Structural basis of dcp2 recognition and activation by dcp1

A critical step in mRNA degradation is the removal of the 5' cap structure, which is catalyzed by the Dcp1-Dcp2 complex. The crystal structure of an S. pombe Dcp1p-Dcp2n complex combined with small-angle X-ray scattering analysis (SAXS) reveals that Dcp2p exists in open and closed conformations, with the closed complex being, or closely resembling, the catalytically more active form. This suggests that a conformational change between these open and closed complexes might control decapping. A bipartite RNA-binding channel containing the catalytic site and Box B motif is identified with a bound ATP located in the catalytic pocket in the closed complex, suggesting possible interactions that facilitate substrate binding. Dcp1 stimulates the activity of Dcp2 by promoting and/or stabilizing the closed complex. Notably, the interface of Dcp1 and Dcp2 is not fully conserved, explaining why the Dcp1-Dcp2 interaction in higher eukaryotes requires an additional factor.     

Modulation of eukaryotic mRNA stability via the cap-binding translation complex eIF4F

Decapping by Dcp1 in Saccharomyces cerevisiae is a key step in mRNA degradation. However, the cap also binds the eukaryotic initiation factor (eIF) complex 4F and its associated proteins. Characterisation of the relationship between decapping and interactions involving eIF4F is an essential step towards understanding polysome disassembly and mRNA decay. Three types of observation suggest how changes in the functional status of eIF4F modulate mRNA stability in vivo. First, partial disruption of the interaction between eIF4E and eIF4G, caused by mutations in eIF4E or the presence of the yeast 4E-binding protein p20, stabilised mRNAs. The interactions of eIF4G and p20 with eIF4E may therefore act to modulate the decapping process. Since we also show that the in vitro decapping rate is not directly affected by the nature of the body of the mRNA, this suggests that changes in eIF4F structure could play a role in triggering decapping during mRNA decay. Second, these effects were seen in the absence of extreme changes in global translation rates in the cell, and are therefore relevant to normal mRNA turnover. Third, a truncated form of eIF4E (Delta196) had a reduced capacity to inhibit Dcp1-mediated decapping in vitro, yet did not change cellular mRNA half-lives. Thus, the accessibility of the cap to Dcp1 in vivo is not simply controlled by competition with eIF4E, but is subject to switching between molecular states with different levels of access.     

New insights into the control of mRNA decapping

mRNA decapping irreversibly targets mRNAs for fast decay. Cap removal is catalyzed by decapping protein Dcp2 but also requires Dcp1. Recently, two groups have provided a first glimpse of the regulation mechanism of this crucial step in gene expression. Resolution of the yeast Dcp2 structure has enabled identification of the residues that are important for its interaction with Dcp1. However, the human decapping machinery seems to be more complex because a third component, Hedls, is required for a functional Dcp1-Dcp2 interaction.     

Targeting an mRNA for decapping: displacement of translation factors and association of the Lsm1p-7p complex on deadenylated yeast mRNAs.

The major pathway of eukaryotic mRNA decay involves deadenylation-dependent decapping followed by 5' to 3' exonucleolytic degradation. By examining interactions among mRNA decay factors, the mRNA, and key translation factors, we have identified a critical transition in mRNP organization that leads to decapping and degradation of yeast mRNAs. This transition occurs after deadenylation and includes loss of Pab1p, eIF4E, and eIF4G from the mRNA and association of the decapping activator complex, Lsm1p-7p, which enhances the coimmunoprecipitation of a decapping enzyme complex (Dcp1p and Dcp2p) with the mRNA. These results define an important rearrangement in mRNP organization and suggest that deadenylation promotes mRNA decapping by both the loss of Pab1p and the recruitment of the Lsm1p-7p complex.     

Nematode m7GpppG and m3(2,2,7)GpppG decapping: activities in Ascaris embryos and characterization of C. elegans scavenger DcpS

A spliced leader contributes the mature 5'ends of many mRNAs in trans-splicing organisms. Trans-spliced metazoan mRNAs acquire an m3(2,2,7)GpppN cap from the added spliced leader exon. The presence of these caps, along with the typical m7GpppN cap on non-trans-spliced mRNAs, requires that cellular mRNA cap-binding proteins and mRNA metabolism deal with different cap structures. We have developed and used an in vitro system to examine mRNA degradation and decapping activities in nematode embryo extracts. The predominant pathway of mRNA decay is a 3' to 5' pathway with exoribonuclease degradation of the RNA followed by hydrolysis of resulting mRNA cap by a scavenger (DcpS-like) decapping activity. Direct decapping of mRNA by a Dcp1/Dcp2-like activity does occur, but is approximately 15-fold less active than the 3' to 5' pathway. The DcpS-like activity in nematode embryo extracts hydrolyzes both m7GpppG and m3(2,2,7)GpppG dinucleoside triphosphates. The Dcp1/Dcp2-like activity in extracts also hydrolyzes these two cap structures at the 5' ends of RNAs. Interestingly, recombinant nematode DcpS differs from its human ortholog in its substrate length requirement and in its capacity to hydrolyze m3(2,2,7)GpppG.     

RNA decapping inside and outside of processing bodies

Decapping is a central step in eukaryotic mRNA turnover. Recent studies have identified several factors involved in catalysis and regulation of decapping. These include the following: an mRNA decapping complex containing the proteins Dcp1 and Dcp2; a nucleolar decapping enzyme, X29, involved in the degradation of U8 snoRNA and perhaps of other capped nuclear RNAs; and a decapping 'scavenger' enzyme, DcpS, that hydrolyzes the cap structure resulting from complete 3'-to-5' degradation of mRNAs by the exosome. Several proteins that stimulate mRNA decapping by the Dcp1:Dcp2 complex co-localize with Dcp1 and Dcp2, together with Xrn1, a 5'-to-3' exonuclease, to structures in the cytoplasm called processing bodies. Recent evidence suggests that the processing bodies may constitute specialized cellular compartments of mRNA turnover, which suggests that mRNA and protein localization may be integral to mRNA decay.     

The DEAD box protein Dhh1 stimulates the decapping enzyme Dcp1

An important control step in the regulation of cytoplasmic mRNA turnover is the removal of the m(7)G cap structure at the 5' end of the message. Here, we describe the functional characterization of Dhh1, a highly conserved member of the family of DEAD box-containing proteins, as a regulator of mRNA decapping in Saccharomyces cerevisiae. Dhh1 is a cytoplasmic protein and is shown to be in a complex with the mRNA degradation factor Pat1/Mtr1 and with the 5'-3' exoribonuclease Xrn1. Dhh1 specifically affects mRNA turnover in the deadenylation-dependent decay pathway, but does not act on the degradation of nonsense-containing mRNAs. Cells that lack dhh1 accumulate degradation intermediates that have lost their poly(A) tail but contain an intact 5' cap structure, suggesting that Dhh1 is required for efficient decapping in vivo. Furthermore, recombinant Dhh1 is able to stimulate the activity of the purified decapping enzyme Dcp1 in an in vitro decapping assay. We propose that the DEAD box protein Dhh1 regulates the access of the decapping enzyme to the m(7)G cap by modulating the structure at the 5' end of mRNAs.     

Phosphorylation of mRNA Decapping Protein Dcp1a by the ERK Signaling Pathway during Early Differentiation of 3T3-L1 Preadipocytes

Background

Turnover of mRNA is a critical step in the regulation of gene expression, and an important step in mRNA decay is removal of the 5′ cap. We previously demonstrated that the expression of some immediate early gene mRNAs is controlled by RNA stability during early differentiation of 3T3-L1 preadipocytes.

Methodology/Principal Findings

Here we show that the mouse decapping protein Dcp1a is phosphorylated via the ERK signaling pathway during early differentiation of preadipocytes. Mass spectrometry analysis and site-directed mutagenesis combined with a kinase assay identified ERK pathway–mediated dual phosphorylation at Ser 315 and Ser 319 of Dcp1a. To understand the functional effects of Dcp1a phosphorylation, we examined protein-protein interactions between Dcp1a and other decapping components with co-immunoprecipitation. Dcp1a interacted with Ddx6 and Edc3 through its proline-rich C-terminal extension, whereas the conserved EVH1 (enabled vasodilator-stimulated protein homology 1) domain in the N terminus of Dcp1a showed a stronger interaction with Dcp2. Once ERK signaling was activated, the interaction between Dcp1a and Ddx6, Edc3, or Edc4 was not affected by Dcp1a phosphorylation. Phosphorylated Dcp1a did, however, enhanced interaction with Dcp2. Protein complexes immunoprecipitated with the recombinant phosphomimetic Dcp1a(S315D/S319D) mutant contained more Dcp2 than did those immunoprecipitated with the nonphosphorylated Dcp1a(S315A/S319A) mutant. In addition, Dcp1a associated with AU-rich element (ARE)-containing mRNAs such as MAPK phosphatase-1 (MKP-1), whose mRNA stability was analyzed under the overexpression of Dcp1a constructs in the Dcp1a knockdown 3T3-L1 cells.

Conclusions/Significance

Our findings suggest that ERK-phosphorylated Dcp1a enhances its interaction with the decapping enzyme Dcp2 during early differentiation of 3T3-L1 cells.     

Dcp1 links coactivators of mRNA decapping to Dcp2 by proline recognition

Cap hydrolysis is a critical step in several eukaryotic mRNA decay pathways and is carried out by the evolutionarily conserved decapping complex containing Dcp2 at the catalytic core. In yeast, Dcp1 is an essential activator of decapping and coactivators such as Edc1 and Edc2 are thought to enhance activity, though their mechanism remains elusive. Using kinetic analysis we show that a crucial function of Dcp1 is to couple the binding of coactivators of decapping to activation of Dcp2. Edc1 and Edc2 bind Dcp1 via its EVH1 proline recognition site and stimulate decapping by 1000-fold, affecting both the K(M) for mRNA and rate of the catalytic step. The C-terminus of Edc1 is necessary and sufficient to enhance the catalytic step, while the remainder of the protein likely increases mRNA binding to the decapping complex. Lesions in the Dcp1 EVH1 domain or the Edc1 proline-rich sequence are sufficient to block stimulation. These results identify a new role of Dcp1, which is to link the binding of coactivators to substrate recognition and activation of Dcp2.     

Structural and functional control of the eukaryotic mRNA decapping machinery

The regulation of mRNA degradation is critical for proper gene expression. Many major pathways for mRNA decay involve the removal of the 5′ 7-methyl guanosine (m⁷G) cap in the cytoplasm to allow for 5′-to-3′ exonucleolytic decay. The most well studied and conserved eukaryotic decapping enzyme is Dcp2, and its function is aided by co-factors and decapping enhancers. A subset of these factors can act to enhance the catalytic activity of Dcp2, while others might stimulate the remodeling of proteins bound to the mRNA substrate that may otherwise inhibit decapping. Structural studies have provided major insights into the mechanisms by which Dcp2 and decapping co-factors activate decapping. Additional mRNA decay factors can function by recruiting components of the decapping machinery to target mRNAs. mRNA decay factors, decapping factors, and mRNA substrates can be found in cytoplasmic foci named P bodies that are conserved in eukaryotes, though their function remains unknown. In addition to Dcp2, other decapping enzymes have been identified, which may serve to supplement the function of Dcp2 or act in independent decay or quality control pathways. This article is part of a Special Issue entitled: RNA Decay mechanisms.     

Transcript-specific decapping and regulated stability by the human Dcp2 decapping protein

mRNA decapping is a critical step in the control of mRNA stability and gene expression and is carried out by the Dcp2 decapping enzyme. Dcp2 is an RNA binding protein that must bind RNA in order to recognize the cap for hydrolysis. We demonstrate that human Dcp2 (hDcp2) preferentially binds to a subset of mRNAs and identify sequences at the 5' terminus of the mRNA encoding Rrp41, a core subunit component of the RNA exosome, as a specific hDcp2 substrate. A 60-nucleotide element at the 5' end of Rrp41 mRNA was identified and shown to confer more efficient decapping on a heterologous RNA both in vitro and upon transfection into cells. Moreover, reduction of hDcp2 protein levels in cells resulted in a selective stabilization of the Rrp41 mRNA, confirming it as a downstream target of hDcp2 regulation. These findings demonstrate that hDcp2 can specifically bind to and regulate the stability of a subset of mRNAs, and its intriguing regulation of the 3'-to-5' exonuclease exosome subunit suggests a potential interplay between 5'-end mRNA decapping and 3'-end mRNA decay.     

Mutational analysis of a Dcp2-binding element reveals general enhancement of decapping by 5′-end stem-loop structures

mRNA decapping is a critical step in the control of mRNA stability and gene expression and is carried out by the Dcp2 protein. Dcp2 is an RNA-binding protein that must bind the RNA in order to recognize the cap for hydrolysis. We previously demonstrated that a 60 nucleotide (nt) element at the 5′ end of the mRNA encoding Rrp41 is preferentially bound and decapped by Dcp2. Here, we demonstrate that enhanced decapping of this element is dependent on the structural integrity of its first 33 nt and not its primary sequence. The structure consists of a stem-loop positioned <10 nt from the 5′ end of the mRNA. The generality of a stem-loop structure in enhanced Dcp2-mediated decapping was underscored by the identification of additional potential Dcp2 substrate mRNAs by a global analysis of human mRNAs containing a similar predicted stem-loop structure at their respective 5′ end. These studies suggest a general role for 5′ stem-loops in enhancing decapping activity and the utilization of this structure as a predictive tool for Dcp2 target substrates. These studies also demonstrate that Dcp2 alone in the absence of additional proteins can preferentially associate with and modulate mRNA decapping.     

Identification of an mRNA-decapping regulator implicated in X-linked mental retardation

Two major decapping enzymes are involved in the decay of eukaryotic mRNA, Dcp2 and DcpS. Despite the detection of robust DcpS decapping activity in cell extract, minimal to no decapping is detected from human Dcp2 (hDcp2) in extract. We now demonstrate that one reason for the lack of detectable hDcp2 activity in extract is due to the presence of inhibitory trans factor(s). Furthermore, we demonstrate that a previously identified testis-specific protein of unknown function implicated in nonspecific X-linked mental retardation, VCX-A, can function as an inhibitor of hDcp2 decapping in vitro and in cells. VCX-A is a noncanonical cap-binding protein that binds to capped RNA but not cap structure lacking an RNA. Its cap association is enhanced by hDcp2 to further augment the ability of VCX-A to inhibit decapping. Our data demonstrate that VCX-A can regulate mRNA stability and that it is an example of a tissue-specific decapping regulator.     

Backbone and sidechain methyl Ile (δ1), Leu and Val resonance assignments of the catalytic domain of the yeast mRNA decapping enzyme, Dcp2

Eukaryotic mRNA decapping by Dcp2 is the penultimate step in several mRNA decay pathways. To understand regulation of Dcp2 by ligand interactions, we have assigned the backbone and sidechain methyl Ile (δ1), Leu and Val chemical shifts of the catalytic domain of the S. Cerevisiae enzyme.     

Analysis of recombinant yeast decapping enzyme

A critical step in the turnover of yeast mRNAs is decapping. Two yeast proteins, Dcp1p and Dcp2p, are absolutely required for decapping, although their precise roles in the decapping reaction have not been established. To determine the function of both Dcp1p and Dcp2p in decapping, we purified recombinant versions of these proteins from Escherichia coli and examined their properties. These experiments demonstrate that copurification of Dcp1p and Dcp2p yields active decapping enzyme under a variety of conditions. Moreover, Dcp2p alone can have decapping activity under some biochemical conditions. This suggests that Dcp2p can be a catalytic subunit of the decapping complex, and Dcp1p may function to enhance Dcp2p activity, or as an additional active subunit. In addition, recombinant Dcp1p/Dcp2p prefers long mRNA substrates and is sensitive to inhibition by sequestration of the 5' end but not the 3' end of the substrate. This suggests that Dcp1p/Dcp2p contains an additional RNA-binding site spatially distinct from the active site. Finally, using two RNA-binding proteins that enhance decapping in vivo (Edc1p and Edc2p), we can reconstitute the activation of decapping with recombinant proteins. This indicates that the Edc1 and Edc2 proteins act directly on the decapping enzyme.     

Multiple Nudix family proteins possess mRNA decapping activity

RNA decapping is an important contributor to gene expression and is a critical determinant of mRNA decay. The recent demonstration that mammalian cells harbor at least two distinct decapping enzymes that preferentially modulate a subset of mRNAs raises the intriguing possibility of whether additional decapping enzymes exist. Because both known decapping proteins, Dcp2 and Nudt16, are members of the Nudix hydrolase family, we set out to determine whether other members of this family of proteins also contain intrinsic RNA decapping activity. Here we demonstrate that six additional mouse Nudix proteins—Nudt2, Nudt3, Nudt12, Nudt15, Nudt17, and Nudt19—have varying degrees of decapping activity in vitro on both monomethylated and unmethylated capped RNAs. The decapping products from Nudt17 and Nudt19 were analogous to Dcp2 and predominantly generated m⁷GDP, while cleavage by Nudt2, Nudt3, Nudt12, and Nudt15 was more pleiotropic and generated both m⁷GMP and m⁷GDP. Interestingly, all six Nudix proteins as well as both Dcp2 and Nudt16 could hydrolyze the cap of an unmethylated capped RNA, indicating that decapping enzymes may be less constrained for the presence of the methyl moiety. Investigation of Saccharomyces cerevisiae Nudix proteins revealed that the yeast homolog of Nudt3, Ddp1p, also possesses decapping activity in vitro. Moreover, the bacterial Nudix pyrophosphohydrolase RppH displayed RNA decapping activity and released m⁷GDP product comparable to Dcp2, indicating that decapping is an evolutionarily conserved activity that preceded mammalian cap formation. These findings demonstrate that multiple Nudix family hydrolases may function in mRNA decapping and mRNA stability.     

The structural basis of Edc3- and Scd6-mediated activation of the Dcp1:Dcp2 mRNA decapping complex

The Dcp1:Dcp2 decapping complex catalyses the removal of the mRNA 5' cap structure. Activator proteins, including Edc3 (enhancer of decapping 3), modulate its activity. Here, we solved the structure of the yeast Edc3 LSm domain in complex with a short helical leucine-rich motif (HLM) from Dcp2. The motif interacts with the monomeric Edc3 LSm domain in an unprecedented manner and recognizes a noncanonical binding surface. Based on the structure, we identified additional HLMs in the disordered C-terminal extension of Dcp2 that can interact with Edc3. Moreover, the LSm domain of the Edc3-related protein Scd6 competes with Edc3 for the interaction with these HLMs. We show that both Edc3 and Scd6 stimulate decapping in vitro, presumably by preventing the Dcp1:Dcp2 complex from adopting an inactive conformation. In addition, we show that the C-terminal HLMs in Dcp2 are necessary for the localization of the Dcp1:Dcp2 decapping complex to P-bodies in vivo. Unexpectedly, in contrast to yeast, in metazoans the HLM is found in Dcp1, suggesting that details underlying the regulation of mRNA decapping changed throughout evolution.