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NAR Top Articles - Structural Biology

Structural Biology

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January 2015

A competitive formation of DNA:RNA hybrid G-quadruplex is responsible to the mitochondrial transcription termination at the DNA replication priming site
K. W. Zheng, R. Y. Wu, Y. D. He, S. Xiao, J. Y. Zhang, J. Q. Liu, Y. H. Hao and Z. Tan
Nucleic Acids Res. (2014) 42 (16): 10832-10844
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Human mitochondrial DNA contains a distinctive guanine-rich motif denoted conserved sequence block II (CSB II) that stops RNA transcription, producing prematurely terminated transcripts to prime mitochondrial DNA replication. Recently, we reported a general phenomenon that DNA:RNA hybrid G-quadruplexes (HQs) readily form during transcription when the non-template DNA strand is guanine-rich and such HQs in turn regulate transcription. In this work, we show that transcription of mitochondrial DNA leads to the formation of a stable HQ or alternatively an unstable intramolecular DNA G-quadruplex (DQ) at the CSB II. The HQ is the dominant species and contributes to the majority of the premature transcription termination. Manipulating the stability of the DQ has little effect on the termination even in the absence of HQ; however, abolishing the formation of HQs by preventing the participation of either DNA or RNA abolishes the vast majority of the termination...

Structural basis of lariat RNA recognition by the intron debranching enzyme Dbr1
E. J. Montemayor, A. Katolik, N. E. Clark, A. B. Taylor, J. P. Schuermann, D. J. Combs, R. Johnsson, S. P. Holloway, S. W. Stevens, M. J. Damha and P. J. Hart
Nucleic Acids Res. (2014) 42 (16): 10845-10855
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The enzymatic processing of cellular RNA molecules requires selective recognition of unique chemical and topological features. The unusual 2',5'-phosphodiester linkages in RNA lariats produced by the spliceosome must be hydrolyzed by the intron debranching enzyme (Dbr1) before they can be metabolized or processed into essential cellular factors, such as snoRNA and miRNA. Dbr1 is also involved in the propagation of retrotransposons and retroviruses, although the precise role played by the enzyme in these processes is poorly understood. Here, we report the first structures of Dbr1 alone and in complex with several synthetic RNA compounds that mimic the branchpoint in lariat RNA. The structures, together with functional data on Dbr1 variants, reveal the molecular basis for 2',5'-phosphodiester recognition and explain why the enzyme lacks activity toward 3',5'-phosphodiester linkages. The findings illuminate structure/function relationships in a unique enzyme that is central to eukaryotic RNA metabolism and set the stage for the rational design of inhibitors...

Molecular insights into replication initiation by Q{beta} replicase using ribosomal protein S1
D. Takeshita, S. Yamashita and K. Tomita
Nucleic Acids Res. (2014) 42 (16): 10809-10822
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Ribosomal protein S1, consisting of six contiguous OB-folds, is the largest ribosomal protein and is essential for translation initiation in Escherichia coli. S1 is also one of the three essential host-derived subunits of Qbeta replicase, together with EF-Tu and EF-Ts, for Qbeta RNA replication in E. coli. We analyzed the crystal structure of Qbeta replicase, consisting of the virus-encoded RNA-dependent RNA polymerase (beta-subunit), EF-Tu, EF-Ts and the N-terminal half of S1, which is capable of initiating Qbeta RNA replication. Structural and biochemical studies revealed that the two N-terminal OB-folds of S1 anchor S1 onto the beta-subunit, and the third OB-fold is mobile and protrudes beyond the surface of the beta-subunit. The third OB-fold mainly interacts with a specific RNA fragment derived from the internal region of Qbeta RNA, and its RNA-binding ability is required for replication initiation of Qbeta RNA. Thus, the third mobile OB-fold of S1, which is spatially anchored near the surface of the beta-subunit, primarily recruits the Qbeta RNA toward the beta-subunit, leading to the specific and efficient replication initiation of Qbeta RNA...

Molecular insights into the interaction of the ribosomal stalk protein with elongation factor 1{alpha}
K. Ito, T. Honda, T. Suzuki, T. Miyoshi, R. Murakami, M. Yao and T. Uchiumi
Nucleic Acids Res. (2014) 42 (22): 14042-14052
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In all organisms, the large ribosomal subunit contains multiple copies of a flexible protein, the so-called 'stalk'. The C-terminal domain (CTD) of the stalk interacts directly with the translational GTPase factors, and this interaction is required for factor-dependent activity on the ribosome. Here we have determined the structure of a complex of the CTD of the archaeal stalk protein aP1 and the GDP-bound archaeal elongation factor aEF1alpha at 2.3 A resolution. The structure showed that the CTD of aP1 formed a long extended alpha-helix, which bound to a cleft between domains 1 and 3 of aEF1alpha, and bridged these domains. This binding between the CTD of aP1 and the aEF1alpha*GDP complex was formed mainly by hydrophobic interactions. The docking analysis showed that the CTD of aP1 can bind to aEF1alpha*GDP located on the ribosome. An additional biochemical assay demonstrated that the CTD of aP1 also bound to the aEF1alpha*GTP*aminoacyl-tRNA complex. These results suggest that the CTD of aP1 interacts with aEF1alpha at various stages in translation...

Attachment site recognition and regulation of directionality by the serine integrases
K. Rutherford, P. Yuan, K. Perry, R. Sharp and G. D. Van Duyne
Nucleic Acids Res. (2013) 41 (17): 8341-8356
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Serine integrases catalyze the integration of bacteriophage DNA into a host genome by site-specific recombination between 'attachment sites' in the phage (attP) and the host (attB). The reaction is highly directional; the reverse excision reaction between the product attL and attR sites does not occur in the absence of a phage-encoded factor, nor does recombination occur between other pairings of attachment sites. A mechanistic understanding of how these enzymes achieve site-selectivity and directionality has been limited by a lack of structural models. Here, we report the structure of the C-terminal domains of a serine integrase bound to an attP DNA half-site. The structure leads directly to models for understanding how the integrase-bound attP and attB sites differ, why these enzymes preferentially form attP x attB synaptic complexes to initiate recombination, and how attL x attR recombination is prevented. In these models, different domain organizations on attP vs. attB half-sites allow attachment-site specific interactions to form between integrase subunits via an unusual protruding coiled-coil motif...

Atomic force microscopy study of DNA flexibility on short length scales: smooth bending versus kinking
A. K. Mazur and M. Maaloum
Nucleic Acids Res. (2014) 42 (22): 14006-14012
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The apparently anomalous flexibility of DNA on short length scales has attracted a lot of attention in recent years. We use atomic force microscopy (AFM) in solution to directly study the DNA bending statistics for small lengths down to one helical turn. The accuracy of experimental estimates could be improved due to a large data volume and a refined algorithm for image processing and measuring bend angles. It is found that, at length scales beyond two helical turns (7 nm), DNA is well described by the harmonic worm-like chain (WLC) model with the bending persistence length of 56 nm. Below this threshold, the AFM data are also described by the WLC model assuming that the accuracy of measured bend angles is limited by the physical width of the double helix. We conclude that the double helical DNA behaves as a uniform elastic rod even at very short length scales. Strong bends due to kinks, melting bubbles and other deviations from the WLC model are statistically negligible.

Architecture and ssDNA interaction of the Timeless-Tipin-RPA complex
J. Witosch, E. Wolf and N. Mizuno
Nucleic Acids Res. (2014) 42 (20): 12912-12927
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The Timeless-Tipin (Tim-Tipin) complex, also referred to as the fork protection complex, is involved in coordination of DNA replication. Tim-Tipin is suggested to be recruited to replication forks via Replication Protein A (RPA) but details of the interaction are unknown. Here, using cryo-EM and biochemical methods, we characterized complex formation of Tim-Tipin, RPA and single-stranded DNA (ssDNA). Tim-Tipin and RPA form a 258 kDa complex with a 1:1:1 stoichiometry. The cryo-EM 3D reconstruction revealed a globular architecture of the Tim-Tipin-RPA complex with a ring-like and a U-shaped domain covered by a RPA lid. Interestingly, RPA in the complex adopts a horse shoe-like shape resembling its conformation in the presence of long ssDNA (>30 nucleotides). Furthermore, the recruitment of the Tim-Tipin-RPA complex to ssDNA is modulated by the RPA conformation and requires RPA to be in the more compact 30 nt ssDNA binding mode. The dynamic formation and disruption of the Tim-Tipin-RPA-ssDNA complex implicates the RPA-based recruitment of Tim-Tipin to the replication fork.

The bacterial antitoxin HipB establishes a ternary complex with operator DNA and phosphorylated toxin HipA to regulate bacterial persistence
Y. Wen, E. Behiels, J. Felix, J. Elegheert, B. Vergauwen, B. Devreese and S. N. Savvides
Nucleic Acids Res. (2014) 42 (15): 10134-10147
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Nearly all bacteria exhibit a type of phenotypic growth described as persistence that is thought to underlie antibiotic tolerance and recalcitrant chronic infections. The chromosomally encoded high-persistence (Hip) toxin-antitoxin proteins HipASO and HipBSO from Shewanella oneidensis, a proteobacterium with unusual respiratory capacities, constitute a type II toxin-antitoxin protein module. Here we show that phosphorylated HipASO can engage in an unexpected ternary complex with HipBSO and double-stranded operator DNA that is distinct from the prototypical counterpart complex from Escherichia coli. The structure of HipBSO in complex with operator DNA reveals a flexible C-terminus that is sequestered by HipASO in the ternary complex, indicative of its role in binding HipASO to abolish its function in persistence. The structure of HipASO in complex with a non-hydrolyzable ATP analogue shows that HipASO autophosphorylation is coupled to an unusual conformational change of its phosphorylation loop. However, HipASO is unable to phosphorylate the translation factor Elongation factor Tu...

U2AF65 adapts to diverse pre-mRNA splice sites through conformational selection of specific and promiscuous RNA recognition motifs
J. L. Jenkins, A. A. Agrawal, A. Gupta, M. R. Green and C. L. Kielkopf
Nucleic Acids Res. (2013) 41 (6): 3859-3873
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Degenerate splice site sequences mark the intron boundaries of pre-mRNA transcripts in multicellular eukaryotes. The essential pre-mRNA splicing factor U2AF(65) is faced with the paradoxical tasks of accurately targeting polypyrimidine (Py) tracts preceding 3' splice sites while adapting to both cytidine and uridine nucleotides with nearly equivalent frequencies. To understand how U2AF(65) recognizes degenerate Py tracts, we determined six crystal structures of human U2AF(65) bound to cytidine-containing Py tracts. As deoxy-ribose backbones were required for co-crystallization with these Py tracts, we also determined two baseline structures of U2AF(65) bound to the deoxy-uridine counterparts and compared the original, RNA-bound structure. Local structural changes suggest that the N-terminal RNA recognition motif 1 (RRM1) is more promiscuous for cytosine-containing Py tracts than the C-terminal RRM2. These structural differences between the RRMs were reinforced by the specificities of wild-type and site-directed mutant U2AF(65) for region-dependent cytosine- and uracil-containing RNA sites...

Structural basis for inhibition of DNA replication by aphidicolin
A. G. Baranovskiy, N. D. Babayeva, Y. Suwa, J. Gu, Y. I. Pavlov and T. H. Tahirov
Nucleic Acids Res. (2014) 42 (22): 14013-14021
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Natural tetracyclic diterpenoid aphidicolin is a potent and specific inhibitor of B-family DNA polymerases, haltering replication and possessing a strong antimitotic activity in human cancer cell lines. Clinical trials revealed limitations of aphidicolin as an antitumor drug because of its low solubility and fast clearance from human plasma. The absence of structural information hampered the improvement of aphidicolin-like inhibitors: more than 50 modifications have been generated so far, but all have lost the inhibitory and antitumor properties. Here we report the crystal structure of the catalytic core of human DNA polymerase alpha (Pol alpha) in the ternary complex with an RNA-primed DNA template and aphidicolin. The inhibitor blocks binding of dCTP by docking at the Pol alpha active site and by rotating the template guanine. The structure provides a plausible mechanism for the selectivity of aphidicolin incorporation opposite template guanine and explains why previous modifications of aphidicolin failed to improve its affinity for Pol alpha. With new structural information, aphidicolin becomes an attractive lead compound for the design of novel derivatives with enhanced inhib

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