pair opening in premelting thermal fluctuations. This suggests that spontaneous base flipping in DNA due to thermal fluctuation may be achieved. Analysis of several crystal HhaI and HaeIII methyltransferase DNA duplex structures showed that the enzyme induced DNA backbone distortion severely restricts the movement of the flipped base, which indicates that during base flipping the backbone needs to adopt a substantially different conformation than that observed in the x-ray (enzyme-bound) structures. Our results suggest the possible role of thermally induced transient base opening in facilitating recognition and binding of methyltransferases and other enzymes.

Protein-mediated base flipping

Since the discovery that the DNA methyltransferase M.HhaI utilises a base flipping mechanism to expose its target cytosine during catalysis, this phenomenon has been observed in other nucleic acid modifying enzymes. The crystallographic analyses of such enzyme-DNA complexes have revealed the molecular features of extrahelical base stabilisation, but have been less informative about the flipping process itself.

Dynamic modes of the flipped-out cytosine during HhaI methyltransferase-DNA interactions in solution

Flipping of a nucleotide out of a B-DNA helix into the active site of an enzyme has been observed for the HhaI and HaeIII cytosine-5 methyltransferases (M.HhaI and M.HaeIII) and for numerous DNA repair enzymes. Here we studied the base flipping motions in the binary M. HhaI-DNA and the ternary M.HhaI-DNA-cofactor systems in solution. Two 5-fluorocytosines were introduced into the DNA in the places of the target cytosine and, as an internal control, a cytosine positioned two nucleotides upstream of the recognition sequence 5'-GCGC-3'. The 19F NMR spectra combined with gel mobility data show that interaction with the enzyme induces partition of the target base among three states, i.e. stacked in the B-DNA, an ensemble of flipped-out forms and the flipped-out form locked in the enzyme active site. Addition of the cofactor analogue S-adenosyl-L-homocysteine greatly enhances the trapping of the target cytosine in the catalytic site. Distinct dynamic modes of the target cytosine have thus been identified along the reaction pathway, which includes novel base-flipping intermediates that were not observed in previous X-ray structures. The new data indicate that flipping of the target base out of the DNA helix is not dependent on binding of the cytosine in the catalytic pocket of M.HhaI, and suggest an active role of the enzyme in the opening of the DNA duplex.

The crystal structure of HaeIII methyltransferase convalently complexed to DNA: an extrahelical cytosine and rearranged base pairing

Many organisms expand the information content of their genome through enzymatic methylation of cytosine residues. Here we report the 2.8 A crystal structure of a bacterial DNA (cytosine-5)-methyltransferase (DCMtase), M. HaeIII, bound covalently to DNA. In this complex, the substrate cytosine is extruded from the DNA helix and inserted into the active site of the enzyme, as has been observed for another DCMtase, M. HhaI. The DNA is bound in a cleft between the two domains of the protein and is distorted from the characteristic B-form conformation at its recognition sequence. A comparison of structures shows a variation in the mode of DNA recognition: M. HaeIII differs from M. HhaI in that the remaining bases in its recognition sequence undergo an extensive rearrangement in their pairing. In this process, the bases are unstacked, and a gap 8 A long opens in the DNA.

DNA modification by methyltransferases

Enzymatic methylation of DNA plays important roles in both prokaryotes and eukaryotes. Structural study of the HhaI DNA methyltransferase has provided considerable insight into the chemistry of C5-cytosine methylation. The DNA-protein complex reveals a substrate cytosine flipped out of the double helix during the reaction, and a novel two-loop DNA-binding motif used for both sequence recognition and flipping the base. Structural comparison of HhaI C5-cytosine methyltransferase, TaqI N6-adenine methyltransferase, and catechol O-methyltransferase reveals a common catalytic domain structure, which might be universal among S-adenosyl-L-methionine (SAM)-dependent methyltransferases.