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NAR Top Articles - Structural Biology

Structural Biology

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October 2015

U2AF65 adapts to diverse pre-mRNA splice sites through conformational selection of specific and promiscuous RNA recognition motifs
Jenkins, JL; Agrawal, AA; Gupta, A; Green, MR; Kielkopf, CL
Nucleic Acids Res. 2013, 41, 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 30 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
Baranovskiy, AG; Babayeva, ND; Suwa, Y; Gu, JY; Pavlov, YI; Tahirov, TH
Nucleic Acids Res. 2014, 42, 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 inhibitory properties

Internally labeled Cy3/Cy5 DNA constructs show greatly enhanced photo-stability in single-molecule FRET experiments
Lee, W; von Hippel, PH; Marcus, AH
Nucleic Acids Res. 2014, 42, 5967-5977
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DNA constructs labeled with cyanine fluorescent dyes are important substrates for single-molecule (sm) studies of the functional activity of protein-DNA complexes. We previously studied the local DNA backbone fluctuations of replication fork and primer-template DNA constructs labeled with Cy3/Cy5 donor-acceptor Forster resonance energy transfer (FRET) chromophore pairs and showed that, contrary to dyes linked 'externally' to the bases with flexible tethers, direct 'internal' (and rigid) insertion of the chromophores into the sugar-phosphate backbones resulted in DNA constructs that could be used to study intrinsic and protein-induced DNA backbone fluctuations by both smFRET and sm Fluorescent Linear Dichroism (smFLD). Here we show that these rigidly inserted Cy3/Cy5 chromophores also exhibit two additional useful properties, showing both high photo-stability and minimal effects on the local thermodynamic stability of the DNA constructs. The increased photo-stability of the internal labels significantly reduces the proportion of false positive smFRET conversion 'background' signals, thereby simplifying interpretations of both smFRET and smFLD experiments...

Structural basis of lariat RNA recognition by the intron debranching enzyme Dbr1
Montemayor, EJ; Katolik, A; Clark, NE; Taylor, AB; Schuermann, JP; Combs, DJ; Johnsson, R; Holloway, SP; Stevens, SW; Damha, MJ; Hart, PJ
Nucleic Acids Res. 2014, 42, 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 that may represent novel therapeutic agents...

Elastic properties and secondary structure formation of single-stranded DNA at monovalent and divalent salt conditions
Bosco, A; Camunas-Soler, J; Ritort, F
Nucleic Acids Res. 2014, 42, 2064-2074
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Single-stranded DNA (ssDNA) plays a major role in several biological processes. It is therefore of fundamental interest to understand how the elastic response and the formation of secondary structures are modulated by the interplay between base pairing and electrostatic interactions. Here we measure force-extension curves (FECs) of ssDNA molecules in optical tweezers set up over two orders of magnitude of monovalent and divalent salt conditions, and obtain its elastic parameters by fitting the FECs to semiflexible models of polymers. For both monovalent and divalent salts, we find that the electrostatic contribution to the persistence length is proportional to the Debye screening length, varying as the inverse of the square root of cation concentration. The intrinsic persistence length is equal to 0.7nm for both types of salts, and the effectivity of divalent cations in screening electrostatic interactions appears to be 100-fold as compared with monovalent salt, in line with what has been recently reported for single-stranded RNA. Finally, we propose an analysis of the FECs using a model that accounts for the effective thickness of the filament at low salt condition...

Structural insights into Paf1 complex assembly and histone binding
Chu, XL; Qin, XH; Xu, HS; Li, L; Wang, Z; Li, FZ; Xie, XQ; Zhou, H; Shen, YQ; Long, JF
Nucleic Acids Res. 2013, 41, 10619-10629
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The highly conserved Paf1 complex (PAF1C) plays critical roles in RNA polymerase II transcription elongation and in the regulation of histone modifications. It has also been implicated in other diverse cellular activities, including posttranscriptional events, embryonic development and cell survival and maintenance of embryonic stem cell identity. Here, we report the structure of the human Paf1/Leo1 subcomplex within PAF1C. The overall structure reveals that the Paf1 and Leo1 subunits form a tightly associated heterodimer through antiparallel beta-sheet interactions. Detailed biochemical experiments indicate that Leo1 binds to PAF1C through Paf1 and that the Ctr9 subunit is the key scaffold protein in assembling PAF1C. Furthermore, we show that the Paf1/Leo1 heterodimer is necessary for its binding to histone H3, the histone octamer, and nucleosome in vitro. Our results shed light on the PAF1C assembly process and substrate recognition during various PAF1C-coordinated histone modifications.

Structure of human RNA N6-methyladenine demethylase ALKBH5 provides insights into its mechanisms of nucleic acid recognition and demethylation
Aik, W; Scotti, JS; Choi, H; Gong, LZ; Demetriades, M; Schofield, CJ; McDonough, MA
Nucleic Acids Res. 2014, 42, 4741-4754
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ALKBH5 is a 2-oxoglutarate (2OG) and ferrous iron-dependent nucleic acid oxygenase (NAOX) that catalyzes the demethylation of N-6-methyladenine in RNA. ALKBH5 is upregulated under hypoxia and plays a role in spermatogenesis. We describe a crystal structure of human ALKBH5 (residues 66-292) to 2.0 A resolution. ALKBH5(66-292) has a double-stranded beta-helix core fold as observed in other 2OG and iron-dependent oxygenase family members. The active site metal is octahedrally coordinated by an HXD...H motif (comprising residues His204, Asp206 and His266) and three water molecules. ALKBH5 shares a nucleotide recognition lid and conserved active site residues with other NAOXs. A large loop (beta IV-V) in ALKBH5 occupies a similar region as the L1 loop of the fat mass and obesity-associated protein that is proposed to confer single-stranded RNA selectivity. Unexpectedly, a small molecule inhibitor, IOX3, was observed covalently attached to the side chain of Cys200 located outside of the active site. Modelling substrate into the active site based on other NAOX-nucleic acid complexes reveals conserved residues...

Formation of circular polyribosomes on eukaryotic mRNA without cap-structure and poly(A)-tail: a cryo electron tomography study
Afonina, ZA; Myasnikov, AG; Shirokov, VA; Klaholz, BP; Spirin, AS
Nucleic Acids Res. 2014, 42, 9461-9469
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The polyribosomes newly formed on recombinant GFP-encoding mRNAs in a wheat germ cell-free translation system were analyzed using cryo-electron tomography, with sub-tomogram averaging of polysomal ribosomes and reconstruction of 3D structures of individual polyribosomes. The achieved level of resolution in the reconstructed polyribosomes allowed deducing the mRNA path by connecting adjacent exit and entry sites at the ribosomes inside each polyribosome. In this way, the circularity of a significant fraction (about 50%) of translating polyribosomes was proved in the case of the capped poly(A)-tailed mRNA, in agreement with the existing paradigm of the circularization via interaction of cap-bound initiation factor eIF4F with poly(A)-binding protein. However, translation of the capped mRNA construct without poly(A) tail, but with unspecific 3'-UTR derived from non-coding plasmid sequence, also led to the formation of circular polyribosomes in similar proportion (40%). Moreover, the polyribosomes formed on the uncapped non-polyadenylated mRNA with non-synergistic 5' and 3'-UTRs proved to be circular as well...

Structural studies of p53 inactivation by DNA-contact mutations and its rescue by suppressor mutations via alternative protein-DNA interactions
Eldar, A; Rozenberg, H; Diskin-Posner, Y; Rohs, R; Shakked, Z
Nucleic Acids Res. 2013, 41, 8748-8759
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A p53 hot-spot mutation found frequently in human cancer is the replacement of R273 by histidine or cysteine residues resulting in p53 loss of function as a tumor suppressor. These mutants can be reactivated by the incorporation of second-site suppressor mutations. Here, we present high-resolution crystal structures of the p53 core domains of the cancer-related proteins, the rescued proteins and their complexes with DNA. The structures show that inactivation of p53 results from the incapacity of the mutated residues to form stabilizing interactions with the DNA backbone, and that reactivation is achieved through alternative interactions formed by the suppressor mutations. Detailed structural and computational analysis demonstrates that the rescued p53 complexes are not fully restored in terms of DNA structure and its interface with p53...

A competitive formation of DNA:RNA hybrid G-quadruplex is responsible to the mitochondrial transcription termination at the DNA replication priming site
Zheng, KW; Wu, RY; He, YD; Xiao, S; Zhang, JY; Liu, JQ; Hao, YH; Tan, Z
Nucleic Acids Res. 2014, 42, 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. These results demonstrate that the type of G-quadruplexes (HQ or DQ) is a crucial determinant in directing the transcription termination at the CSB II...

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