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X-RAY CRYSTALLOGRAPHIC STUDY OF YEAST DCP1 AND DCP2 PROTEINS: INSIGHTS INTO THE MECHANISM AND REGULATION OF EUKARYOTIC mRNA DECAPPING SHE MEIPEI (B.Sc.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF BIOLOGICAL SCIENCES NATIONAL UNIVERSITY OF SINGAPORE Acknowledgements I would like to express my gratitude to all the people that make my graduate research possible and enjoyable. First of all, I would like to thank my supervisor Dr. Haiwei Song for giving me the opportunity to work on this interesting project and for his invaluable support and guidance, without which this thesis would not have been possible. I am greatly indebted to our collaborators Dr. Carolyn Decker and Professor Roy Parker at the University of Arizona for their contribution to this study (chapters 3.1.3 and 3.2.6). I would like to thank current and former SHW lab members for their constant help and comradeship, especially Cheng Zhihong, Kong Chunguang, Chen Nan, Zhou Zhihong, and Wu Mousheng. And I want to thank Sharon Ling for proofreading the manuscript. I would also like to thank previous supervisors in my first year of rotation, Dr. Qi Xie and Dr. Mohan Balasubramanian at the ex-Institute of Molecular Agrobiology, for their guidance and patience. I acknowledge the Institute of Molecular and Cell Biology for financial support and beamline scientists at SPring-8, ESRF and DESY for technical support. Finally, I am thankful to my father, my mother and my sister for their support and understanding throughout the years. I TABLE OF CONTENT Acknowledgments I Table of content II Summary V Abbreviations VII List of Figures X List of Tables XI Chapter 1. Introduction mRNA turnover 1.1 1.1.1 The life cycle of mRNA 1.1.2 Biological significance of mRNA decay 1.1.3 General mRNA decay pathways 1.1.4 Specialized mRNA decay pathways 1.1.5 cis-trans interaction affecting mRNA stability 12 1.1.6 mRNA degradation and diseases 13 5’-3’ decay pathway and processing bodies 1.2 14 1.2.1 Components of mRNA 5’decay machinery 14 1.2.2 mRNA cytoplasmic processing bodies 17 mRNA decapping enzymes 1.3 1.3.1 1.3.1.1 1.3.2 1.3.2.1 1.3.3 1.4 21 Characteristics of Dcp1 protein 21 The EVH1 domain 23 Characteristics of Dcp2 protein 25 The Nudix pyrophosphatase 27 Other decapping enzymes 29 Rationales of my study 33 II Chapter Material and Methods 2.1 Experimental materials 34 2.2 Molecular cloning 42 2.3 Protein analysis 46 2.4 Functional Analysis 47 2.5 Protein purification, crystallization and structure determination 53 Chapter Results Part I: Crystal structure of scDcp1p 63 3.1.1 Structural overview 63 3.1.2 scDcp1p belongs to the EVH1 family 63 3.1.3 Analysis on scDcp1p surface to identify regions for potential Dcp2p binding 67 3.1 3.2 Part II: Crystal structure of spDcp2n 75 3.2.1 Structural overview 75 3.2.2 Nudix domain as the catalytic center 77 3.2.3 The function of the N-terminal domain 81 3.2.4 The Dcp1 binding site 86 3.2.5 spDcp1p stimulates spDcp2p decapping activity 87 3.2.6 in vivo and in vitro study of S. cerevisiae Dcp2 protein 90 Part III: Structural basis for S. pombe Dcp1p and Dcp2p interaction 3.3 94 3.3.1 Structural overview of the S. pombe Dcp1p-Dcp2NT complex 94 3.3.2 The protein-protein interface in the Dcp1p-Dcp2NT complex 96 Chapter 4.1 Discussion Comparison of putative PRS binding site of scDcp1p with other EVH1 domains 98 III 4.2 The Dcp1-Dcp2 complex in lower and higher eukaryotes 99 4.3 Implication on the assembly and regulation of 5’ mRNA decay machinery 100 References 102 List and reprints of publications 113 IV Summary mRNA degradation is important in post-transcriptional gene regulation. There are two major mRNA decay pathways in eukaryotes, both initiated by the shortening of the poly(A) tail in the 3’ end of mRNA. After deadenylation, the transcript can be degraded in the 3’ pathway by the exosome complex. Alternatively, it can be degraded in a 5’ pathway, in which the 5’ guanosine cap is removed by the decapping enzyme and the transcript is hydrolyzed by the 5’→3’ exonuclease. The enzymes and factors involved in the 5’ decay pathway are co-localized into the cytoplasmic processing bodies, whereby nonsense-mediated mRNA decay and RNA interference also take place. As a rate-limiting step of 5’ decay pathway, the decapping reaction is carried out by the Dcp1-Dcp2 holoenzyme. Dcp2 is a Nudix pyrophosphatase and Dcp1 stimulates the activity of Dcp2. The crystal structures of yeast Dcp1 and Dcp2 proteins are presented in this study. The structure of the S.cerevisiae Dcp1 protein shows that it resembles the EVH1 domain, a protein-protein interaction module. Two highly conserved patches have been identified on the surface of Dcp1p: one corresponds to the ligand recognition site of the EVH1 family and the other is specific to Dcp1 proteins. Biochemical assays demonstrated that these two patches are not required for direct Dcp2p binding but it could be a putative binding site for other regulators. The N-terminal 300 residues of S.cerevisiae Dcp2p are necessary and sufficient for mRNA decapping. The crystal structure of the corresponding region of S. pombe Dcp2(1-266) shows that it consists of an N-terminal helical domain followed by a Nudix domain. The Nudix domain is the catalytic domain, containing a Nudix motif characteristic of this family of pyrophosphatases. Mutagenesis study confirmed the V significance of two glutamic acid residues, Glu143 and Glu147, inside the Nudix motif. A third glutamic acid residue, Glu192, critical for decapping and outside of the Nudix motif was also identified based on structural analysis. The N-terminal domain is indispensable for mRNA turnover in vivo. In vitro, this region not only contributes to the decapping activity of the Nudix domain but also mediates the Dcp1-Dcp2 complex formation. A portion of the large conserved patch on the Dcp2 N-terminal domain was identified to be critical for Dcp1p binding by GST pull-down assay. The equivalent residues in S. cerevisiae Dcp2p critical for Dcp1p binding was demonstrated by yeast two-hybrid. Importantly, the association of Dcp1p to the Nterminal domain of Dcp2 is shown to be required for the stimulation of the Dcp2 protein activity. The crystal structure of S. pombe Dcp1p in complex with the N-terminal domain of Dcp2p confirmed the previous finding that the highly conserved residues on Dcp2 N-terminal domain are cricital for Dcp1p binding. In contrast to Dcp2p, Dcp1p binds to Dcp2p using mainly variant residues, suggesting that the direct interaction of Dcp1p with Dcp2p is not conserved across species, consistent with the notion that the binding of Dcp1 to Dcp2 in higher eukaryotes requires an additional factor. Based on these studies, the implication on mRNA decapping mechanism and regulation is discussed. VI Abbreviations 3AT 3-aminotriazole ADPRP ADP-ribose pyrophosphatase AMD ARE-mediated mRNA decay Ap4AP Ap4A pyrophosphatase ARE AU-rich elements ATP adenosine triphosphate CBC cap-binding complex CD circular dichroism CPSF cleavage and polyadenylation specificity factor CTD C-terminal domain DSE downstream sequence elements DTT dithiothreitol EDTA ethylene diamine tetra acetic acid EG ethylene glycol EJC exon junction complex EVH1 Enabled/VASP homology GST glutathione S-transferase GTP guanosine triphosphate HIT histidine triad IRES internal ribosome entry site Lsm Sm-like m7GDP 7-methylated guanosine diphosphate m7GMP 7-methylated guanosine monophosphate miRNA microRNA VII MPD 2-methyl-2, 4-pentanediol mRNA messenger RNA mRNP messenger ribonucleoprotein particle NCS non-crystallographic symmetry NGD no-go decay NMD nonsense-mediated mRNA decay NPC nuclear pore complex NSD non-stop decay NTD N-terminal domain PABP poly(A) binding protein PBC primary biliary cirrhosis P-body processing body PDB Protein Data Bank PH pleckstrin homology PMSF phenylmethanesulfonyl fluoride PRS proline-rich sequence PTC premature termination codon RISC RNA-induced silencing complex RNAi RNA interference RNAP II RNA polymerase II RNP ribonucleoprotein particle r.m.s.d root mean squared deviation rRNA ribosomal RNA SAD single wavelength anomalous dispersion scDcp1p S. cerevisiae Dcp1p VIII scDcp2p S.cerevisiae Dcp2p scDcp2ΔN S.cerevisiae Dcp2(102-970) scDcp2n S.cerevisiae Dcp2(1-300) SDS sodium dodecyl sulfate SeMet Seleno-L-methionine siRNA small interference RNA snoRNA small nucleolar RNA snRNA small nuclear RNA snRNP small nuclear ribonucleoprotein particle spDcp1p S. pombe Dcp1p spDcp2p S. pombe Dcp2p spDcp2n S. pombe Dcp2(1-266) TGF transforming growth factor TLC thin layer chromatography TMLA trimethyl lead acetate TNF-α tumor nercosis factor-α tRNA transfer RNA ts temperature sensitive TTP tristetraprolin UTR un-translated region WASP Wiskott-Aldrich syndrome protein IX 4. Discussion 4.1 Comparison of putative PRS binding site of scDcp1p with other EVH1 domains The structures of the yeast Dcp1 proteins revealed the core folding of the EVH1 domain with external helices. All previously characterized EVH1 domains utilize a highly conserved cluster of surface-exposed aromatic residues to recognize the PRS ligands with remarkably low affinity but high specificity (Ball et al., 2002). All EVH1 domains can be divided into three general classes based on their target PRS ligands, and the specificity is achieved by the variation of residues in the PRS binding site. Comparison of the putative PRS-binding site of Dcp1p with other EVH1 domains with known ligands helps to understand the characteristics of the site and to predict the sequence of a possible ligand. Mena, Homer and N-WASP are representatives of three different classes of the EVH1 domains and the comparison of residue composition and arrangement has revealed some similarities and marked differences as well (Figure 28). In scDcp1p, the potential PRS-binding site is lined with conserved aromatic residues including Trp49, Trp56, Leu190, Tyr47 and Trp204, of which only Trp56 is invariant in all four EVH1 proteins. Trp49 of Dcp1p is analogous to Tyr16 of the Mena protein; Tyr47 matches with Phe14 of Homer and Tyr46 of N-WASP; and Leu190 is the structural counterpart of phenylalanine of other EVH1 proteins. A unique feature of the Dcp1p pocket is Trp204, a highly conserved residue in all the Dcp1 proteins, but has no aromatic counterpart in other classes of EVH1 domains (Figure 11c). Thus the hydrophobic surface is much extended in Dcp1p, consisting of five conserved residues instead of three in other EVH1 domains. This observation suggests that 98 Dcp1p may represent a novel class of EVH1 domain and has specificity distinct from that of other EVH1 domains towards ligands. Dcp1 vs Mena (Class I) Dcp1 vs Homer (Class II) Dcp1 vs N-WASP (Class III) Figure 28. The potential ligand-binding site of scDcp1p. The comparison of PRSbinding site in Dcp1 protein (cyan) with that of other EVH1 domain-containing proteins representing three classes of proline-rich sequences is shown in Cα traces. The side chains of conserved residues are shown as sticks. 4.2 The Dcp1 and Dcp2 interaction in lower and higher eukaryotes In yeast, Dcp1p strongly associates with Dcp2p to form a holoenzyme with stimulated activity (Steiger et al., 2003). However, such direct interaction and activation effect were not observed with human or fly proteins in vitro (LykkeAndersen 2002; Cohen et al., 2005). This discrepancy is partly clarified based on the crystal structures of the S. pombe Dcp1p-Dcp2NT complex. As shown in Chapter 3.3.2, although the surface of Dcp2p involved in Dcp1p binding is highly conserved, the residues in Dcp1p that are involved in Dcp2p association have a higher degree of variation in higher eukaryotes than in yeast. Only three out of nine spDcp1p residues depicted in Figure 27 are identical in hDcp1a, versus seven invariant residues in scDcp1p. Accordingly, scDcp1p can form a heterogeneous complex with spDcp2n (residues 1-266) in vitro (data not shown). It is very likely that the protein-protein 99 interface seen in yeast is not preserved in other species or that the affinity is weak. Thus Dcp1 and Dcp2 proteins in higher eukaryotes might adopt a different mode of interaction. In line with this view, H. sapiens protein Hedls or A. thaliana VARICOSE, which lacks a homolog in yeast, has been shown to bridge the interactions between the Dcp1 and Dcp2 proteins in human and plant respectively (Fenger-GrØn et al., 2005; Xu et al., 2006; Simon et al., 2006). The divergence of the decapping proteins interaction in yeast and human, together with the observation that the human P-body contains additional components, reflects the evolution and developing complexity of the RNA degradation machinery from lower to higher eukaryotes. 4.3 Implication on the assembly and regulation of 5’ mRNA decay machinery The Dcp1p-Dcp2p complex is part of the 5’ mRNA decay machinery residing in the cytoplasmic P-bodies, including Dhh1p, Edc1p, Edc2p, Edc3p, Pat1p and Lsm17p, all of which are activators of decapping (Coller & Parker, 2004, Figure 29). In this study, several conserved surfaces on the Dcp1 and Dcp2 proteins are revealed, including patch and patch of Dcp1p and the conserved regions on the N-terminal domain and the Nudix domain of Dcp2p. Presumably, all or some of these surfaces are involved in binding other factors in the P-bodies. The 5’ decay factors participate at different steps of mRNA degeneration including translation repression, deadenylation and P-body formation (Coller & Parker, 2004). The interactions between these factors and the Dcp1p-Dcp2p complex suggest that mRNA degeneration is a coordinated and regulated process. And the activation effects on the Dcp1p-Dcp2p complex could be carried out through several mechanisms: by promoting the active conformation of the Dcp1p-Dcp2p holoenzyme 100 through protein-protein interaction; by reducing the inhibitory structures of mRNA; or by facilitating the access of the 5’ termini of mRNAs to the catalytic center. And the structural and functional analysis on the Dcp1 and Dcp2 proteins in this study provide a molecular basis for further investigation in the field of mRNA decay study. m7Gppp AAAAA m7Gppp Translation termination factors AAAAA deadenylases Dhh1p, Pat1p ribosome Dhh1p, Pat1p Dcp1p Dcp1p Dcp2p Lsm1-7p OH Dcp1p Dcp2p Pat1p Dhh1p Dcp2p OH Pat1p p pp Dhh1p 7G m Edc3p Dcp2p Lsm1-7p Pat1p Dhh1p Edc3p Edc3p m7Gppp Lsm1-7p OH Xrn1p m7Gpp mRNA processing body Figure 29. The model for mRNA decapping mechanism. In the cytoplasm, the closed-loop mRNA serves as the template for protein synthesis. When translation is terminated, the poly(A) tail of mRNA is degraded by the deadenylase complex. The translation repressors, Pat1 and Dhh1 proteins, link the translation termination with the decapping step and mediate mRNA into the cytoplasmic processing body (grey area). 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Nat Struct Mol Biol. 11(3):249-56. 113 [...]... 23 spDcp1p stimulates the decapping activity of spDcp2n 88 X Figure 24 In vivo decapping assay of the wild-type and mutants spDcp2n 92 Figure 25 Analysis of scDcp1p-scDcp2n interaction and decapping activity 93 Figure 26 Crystal structure of the S pombe Dcp1p-Dcp2NT complex 95 Figure 27 The protein-protein interface in the Dcp1p-Dcp2NT complex 97 Figure 28 The Potential ligand-binding site of scDcp1p... Edc3p and other auxiliary RNA-binding proteins Figure is modified from Coller and Parker, 2004 Dcp1p/Dcp2p) Decapping enzyme complex are composed of two subunits: the regulatory unit Dcp1p and the catalytic unit Dcp2p The features of these two proteins will be addressed in Chapter 1.3 Dhh1p is a DEAD-box RNA helicase of 58 kDa that actively represses translation and stimulates the assembly of the decapping. .. After rounds of translation, mRNA is finally destabilized and degraded mRNA decay is generally initiated by the removal of the poly(A) tail, thereafter the mRNA body is subjected to exonucleolytic digestion from either the 5’ end or 3’ end The mechanism and regulation of mRNA decay will be addressed in the following chapter 1.1.2 Biological significance of mRNA decay As the final step of mRNA metabolism,... Activity of mutants in the hydrophobic surface patch of scDcp1p 74 Figure 17 Crystal structure of spDcp2n 76 Figure 18 Comparison of spDcp2n with other Nudix enzymes 77 Figure 19 The Nudix motif of spDcp2n is the catalytic center 80 Figure 20 Functional analysis of two individual domains of spDcp2n 82 Figure 21 Sequence alignment and surface view of spDcp2n 83 Figure 22 The spDcp1p binding region in the. .. degraded from the 3’ end The 3’ pathway requires the exosome, which is a complex containing multiple 3' to 5' exoribonucleases and RNA binding proteins (Allmang et al., 1999; Mitchell et al., 1997) Together with the Ski complex (including Ski2p, Ski3p, Ski8p and the adapter Ski7p), exosome degrades mRNA to release the cap and the remaining mRNA of only a few nucleotides in length (Anderson and Parker,... Purification of recombinant spDcp2n 55 Figure 10 Purification of the S pombe Dcp1p-Dcp2NT complex 56 Figure 11 Crystal structure of scDcp1p 64 Figure 12 Comparison of scDcp1p with the homologous EVH1/PH domains 66 Figure 13 Surface representation of scDcp1p 68 Figure 14 Activity of mutants in the conserved patch 1 of scDcp1p 70 Figure 15 Activity of mutants in the conserved patch 2 of scDcp1p 72 Figure... mediating the posterior localization of osk mRNA to the oocyte (Lin et al., 2006) 231 S.Cerevisiae Dcp1p S Pombe Dcp1p D.Melanogaster Dcp1 H.sapiens Dcp1a H.sapiens Dcp1b 127 372 582 617 Figure5 Schematic diagram of Dcp1 proteins Domain organization of S.cerevisiae Dcp1p, S pombe Dcp1p, D melanogaster Dcp1, H sapiens Dcp1a and Dcp1b respectively The conserved N-terminal regions among all Dcp1 proteins. .. rendering the mRNA more stability than the wild-type one In α-thalassemia, certain variant causes a translation read-through and the displacement of the stabilizing factor bound to the 3’ARE This is associated with a decrease in the mRNA level and hence the development of the disease 1.2 mRNA 5’ decay machinery and processing bodies The machinery of the 5’ decay pathway is composed of multiple proteins. .. converging input stimuli to control the quantity and quality of mRNA Firstly, the half-lives of eukaryotic mRNAs may vary up to three orders of magnitude and they are correlated with the physical functions of encoded proteins (Sachs, 1993; Wang et al., 2002a) For example, the transcripts of structural proteins tend to have longer halflives, while the transcripts of proteins in signaling pathways usually... (Sheth and Parker, 2003) In contrast, there is no specific locus for the 3’ decay pathway, and the exosome and Ski proteins are evenly distributed in the cytosol In higher eukaryotes, extra components that have no yeast counterparts are identified in P-bodies One of them is Ge-1, or Hedls Ge-1 stimulates hDcp2 decapping activity and is suggested to mediate the hDcp1a and hDcp2 protein complex formation . X-RAY CRYSTALLOGRAPHIC STUDY OF YEAST DCP1 AND DCP2 PROTEINS: INSIGHTS INTO THE MECHANISM AND REGULATION OF EUKARYOTIC mRNA DECAPPING SHE MEIPEI (B.Sc.) A THESIS. by the Dcp1- Dcp2 holoenzyme. Dcp2 is a Nudix pyrophosphatase and Dcp1 stimulates the activity of Dcp2. The crystal structures of yeast Dcp1 and Dcp2 proteins are presented in this study. The. from either the 5’ end or 3’ end. The mechanism and regulation of mRNA decay will be addressed in the following chapter. 1.1.2 Biological significance of mRNA decay As the final step of mRNA