Protein gates in DNA topoisomerase II

The difficult case of crystallization and structure solution for the ParC55 breakage-reunion domain of topoisomerase IV from Streptococcus pneumoniae

Background

Streptococcus pneumoniae is the major cause of community-acquired pneumonia and is also associated with bronchitis, meningitis, otitis and sinusitis. The emergence and increasing prevalence of resistance to penicillin and other antibiotics has led to interest in other anti-pneumonococcal drugs such as quinolones that target the enzymes DNA gyrase and topoisomerase IV. During crystallization and in the avenues to finding a method to determine phases for the structure of the ParC55 breakage-reunion domain of topoisomerase IV from Streptococcus pneumoniae, obstacles were faced at each stage of the process. These problems included: majority of the crystals being twinned, either non-diffracting or exhibiting a high mosaic spread. The crystals, which were grown under conditions that favoured diffraction, were difficult to flash-freeze without loosing diffraction. The initial structure solution by molecular replacement failed and the approach proved to be unviable due to the complexity of the problem. In the end the successful structure solution required an in-depth data analysis and a very detailed molecular replacement search.

Methodology/Principal Findings

Crystal anti-twinning agents have been tested and two different methods of flash freezing have been compared. The fragility of the crystals did not allow the usual method of transferring the crystals into the heavy atom solution. Consequently, it was necessary to co-crystallize in the presence of the heavy atom compound. The multiple isomorphous replacement approach was unsuccessful because the 7 cysteine mutants which were engineered could not be successfully derivatized. Ultimately, molecular replacement was used to solve the structure by sorting through a large number of solutions in space group P1 using CNS.

Conclusions/Significance

The main objective of this paper is to describe the obstacles which were faced and overcome in order to acquire data sets on such difficult crystals and determine phases for successful structure solution.

Breakage-reunion domain of Streptococcus pneumoniae topoisomerase IV: crystal structure of a gram-positive quinolone target

The 2.7 A crystal structure of the 55-kDa N-terminal breakage-reunion domain of topoisomerase (topo) IV subunit A (ParC) from Streptococcus pneumoniae, the first for the quinolone targets from a gram-positive bacterium, has been solved and reveals a 'closed' dimer similar in fold to Escherichia coli DNA gyrase subunit A (GyrA), but distinct from the 'open' gate structure of Escherichia coli ParC. Unlike GyrA whose DNA binding groove is largely positively charged, the DNA binding site of ParC exhibits a distinct pattern of alternating positively and negatively charged regions coincident with the predicted positions of the grooves and phosphate backbone of DNA. Based on the ParC structure, a new induced-fit model for sequence-specific recognition of the gate (G) segment by ParC has been proposed. These features may account for the unique DNA recognition and quinolone targeting properties of pneumococcal type II topoisomerases compared to their gram-negative counterparts.

Dissection of the bacteriophage Mu strong gyrase site (SGS): significance of the SGS right arm in Mu biology and DNA gyrase mechanism

The bacteriophage Mu strong gyrase site (SGS), required for efficient phage DNA replication, differs from other gyrase sites in the efficiency of gyrase binding coupled with a highly processive supercoiling activity. Genetic studies have implicated the right arm of the SGS as a key structural feature for promoting rapid Mu replication. Here, we show that deletion of the distal portion of the right arm abolishes efficient binding, cleavage, and supercoiling by DNA gyrase in vitro. DNase I footprinting analysis of the intact SGS revealed an adenylyl imidodiphosphate-dependent change in protection in the right arm, indicating that this arm likely forms the T segment that is passed through the cleaved G segment during the supercoiling reaction. Furthermore, in an SGS derivative with an altered right-arm sequence, the left arm showed these changes, suggesting that the selection of a T segment by gyrase is determined primarily by the sequences of the arms. Analysis of the sequences of the SGS and other gyrase sites suggests that the choice of T segment correlates with which arm possesses the more extensive set of phased anisotropic bending signals, with the Mu right arm possessing an unusually extended set of such signals. The implications of these observations for the structure of the gyrase-DNA complex and for the biological function of the Mu SGS are discussed.

Nucleotide-dependent domain movement in the ATPase domain of a human type IIA DNA topoisomerase

Type IIA DNA topoisomerases play multiple essential roles in the management of higher-order DNA structure, including modulation of topological state, chromosome segregation, and chromatin condensation. These diverse physiologic functions are all accomplished through a common molecular mechanism, wherein the protein catalyzes transient cleavage of a DNA duplex (the G-segment) to yield a double-stranded gap through which another duplex (the T-segment) is passed. The overall process is orchestrated by the opening and closing of molecular "gates" in the topoisomerase structure, which is regulated by ATP binding, hydrolysis, and release of ADP and inorganic phosphate. Here we present two crystal structures of the ATPase domain of human DNA topoisomerase IIalpha in different nucleotide-bound states. Comparison of these structures revealed rigid-body movement of the structural modules within the ATPase domain, suggestive of the motions of a molecular gate.

Monoclonal antibodies to mycobacterial DNA gyrase A inhibit DNA supercoiling activity

DNA gyrase is an essential type II topoisomerase found in bacteria. We have previously characterized DNA gyrase from Mycobacterium tuberculosis and Mycobacterium smegmatis. In this study, several monoclonal antibodies were generated against the gyrase A subunit (GyrA) of M. smegmatis. Three, MsGyrA:C3, MsGyrA:H11 and MsGyrA:E9, were further analyzed for their interaction with the enzyme. The monoclonal antibodies showed high degree of cross-reactivity with both fast-growing and slow-growing mycobacteria. In contrast, none recognized Escherichia coli GyrA. All the three monoclonal antibodies were of IgG1 isotype falling into two distinct types with respect to epitope recognition and interaction with the enzyme. MsGyrA:C3 and MsGyrA:H11 IgG, and their respective Fab fragments, inhibited the DNA supercoiling activity catalyzed by mycobacterial DNA gyrase. The epitope for the neutralizing monoclonal antibodies appeared to involve the region towards the N-terminus (residues 351-415) of the enzyme in a conformation-dependent manner. These monoclonal antibodies would serve as valuable tools for structure-function analysis and immunocytological studies of mycobacterial DNA gyrase. In addition, they would be useful for designing peptide inhibitors against DNA gyrase.

The 2-pyridone antibacterial agents: bacterial topoisomerase inhibitors

Many attempts have been made to prepare analogs of 4-quinolone antibacterial agents bearing novel ring systems, which might retain the favorable properties of these widely used antibacterial agents and at the same time increase activity against multidrug-resistant bacteria, streptococci, and anaerobic microorganisms. One such attempt involved bioisosteric exchange of the 1-N atom and 4a-C atom of naphthyridones, quinolones, and benzoxazines to produce a family of highly active pyridopyrimidines, quinolizines, and ofloxacin bioisosteres. These new antibacterial agents have been named collectively as the 2-pyridones. Many hundreds of 2-pyridones have been synthesized and evaluated in vitro and in vivo, and selected members are advancing toward human clinical trials. Preparation of these bioisosteres required the development of enabling chemistry, as previous methods were unsuccessful in producing the needed core structures. This review compares the structure-activity relationships of these agents with known trends among 4-quinolones, from which it is seen that there are many parallels, but also some significant departures as well. Generally, 2-pyridones are more highly active in vitro and in vivo and more water soluble than comparable 4-quinolones. These properties are posited to arise from electronic and conformational alternations in these new substances. Selected members show excellent pharmacodynamic properties, justifying the view that this is a very promising new class of totally synthetic antibacterial agents.

Symmetry and chirality in topoisomerase II-DNA crossover recognition

Several experimental data support the notion that the recognition of DNA crossovers play an important role in the multiple functions of topoisomerase II. Here, a theoretical analysis of the possible modes of assembly of yeast topoisomerase II with right and left-handed tight DNA crossovers is performed, using the crystal coordinates of the docking partners. The DNA crossovers are assumed to be clamped into the central hole of the enzyme. Taking into account the rules for building symmetric ternary complexes and the structural constraints imposed by DNA-DNA and protein-DNA interactions, this analysis shows that two geometric solutions could exist, depending on the chirality of the DNA crossovers. In the first one, the two DNA segments are symmetrically recognized by the enzyme while each single double helix binds asymmetrically the protein dimer. In the second one, each double helix is symmetrically recognized by the protein around its dyad axis, while the two DNA segments have their own binding modes. The finding of potential DNA-binding domains which could interact with the crossovers provides structural supports for each model. The structural similarity of a loop containing a cluster of conserved basic residues pointing into the central hole of topoisomerase II and the second DNA-binding site of histone H5 which binds DNA crossover is of particular interest. Each solution, which is consistent with different sets of experimental data found in the literature, could either correspond to different functions of the enzyme or different steps of the reaction. This work provides structural insights for better understanding the role of chirality and symmetry in topoisomerase II-DNA crossover recognition, suggests testable experiments to further elucidate the structure of ternary complexes, and raises new questions about the relationships between the mechanism of strand-passage and strand-exchange catalyzed by the enzyme.

DNA binding properties of minor groove binders and their influence on the topoisomerase II cleavage reaction

We present titrations of the human delta beta-globin gene region with DNA minor groove binders netropsin, bisnetropsin, distamycin, chromomycin and four bis-quaternary ammonium compounds in the presence of calf thymus topoisomerase II and DNase I. With increasing ligand concentration, stimulation and inhibition of enzyme activity were detected and quantitatively evaluated. Additionally we show a second type of stimulation, the appearance of strong new topoisomerase II cleavage sites at high ligand concentrations. The specific binding sites of the minor groove binders of the DNA sequence and their microscopic binding constants were determined from DNase I footprints. A binding mechanism for minor groove binders is proposed in order to explain these results especially when ligand concentration is increased.