Tumor Suppressor p53-Mediated Structural Reorganization of the Transcriptional Coactivator p300.

Transcriptional coactivator p300, a critical player in eukaryotic gene regulation, primarily functions as a histone acetyltransferase (HAT). It is also an important player in acetylation of a number of nonhistone proteins, p53 being the most prominent one. Recruitment of p300 to p53 is pivotal in the regulation of p53-dependent genes. Emerging evidence suggests that p300 adopts an active conformation upon binding to the tetrameric p53, resulting in its enhanced acetylation activity. As a modular protein, p300 consists of multiple well-defined domains, where the structured domains are interlinked with unstructured linker regions. A crystal structure of the central domain of p300 encompassing Bromo, RING, PHD, and HAT domains demonstrates a compact module, where the HAT active site stays occluded by the RING domain. However, although p300 has a significant role in mediating the transcriptional activity of p53, only a few structural details on the complex of these two full-length proteins are available. Here, we present a cryo-electron microscopy (cryo-EM) study on the p300-p53 complex. The three-dimensional cryo-EM density map of the p300-p53 complex, when compared to the cryo-EM map of free p300, revealed that substantial change in the relative arrangement of Bromo and HAT domains occurs upon complex formation, which is likely required for exposing HAT active site and subsequent acetyltransferase activity. Our observation correlates well with previous studies showing that the presence of Bromodomain is obligatory for effective acetyltransferase activity of HAT. Thus, our result sheds new light on the mechanism whereby p300, following binding with p53, gets activated.

Enhanced basepair dynamics pre-disposes protein-assisted flips of key bases in DNA strand separation during transcription initiation.

Localized separation of strands of duplex DNA is a necessary step in many DNA-dependent processes, including transcription and replication. Little is known about how these strand separations occur. The strand-separated E.coli RNA polymerase-promoter open-complex structure showed four bases of the non-template strand, the master base -11A, -7, -6 and +2, in a flipped state and inserted into protein pockets. To explore whether any property of these bases in the duplex state pre-disposes them to flipping, NMR studies were performed on a wild-type promoter in the duplex state. Measurement of relaxation times indicates that a limited number of base pairs, including the flipped ones, have faster opening rates than the rest. Molecular dynamics studies also show an inherently high dynamic character of the -11A:T base pair in the wild-type strand-paired state. In order to explore the role of the RNA polymerase in the flipping process, we have used 2-aminopurine as a fluorescent probe. Slower kinetics of the increase of 2-aminopurine fluorescence was observed with RNA polymerases containing several mutant σ70s. This may be interpreted as the protein playing an important role in enhancing the flipping rate. These results suggest that flipping of -11A, and perhaps other flipped bases observed in the open-complex, is facilitated by its inherent proclivity to open-up with further assistance from the protein, thus leading to a strand-open state. Other DNA-based processes that require strand-separation may use similar pathways for strand separation. We conclude that not only basepair stability, but also dynamics may play an important role in the strand-separation.

Specific DNA sequences allosterically enhance protein-protein interaction in a transcription factor through modulation of protein dynamics: implications for specificity of gene regulation.

Most genes are regulated by multiple transcription factors, often assembling into multi-protein complexes in the gene regulatory region. Understanding of the molecular origin of specificity of gene regulatory complex formation in the context of the whole genome is currently inadequate. A phage transcription factor λ-CI forms repressive multi-protein complexes by binding to multiple binding sites in the genome to regulate the lifecycle of the phage. The protein-protein interaction between two DNA-bound λ-CI molecules is stronger when they are bound to the correct pair of binding sites, suggesting allosteric transmission of recognition of correct DNA sequences to the protein-protein interaction interface. Exploration of conformation and dynamics by time-resolved fluorescence anisotropy decay and molecular dynamics suggests a change in protein dynamics to be a crucial factor in mediating allostery. A lattice-based model suggests that DNA-sequence induced allosteric effects could be crucial underlying factors in differentially stabilizing the correct site-specific gene regulatory complexes. We conclude that transcription factors have evolved multiple mechanisms to augment the specificity of DNA-protein interactions in order to achieve an extraordinarily high degree of spatial and temporal specificities of gene regulatory complexes, and DNA-sequence induced allostery plays an important role in the formation of sequence-specific gene regulatory complexes.

Ultrafast differential flexibility of Cro-protein binding domains of two operator DNAs with different sequences.

The nature of the interface of specific protein-DNA complexes has attracted immense interest in contemporary molecular biology. Although extensive studies on the role of flexibility of DNA in the specific interaction in the genetic regulatory activity of lambda Cro (Cro-protein) have been performed, the exploration of quantitative features remains deficient. In this study, we have mutated (site directed mutagenesis: SDM) Cro-protein at the 37th position with a cysteine residue (G37C) retaining the functional integrity of the protein and labelled the cysteine residue, which is close to the interface, with a fluorescent probe (AEDANS), for the investigation of its interface with operator DNAs (OR3 and OR2). We have employed picosecond resolved polarization gated fluorescence spectroscopy and the well known strategy of solvation dynamics for the exploration of physical motions of the fluorescent probes and associated environments, respectively. Even though this particular probe on the protein (AEDANS) shows marginal changes in its structural flexibility upon interaction with the DNAs, a non-covalent DNA bound probe (DAPI), which binds to the minor groove, shows a major differential alteration in the dynamical flexibility in the OR3-Cro complex when compared to that of the OR2 complex with the Cro-protein. We attempt to correlate the observed significant structural fluctuation of the Cro-protein binding domain of OR3 for the specificity of the protein to the operator DNA.

Structural and mutational analysis of archaeal ATP-dependent RNA ligase identifies amino acids required for RNA binding and catalysis.

An ATP-dependent RNA ligase from Methanobacterium thermoautotrophicum (MthRnl) catalyzes intramolecular ligation of single-stranded RNA to form a closed circular RNA via covalent ligase-AMP and RNA-adenylylate intermediate. Here, we report the X-ray crystal structures of an MthRnl•ATP complex as well as the covalent MthRnl-AMP intermediate. We also performed structure-guided mutational analysis to survey the functions of 36 residues in three component steps of the ligation pathway including ligase-adenylylation (step 1), RNA adenylylation (step 2) and phosphodiester bond synthesis (step 3). Kinetic analysis underscored the importance of motif 1a loop structure in promoting phosphodiester bond synthesis. Alanine substitutions of Thr117 or Arg118 favor the reverse step 2 reaction to deadenylate the 5'-AMP from the RNA-adenylate, thereby inhibiting step 3 reaction. Tyr159, Phe281 and Glu285, which are conserved among archaeal ATP-dependent RNA ligases and are situated on the surface of the enzyme, are required for RNA binding. We propose an RNA binding interface of the MthRnl based on the mutational studies and two sulfate ions that co-crystallized at the active site cleft in the MthRnl-AMP complex.

Crystal structure analysis of NP24-I: a thaumatin-like protein.

The crystal structure of NP24-I, an isoform of the thaumatin-like protein (TLP) NP24 from tomato, has been reported. A prominent acidic cleft is observed between domains I and II of the three-domain structure of this antifungal protein, a feature common to other antifungal TLPs. The defensive role of the TLPs has also been attributed to their beta-1,3-glucanase activity and here too the acidic cleft is reported to play a vital role. NP24 is known to bind beta-glucans and so a linear beta-1,3-glucan molecule has been docked in the interdomain cleft of NP24-I. From the docked complex it is observed that the beta-glucan chain is so positioned in the cleft that a Glu and Asp residue on either side of it may form a catalytic pair to cause the cleavage of a glycosidic bond. NP24 has been reported to be an allergenic protein and an allergenic motif could be identified on the surface of the helical domain II of NP24-I. In addition, some allergenic motifs bearing high similarity/identity with some predicted Ig-E binding motifs of closely related allergenic TLPs like Jun a 3 (Juniperus ashei, from mountain cedar pollen) and banana-TLP have been identified on the molecular surface of NP24-I.

Structural insights into the substrate specificity and activity of ervatamins, the papain-like cysteine proteases from a tropical plant, Ervatamia coronaria.

Multiple proteases of the same family are quite often present in the same species in biological systems. These multiple proteases, despite having high homology in their primary and tertiary structures, show deviations in properties such as stability, activity, and specificity. It is of interest, therefore, to compare the structures of these multiple proteases in a single species to identify the structural changes, if any, that may be responsible for such deviations. Ervatamin-A, ervatamin-B and ervatamin-C are three such papain-like cysteine proteases found in the latex of the tropical plant Ervatamia coronaria, and are known not only for their high stability over a wide range of temperature and pH, but also for variations in activity and specificity among themselves and among other members of the family. Here we report the crystal structures of ervatamin-A and ervatamin-C, complexed with an irreversible inhibitor 1-[l-N-(trans-epoxysuccinyl)leucyl]amino-4-guanidinobutane (E-64), together with enzyme kinetics and molecular dynamic simulation studies. A comparison of these results with the earlier structures helps in a correlation of the structural features with the corresponding functional properties. The specificity constants (k(cat)/K(m)) for the ervatamins indicate that all of these enzymes have specificity for a branched hydrophobic residue at the P2 position of the peptide substrates, with different degrees of efficiency. A single amino acid change, as compared to ervatamin-C, in the S2 pocket of ervatamin-A (Ala67-->Tyr) results in a 57-fold increase in its k(cat)/K(m) value for a substrate having a Val at the P2 position. Our studies indicate a higher enzymatic activity of ervatamin-A, which has been subsequently explained at the molecular level from the three-dimensional structure of the enzyme and in the context of its helix polarizibility and active site plasticity.

A thermostable cysteine protease precursor from a tropical plant contains an unusual C-terminal propeptide: cDNA cloning, sequence comparison and molecular modeling studies.

We report here the cloning and characterization of the entire cDNA of a papain-like cysteine protease from a tropical flowering plant. The 1098-bp ORF of the cDNA codify a protease precursor having a signal peptide of 19 amino acids, a cathepsin-L like N-terminal proregion of 114 amino acids, a mature enzyme part of 208 amino acids and a C-terminal proregion of 24 amino acids. The derived amino acid sequence of the mature part tallies with the thermostable cysteine protease Ervatamin-C--as was aimed at. The C-terminal proregion of the protease has altogether a different sequence pattern not observed in other members of the family and it contains a negatively charged helical zone. The three-dimensional model of the precursor, based on the homology modeling and X-ray structure, shows that the extended peptide stretch region of the N-terminal propeptide, covering the interdomain cleft, contains protruding side chains of positively charged residues. This study also indicates that the negatively charged zone of C-terminal propeptide may interact with the positively charged zone of the N-terminal propeptide in a cooperative manner in the maturation process of this enzyme.

Crystallization and preliminary X-ray diffraction studies of NP24-I, an isoform of a thaumatin-like protein from ripe tomato fruits.

NP24 is a 24 kDa (207-amino-acid) antifungal thaumatin-like protein (TLP) found in tomato fruits. An isoform of the protein, NP24-I, is reported to play a possible role in ripening of the fruit in addition to its antifungal properties. The protein has been isolated and purified and crystallized by the hanging-drop vapour-diffusion method. The crystals belong to the tetragonal space group P4(3), with unit-cell parameters a = b = 61.01, c = 62.90 A and one molecule per asymmetric unit. X-ray diffraction data were processed to a resolution of 2.45 A and the structure was solved by molecular replacement.