[Show abstract][Hide abstract] ABSTRACT: Poly(ADP-ribose) polymerase 1 (PARP-1) plays an important role in DNA repair, but also contributes to other aspects of nucleic acid metabolism, such as transcriptional regulation. Modification of PARP-1 with the small ubiquitin-related modifier (SUMO) affects its function as a transcriptional co-activator of hypoxia-responsive genes and promotes induction of the heat shock-induced HSP70.1 promoter. We now report that PARP-1 sumoylation is strongly influenced by DNA. Consistent with a function in transcription, we show that sumoylation in vitro is enhanced by binding to intact, but not to damaged DNA, in a manner clearly distinct from the mechanism by which DNA damage stimulates PARP-1's catalytic activity. An enhanced affinity of PARP-1 for the SUMO-conjugating enzyme Ubc9 upon binding to DNA is likely responsible for this effect. Sumoylation does not interfere with the catalytic or DNA-binding properties of PARP-1, and structural analysis reveals no significant impact of SUMO on the conformation of PARP-1's DNA-binding domain. In vivo, sumoylated PARP-1 is associated with chromatin, but the modification is not responsive to DNA damage and is not affected by PARP-1 catalytic activity. Our results suggest that PARP-1's alternative modes of DNA recognition serve as a means to differentiate between distinct aspects of the enzyme's function.
DNA repair 07/2013; 12(9). DOI:10.1016/j.dnarep.2013.07.001 · 3.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Polyadenylation regulation and efficient nuclear export of mature mRNPs both require the polyadenosine-RNA-binding protein, Nab2, which contains seven CCCH Zn fingers. We describe here the solution structure of fingers 5-7, which are necessary and sufficient for high-affinity polyadenosine-RNA binding, and identify key residues involved. These Zn fingers form a single structural unit. Structural coherence is lost in the RNA-binding compromised Nab2-C437S mutant, which also suppresses the rat8-2 allele of RNA helicase Dbp5. Structure-guided Nab2 variants indicate that dbp5(rat8-2) suppression is more closely linked to hyperadenylation and suppression of mutant alleles of the nuclear RNA export adaptor, Yra1, than to affinity for polyadenosine-RNA. These results indicate that, in addition to modulating polyA tail length, Nab2 has an unanticipated function associated with generating export-competent mRNPs, and that changes within fingers 5-7 lead to suboptimal assembly of mRNP export complexes that are more easily disassembled by Dbp5 upon reaching the cytoplasm.
[Show abstract][Hide abstract] ABSTRACT: Accurate read-out of chromatin modifications is essential for eukaryotic life. Mutations in the gene encoding X-linked ATRX protein cause a mental-retardation syndrome, whereas wild-type ATRX protein targets pericentric and telomeric heterochromatin for deposition of the histone variant H3.3 by means of a largely unknown mechanism. Here we show that the ADD domain of ATRX, in which most syndrome-causing mutations occur, engages the N-terminal tail of histone H3 through two rigidly oriented binding pockets, one for unmodified Lys4 and the other for di- or trimethylated Lys9. In vivo experiments show this combinatorial readout is required for ATRX localization, with recruitment enhanced by a third interaction through heterochromatin protein-1 (HP1) that also recognizes trimethylated Lys9. The cooperation of ATRX ADD domain and HP1 in chromatin recruitment results in a tripartite interaction that may span neighboring nucleosomes and illustrates how the 'histone-code' is interpreted by a combination of multivalent effector-chromatin interactions.
[Show abstract][Hide abstract] ABSTRACT: Poly(ADP-ribose)polymerase-1 (PARP-1) is a highly abundant chromatin-associated enzyme present in all higher eukaryotic cell nuclei, where it plays key roles in the maintenance of genomic integrity, chromatin remodeling and transcriptional control. It binds to DNA single- and double-strand breaks through an N-terminal region containing two zinc fingers, F1 and F2, following which its C-terminal catalytic domain becomes activated via an unknown mechanism, causing formation and addition of polyadenosine-ribose (PAR) to acceptor proteins including PARP-1 itself. Here, we report a biophysical and structural characterization of the F1 and F2 fingers of human PARP-1, both as independent fragments and in the context of the 24-kDa DNA-binding domain (F1+F2). We show that the fingers are structurally independent in the absence of DNA and share a highly similar structural fold and dynamics. The F1+F2 fragment recognizes DNA single-strand breaks as a monomer and in a single orientation. Using a combination of NMR spectroscopy and other biophysical techniques, we show that recognition is primarily achieved by F2, which binds the DNA in an essentially identical manner whether present in isolation or in the two-finger fragment. F2 interacts much more strongly with nicked or gapped DNA ligands than does F1, and we present a mutational study that suggests origins of this difference. Our data suggest that different DNA lesions are recognized by the DNA-binding domain of PARP-1 in a highly similar conformation, helping to rationalize how the full-length protein participates in multiple steps of DNA single-strand breakage and base excision repair.
[Show abstract][Hide abstract] ABSTRACT: Eukaryotic transcriptional repressors function by recruiting large coregulatory complexes that target histone deacetylase enzymes to gene promoters and enhancers. Transcriptional repression complexes, assembled by the corepressor NCoR and its homolog SMRT, are crucial in many processes, including development and metabolic physiology. The core repression complex involves the recruitment of three proteins, HDAC3, GPS2 and TBL1, to a highly conserved repression domain within SMRT and NCoR. We have used structural and functional approaches to gain insight into the architecture and biological role of this complex. We report the crystal structure of the tetrameric oligomerization domain of TBL1, which interacts with both SMRT and GPS2, and the NMR structure of the interface complex between GPS2 and SMRT. These structures, together with computational docking, mutagenesis and functional assays, reveal the assembly mechanism and stoichiometry of the corepressor complex.
[Show abstract][Hide abstract] ABSTRACT: Addition of poly(ADP-ribose) (PAR) is an important post-translational modification in higher eukaryotes. Several DNA repair and checkpoint proteins possess specific PAR-binding zinc-finger (PBZ) modules critical for function. Here, we present solution structures of the two PBZ modules of aprataxin and PNK-like factor (APLF), revealing a novel type of zinc finger. By combining in vivo PAR-binding data with NMR interaction data using PAR fragments, we propose a structural basis for PBZ-PAR recognition.
[Show abstract][Hide abstract] ABSTRACT: The ubiquitin-associated (UBA) domain of the principal Saccharomyces cerevisiae mRNA nuclear export factor, Mex67, can bind both nuclear pore protein (nucleoporin) FG repeats and Hpr1, a component of the TREX.THO complex that functions to link transcription and export. Using fluorescence resonance energy transfer-based assays, we show here that Hpr1 and the FG repeats interact with overlapping binding sites on the Mex67 UBA domain. We present the solution structure of the Mex67 UBA domain (UBA-Mex67) complexed with a FXFG nucleoporin peptide and define residues engaged in the interaction and those involved in the FXFG-induced conformational change. We show by NMR titration that the binding of Hpr1 produces analogous changes in chemical shifts in similar regions of the UBA domain. Together the data presented here indicate that both Hpr1 and FXFG nucleoporins may bind in a similar way to the UBA-Mex67 domain. However, whereas binding of Hpr1 allows UBA-Mex67 to interact with tetra-ubiquitin, the complex between UBA-Mex67 and FXFG is unable to bind mono- or tetra-ubiquitin, suggesting that both substrate binding and also the nature of the substrate may influence the affinity of the UBA-Mex67 domain for ubiquitin.
[Show abstract][Hide abstract] ABSTRACT: Rds3p, a component of the U2 snRNP subcomplex SF3b, is essential for pre-mRNA splicing and is extremely well conserved in all eukaryotic species. We report here the solution structure of Rds3p, which reveals an unusual knotted fold unrelated to previously known knotted proteins. Rds3p has a triangular shape with a GATA-like zinc finger at each vertex. Pairs of cysteines contributing to each finger are arranged nonsequentially in a permuted arrangement reminiscent of domain-swapping but which here involves segments of subdomains within a single chain. We suggest that the structure arose through a process of segment swapping after gene duplication. The fingers are connected through beta-strands and loops, forming an overall topology strongly resembling a "triquetra knot." The conservation and surface properties of Rds3p suggest that it functions as a platform for protein assembly within the multiprotein SF3b complex of U2 snRNP. The recombinant protein used for structure determination is biologically active, as it restores splicing activity in a yeast splicing extract depleted of native Rds3p.
Proceedings of the National Academy of Sciences 08/2008; 105(28):9621-6. DOI:10.1073/pnas.0802494105 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nuclear abundant poly(A) RNA-binding protein 2 (Nab2) is an essential yeast heterogeneous nuclear ribonucleoprotein that modulates both mRNA nuclear export and poly(A) tail length. The N-terminal domain of Nab2 (residues 1-97) mediates interactions with both the C-terminal globular domain of the nuclear pore-associated protein, myosin-like protein 1 (Mlp1), and the mRNA export factor, Gfd1. The solution and crystal structures of the Nab2 N-terminal domain show a primarily helical fold that is analogous to the PWI fold found in several other RNA-binding proteins. In contrast to other PWI-containing proteins, we find no evidence that the Nab2 N-terminal domain binds to nucleic acids. Instead, this domain appears to mediate protein:protein interactions that facilitate the nuclear export of mRNA. The Nab2 N-terminal domain has a distinctive hydrophobic patch centered on Phe73, consistent with this region of the surface being a protein:protein interaction site. Engineered mutations within this hydrophobic patch attenuate the interaction with the Mlp1 C-terminal domain but do not alter the interaction with Gfd1, indicating that this patch forms a crucial component of the interface between Nab2 and Mlp1.
[Show abstract][Hide abstract] ABSTRACT: The chromatin-associated protein ATRX was originally identified because mutations in the ATRX gene cause a severe form of syndromal X-linked mental retardation associated with alpha-thalassemia. Half of all of the disease-associated missense mutations cluster in a cysteine-rich region in the N terminus of ATRX. This region was named the ATRX-DNMT3-DNMT3L (ADD) domain, based on sequence homology with a family of DNA methyltransferases. Here, we report the solution structure of the ADD domain of ATRX, which consists of an N-terminal GATA-like zinc finger, a plant homeodomain finger, and a long C-terminal alpha-helix that pack together to form a single globular domain. Interestingly, the alpha-helix of the GATA-like finger is exposed and highly basic, suggesting a DNA-binding function for ATRX. The disease-causing mutations fall into two groups: the majority affect buried residues and hence affect the structural integrity of the ADD domain; another group affects a cluster of surface residues, and these are likely to perturb a potential protein interaction site. The effects of individual point mutations on the folding state and stability of the ADD domain correlate well with the levels of mutant ATRX protein in patients, providing insights into the molecular pathophysiology of ATR-X syndrome.
Proceedings of the National Academy of Sciences 08/2007; 104(29):11939-44. DOI:10.1073/pnas.0704057104 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The ubiquitin-associated (UBA) domain of the mRNA nuclear export receptor Mex67 helps in coordinating transcription elongation and nuclear export by interacting both with ubiquitin conjugates and specific targets, such as Hpr1, a component of the THO complex. Here, we analyzed substrate specificity and ubiquitin selectivity of the Mex67 UBA domain. UBA-Mex67 is formed by three helices arranged in a classical UBA fold plus a fourth helix, H4. Deletion or mutation of helix H4 strengthens the interaction between UBA-Mex67 and ubiquitin, but it decreases its affinity for Hpr1. Interaction with Hpr1 is required for Mex67 UBA domain to bind polyubiquitin, possibly by inducing an H4-dependent conformational change. In vivo, deletion of helix H4 reduces cotranscriptional recruitment of Mex67 on activated genes, and it also shows an mRNA export defect. Based on these results, we propose that H4 functions as a molecular switch that coordinates the interaction of Mex67 with ubiquitin bound to specific substrates, defines the selectivity of the Mex67 UBA domain for polyubiquitin, and prevents its binding to nonspecific substrates.
Molecular Biology of the Cell 08/2007; 18(7):2561-8. DOI:10.1091/mbc.E07-02-0153 · 4.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The peripheral stalk of ATP synthase acts as a stator holding the alpha(3)beta(3) catalytic subcomplex and the membrane subunit a against the torque of the rotating central stalk and attached c ring. In bovine mitochondria, the N-terminal domain of the oligomycin sensitivity conferral protein (OSCP-NT; residues 1-120) anchors one end of the peripheral stalk to the N-terminal tails of one or more alpha subunits of the F(1) subcomplex. Here, we present an NMR characterisation of the interaction between OSCP-NT and a peptide corresponding to residues 1-25 of the alpha-subunit of bovine F(1)-ATPase. The interaction site contains adjoining hydrophobic surfaces of helices 1 and 5 of OSCP-NT binding to hydrophobic side-chains of the alpha-peptide.
[Show abstract][Hide abstract] ABSTRACT: Developing Dictyostelium discoideum amoebae form a stalked fruiting body in which individual cells differentiate into either stalk cells or spores. The major known inducer of stalk cell differentiation is the chlorinated polyketide DIF-1 (1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)hexan-1-one); however a mutant blocked in the terminal step of DIF-1 biosynthesis still produces one of the prestalk cell subtypes - the pstA cells - as well as some mature stalk cells. We therefore searched for additional stalk cell-inducing factors in the medium supporting development of this mutant. These factors were purified by solvent extraction and HPLC and identified by mass spectroscopy and NMR. The mutant lacked detectable DIF-2 and DIF-3 (the pentanone and deschloro homologues of DIF-1) but four major stalk cell-inducing activities were detected, of which three were identified. Two compounds were predicted intermediates in DIF-1 biosynthesis: the desmethyl, and desmethyl-monochloro analogues of DIF-1 (dM-DIF-1 and Cl-THPH, respectively), supporting the previously proposed pathway of DIF-1 biosynthesis. The third compound was a novel factor and was identified as 4-methyl-5-pentylbenzene-1,3-diol (MPBD) with the structure confirmed by chemical synthesis. To investigate the potential roles of these compounds as signal molecules, their effects on morphological stalk and spore differentiation were examined in cell culture. All three induced morphological stalk cell differentiation. We found that synthetic MPBD also stimulated spore cell differentiation. Now that these factors are known to be produced and released during development, their biological roles can be pursued further.
[Show abstract][Hide abstract] ABSTRACT: The peripheral stalk of ATP synthase holds the alpha3beta3 catalytic subcomplex stationary against the torque of the rotating central stalk. In bovine mitochondria, the N-terminal domain of the oligomycin sensitivity conferral protein (OSCP-NT; residues 1-120) anchors one end of the peripheral stalk to the N-terminal tails of one or more alpha-subunits of the F1 subcomplex. Here we present the solution structure of OSCP-NT and an NMR titration study of its interaction with peptides representing N-terminal tails of F1 alpha-subunits. The structure comprises a bundle of six alpha-helices, and its interaction site contains adjoining hydrophobic surfaces of helices 1 and 5; residues in the region 1-8 of the alpha-subunit are essential for the interaction. The OSCP-NT is similar to the N-terminal domain of the delta-subunit from Escherichia coli ATP synthase (delta-NT), except that their surface charges differ (basic and acidic, respectively). As the charges of the adjacent crown regions in their alpha3beta3 complexes are similar, the OSCP-NT and delta-NT probably do not contact the crowns extensively. The N-terminal tails of alpha-subunit tails are probably alpha-helical, and so this interface, which is essential for the rotary mechanism of the enzyme, appears to consist of helix-helix interactions.
[Show abstract][Hide abstract] ABSTRACT: P-element transposition in Drosophila is regulated by tissue-specific alternative splicing of the P-element transposase pre-mRNA. In somatic cells, the P-element somatic inhibitor (PSI) protein binds to exon 3 of the pre-mRNA and recruits U1 small nuclear ribonucleoprotein (snRNP) to the F1 pseudo-splice site. This abrogates binding of U1 snRNP to the genuine 5' splice site, thereby preventing excision of the third intron. Two homologous short sequences, referred to as the A and B boxes, near the C terminus of PSI bind to U1-70k protein within U1 snRNP. We have now mapped the AB box-binding site of U1-70k to a short proline-rich sequence at the C terminus. Our NMR study shows that the B box forms an anti-parallel helical hairpin in which four highly conserved aromatic residues form a cluster on one face of the first helix. This hydrophobic cluster interacts extensively with the proline-rich region of the U1-70k protein.
[Show abstract][Hide abstract] ABSTRACT: SMRT (silencing mediator of retinoid acid and thyroid hormone receptor) and NCoR (nuclear receptor corepressor) are transcriptional corepressors that play an essential role in the regulation of development and metabolism. This role is achieved, in part, through the recruitment of a key histone deacetylase (HDAC3), which is itself indispensable for cell viability. The assembly of HDAC3 with the deacetylase activation domain (DAD) of SMRT and NCoR is required for activation of the otherwise inert deacetylase. The DAD comprises an N-terminal DAD-specific motif and a C-terminal SANT (SWI3/ADA2/NCoR/TFIIIB)-like domain. We report here the solution structure of the DAD from SMRT, which reveals a four-helical structure. The DAD differs from the SANT (and MYB) domains in that (i) it has an additional N-terminal helix and (ii) there is a notable hydrophobic groove on the surface of the domain. Structure-guided mutagenesis, combined with interaction assays, showed that residues in the vicinity of the hydrophobic groove are required for interaction with (and hence activation of) HDAC3. Importantly, one surface-exposed lysine is required for activation of HDAC3, but not for interaction. This lysine may play a uniquely important role in the mechanism of activating HDAC3.
Proceedings of the National Academy of Sciences 05/2005; 102(17):6009-14. DOI:10.1073/pnas.0500299102 · 9.67 Impact Factor