Structural aspects of protein-DNA recognition

Theoretical design of a bis-orthopepetide derivative of a tetracationic porphyrin targeted toward a six-base pair sequence of DNA

Tetra-(4-N-methylpyridyl)-porphyrin, (T4MPyP), is a tetracationic porphyrin that binds to G-C sequences of DNA by means of an intercalative mode. In order to extend its selective sequence recognition capacity for bases beyond the intercalation site, and in the major groove, we have undertaken the theoretical design of bis-ortho peptide derivatives of T4MPyP. In these, two ortho N-methylpiperidinium nitrogens are linked to a cationic residue, L-Lys, L-Orn, or L-Arg. The binding energetics of these novel compounds were compared for six distinct double-stranded palindromic hexanucleotide sequences. Four distinct modes of binding were compared: a) major, b) minor groove binding of both peptidic arms; c) a straddling mode in which each arm is in a different groove of DNA; d) exclusive binding of the arms to the sugar-phosphate backbone. For our most promising compound, that with Lys side-chains, a distinctive energetical advantage was computed in favor of an all-major groove binding to sequence d(CCCGGG)2. The corresponding complex is separated by an energy gap of 12 kcal/mol, with respect to the second-best sequence bound in the major groove, d(GGCGCC)2, and of 20 kcal/mol with respect to minor groove binding to sequence d(TACGTA)2. The results obtained with such a prototypic compound indicate that it is fully possible to design sequence selective (> 6 base-pairs) photosensitizers as peptide derivatives of T4MPyP and prompt the engineering of further, more complex analogs thereof.

Exploring the DNA binding domain of gene V protein encoded by bacteriophage M13 with the aid of spin-labeled oligonucleotides in combination with 1H-NMR

The DNA binding domain of the single-stranded DNA binding protein gene V protein encoded by the bacteriophage M13 was studied by means of 1H nuclear magnetic resonance, through use of a spin-labeled deoxytrinucleotide. The paramagnetic relaxation effects observed in the 1H-NMR spectrum of M13 GVP upon binding of the spin-labeled ligand were made manifest by means of 2D difference spectroscopy. In this way, a vast data reduction was accomplished which enabled us to check and extend the analysis of the 2D spectra carried out previously as well as to probe the DNA binding domain and its surroundings. The DNA binding domain is principally situated on two beta-loops. The major loop of the two is the so-called DNA binding loop (residues 16-28) of the protein where the residues which constitute one side of the beta-ladder (in particular, residues Ser20, Tyr26, and Leu28) are closest to the DNA spin-label. The other loop is part of the so-called dyad domain of the protein (residues 68-78), and mainly its residues at the tip are affected by the spin-label (in particular, Phe73). In addition, a part of the so-called complex domain of the protein (residues 44-51) which runs contiguous to the DNA binding loop is in close vicinity to the DNA. The NMR data imply that the DNA binding domain is divided over two monomeric units of the GVP dimer in which the DNA binding loop and the tip of the dyad loop are part of opposite monomers. The view of the GVP-ssDNA binding interaction which emerges from our data differs from previous molecular modeling proposals which were based on the GVP crystal structure (Brayer & McPherson, 1984; Hutchinson et al., 1990). These models implicate the involvement of one or two tyrosines (Tyr34, Tyr41) of the complex loop of the protein to participate in complex formation with ssDNA. In the NMR studies with the spin-labeled oligonucleotides, no indication of such interactions has been found. Other differences between the models and our NMR data are related to the structural differences found when solution and crystal structures are compared.

The activating transcription factor region within the E2A promoter exists in a novel conformation

The ATF (activating transcription factor) binding site within the E2A promoter region of adenovirus is shown to exist in a novel conformation in vitro via nuclear magnetic resonance methods. This novel conformation may be important to the protein DNA recognition process. This conformation has characteristics of both A- and B-form DNA. From circular dichroism and through-space-based NMR experiments, it is clear that the overall helical structure is B-like. However, the 1H-1H coupling constant information indicates that most of the sugar puckers of the individual nucleotides are in the C3'-endo/C4'-exo range which is more characteristic of A-form DNA. The sugar conformation can also be described as a mixture of two states, C3'-endo and C2'-endo, where many of the sugars exist mainly in the C3'-endo state. These data show that the conformation of the sugar puckers does not determine the nature of the overall base stacking on the DNA. Helical parameters were calculated from NOE build-up curves for half of the dinucleotide pairs. Severe spectral overlap on the nuclear Overhauser based spectra prevented determination of the helical parameters for all of the dinucleotide base-pairs. Energy minimization and molecular dynamic simulation methods using the sugar pucker and glycosidic torsion angles determined from the NMR data as constraints were carried out in order to demonstrate that such a conformation was energetically favorable.

Molecular analysis of two ScrR repressors and of a ScrR-FruR hybrid repressor for sucrose and D-fructose specific regulons from enteric bacteria

The scr regulon of pUR400 and the chromosomally encoded scr regulon of Klebsiella pneumoniae KAY2026 are both negatively controlled by a specific repressor (ScrR). As deduced from the nucleotide sequences, both scrR genes encode polypeptides of 334 residues (85.5% identical base pairs, 91.3% identical amino acids), containing an N-terminal helix-turn-helix motif. Comparison with other regulatory proteins revealed 30.6% identical amino acids to FruR, 27.0% to Lacl and 28.1% to GalR. Six scrRs super-repressor mutations define the inducer-binding domain. The scr operator sequences were identified by in vivo titration tests of the sucrose repressor and by in vitro electrophoretic mobility shift assays. D-fructose, an intracellular product of sucrose transport and hydrolysis, and D-fructose 1-phosphate were shown to be molecular inducers of both scr regulons. An active ScrR-FruR hybrid repressor protein was constructed with the N-terminal part of the sucrose repressor of K. pneumoniae and the C-terminal part of the fructose repressor of Salmonella typhimurium LT2. Gel retardation assays showed that the hybrid protein bound to scr-specific operators, and that D-fructose 1-phosphate, the inducer for FruR, was the only inducer. In vivo, neither the operators of the fru operon nor of the pps operon, the natural targets for FruR, were recognized, but the scr operators were. These data and the data obtained from the super-repressor alleles confirm previous models on the binding of repressors of the Lacl family to their operators.

The crystal structure of EcoRV endonuclease and of its complexes with cognate and non-cognate DNA fragments

The crystal structure of EcoRV endonuclease has been determined at 2.5 A resolution and that of its complexes with the cognate DNA decamer GGGATATCCC (recognition sequence underlined) and the non-cognate DNA octamer CGAGCTCG at 3.0 A resolution. Two octamer duplexes of the non-cognate DNA, stacked end-to-end, are bound to the dimeric enzyme in B-DNA-like conformations. The protein--DNA interactions of this complex are prototypic for non-specific DNA binding. In contrast, only one cognate decamer duplex is bound and deviates considerably from canonical B-form DNA. Most notably, a kink of approximately 50 degrees is observed at the central TA step with a concomitant compression of the major groove. Base-specific hydrogen bonds between the enzyme and the recognition base pairs occur exclusively in the major groove. These interactions appear highly co-operative as they are all made through one short surface loop comprising residues 182-186. Numerous contacts with the sugar phosphate backbone extending beyond the recognition sequence are observed in both types of complex. However, the total surface area buried on complex formation is > 1800 A2 larger in the case of cognate DNA binding. Two acidic side chains, Asp74 and Asp90, are close to the reactive phosphodiester group in the cognate complex and most probably provide oxygen ligands for binding the essential cofactor Mg2+. An important role is also indicated for Lys92, which together with the two acidic functions appears to be conserved in the otherwise unrelated structure of EcoRI endonuclease. The structural results give new insight into the physical basis of the remarkable sequence specificity of this enzyme.

Identification and preliminary characterization of a protein motif related to the zinc finger

We have identified a protein motif, related to the zinc finger, which defines a newly discovered family of proteins. The motif was found in the sequence of the human RING1 gene, which is proximal to the major histocompatibility complex region on chromosome six. We propose naming this motif the "RING finger" and it is found in 27 proteins, all of which have putative DNA binding functions. We have synthesized a peptide corresponding to the RING1 motif and examined a number of properties, including metal and DNA binding. We provide evidence to support the suggestion that the RING finger motif is the DNA binding domain of this newly defined family of proteins.

Identification of DNA-binding proteins on human umbilical vein endothelial cell plasma membrane

The binding of anti-DNA antibodies to the endothelial cell is mediated through DNA, which forms a bridge between the immunoglobulin and the plasma membrane. We have shown that 32P-labelled DNA bound to the plasma membrane of human umbilical vein endothelial cells (HUVEC) by a saturable process, which could be competitively inhibited by non-radiolabelled DNA. In addition, DNA-binding was enhanced in HUVEC that had been treated with IL-1 alpha or tumour necrosis factor-alpha (TNF-alpha). DNA-binding proteins of mol. wt 46,000, 92,000, and 84,000 were identified by the binding of 32P-labelled DNA to plasma membrane proteins separated on SDS-PAGE. DNA-binding proteins of mol. wt 46,000 and 84,000 were also present in the cytosol and nucleus. Murine anti-DNA MoAb410 bound to a single band, at mol. wt 46,000, of plasma membrane protein, in the presence of DNA. Our results showed that DNA-binding proteins are present in different cellular fractions of endothelial cells. DNA-binding proteins on the cell membrane could participate in the in situ formation of immune deposits; and their presence in the cell nucleus suggests a potential role in the modulation of cell function.

Crystal structure of the met repressor-operator complex at 2.8 A resolution reveals DNA recognition by beta-strands

The crystal structure of the met repressor-operator complex shows two dimeric repressor molecules bound to adjacent sites 8 base pairs apart on an 18-base-pair DNA fragment. Sequence specificity is achieved by insertion of double-stranded antiparallel protein beta-ribbons into the major groove of B-form DNA, with direct hydrogen-bonding between amino-acid side chains and the base pairs. The repressor also recognizes sequence-dependent distortion or flexibility of the operator phosphate backbone, conferring specificity even for inaccessible base pairs.

Possible relationship between coding recognition amino acid sequence motif or residue(s) and post-translational chemical modification of proteins

1. The "code-sequence" of N-glycosylation site(s), the amino acids located around O-glycosylation site(s), the sequence motifs of several kinases, the sequence motifs of--sulfation, amidation, isoprenylation, myristoylation, palmitoylation and N-acetylation, Aspartic and Asparagine hydroxylation-site, gamma-carboxyglutamate domain, phosphopantetheine attachment site etc. are extensively listed, compared to those reported by "PROSITE" Computer Screen Center and discussed. 2. The structural aspects of protein-DNA recognition are quoted as discussion and conclusion.

On the catalytic mechanism of EcoRI and EcoRV. A detailed proposal based on biochemical results, structural data and molecular modelling

EcoRI and EcoRV have a very similar active site, as is apparent from a comparison of the structures of their respective protein-DNA complexes. Based on structural and mechanistic data, as well as detailed molecular modelling presented here, a mechanism for the DNA cleavage by these enzymes is suggested in which the attacking water molecule is activated by the phosphate group 3' to the scissile phosphodiester bond, and in which the leaving group is protonated by a water molecule associated with the essential cofactor, Mg2+. The mechanism proposed may also apply to other nucleases.

Sequence-specific NMR assignments of the trp repressor from Escherichia coli using three-dimensional 15N/1H heteronuclear techniques

Sequence-specific 15N and 1H assignments for the trp holorepressor from Escherichia coli are reported. The trp repressor consists of two identical 107-residue subunits which are highly helical in the crystal state [Schevitz, R., Otwinowski, Z., Joachimiak, A., Lawson, C. L. & Sigler, P. B. (1985) Nature 317, 782-786]. The high helical content and the relatively large size of the protein (Mr = 25,000) make it difficult to assign even the main-chain resonances by conventional homonuclear two-dimensional NMR methods. However, we have now assigned the main-chain resonances of 94% of the residues by using three-dimensional 15N/1H heteronuclear experiments on a sample of protein uniformly labelled with 15N. The additional resolution obtained by spreading out the signals into three dimensions proved indispensable in making these assignments. In particular, we have been able to resolve signals from residues in the N-terminal region of the A helix for the first time in solution. The observed NOE results confirm that the repressor is highly helical in solution, and contains no extended chain conformations.