An essential component of the decapping enzyme required for normal rates of mRNA turnover

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.

Multiple processing body factors and the ARE binding protein TTP activate mRNA decapping

Decapping is a key step in mRNA turnover. However, the composition and regulation of the human decapping complex is poorly understood. Here, we identify three proteins that exist in complex with the decapping enzyme subunits hDcp2 and hDcp1: hEdc3, Rck/p54, and a protein in decapping we name Hedls. Hedls is important in decapping because it enhances the activity of the catalytic hDcp2 subunit and promotes complex formation between hDcp2 and hDcp1. Specific decapping factors interact with the mRNA decay activators hUpf1 and TTP, and TTP enhances decapping of a target AU-rich element (ARE) RNA in vitro. Each decapping protein localizes in cytoplasmic processing bodies (PBs), and overexpression of Hedls produces aberrant PBs and concomitant accumulation of a deadenylated ARE-mediated mRNA decay intermediate. These observations suggest that multiple proteins involved in human decapping are important subunits of PBs and are activated on ARE-mRNAs by the protein TTP.

Gene set coregulated by the Saccharomyces cerevisiae nonsense-mediated mRNA decay pathway

The nonsense-mediated mRNA decay (NMD) pathway has historically been thought of as an RNA surveillance system that degrades mRNAs with premature translation termination codons, but the NMD pathway of Saccharomyces cerevisiae has a second role regulating the decay of some wild-type mRNAs. In S. cerevisiae, a significant number of wild-type mRNAs are affected when NMD is inactivated. These mRNAs are either wild-type NMD substrates or mRNAs whose abundance increases as an indirect consequence of NMD. A current challenge is to sort the mRNAs that accumulate when NMD is inactivated into direct and indirect targets. We have developed a bioinformatics-based approach to address this challenge. Our approach involves using existing genomic and function databases to identify transcription factors whose mRNAs are elevated in NMD-deficient cells and the genes that they regulate. Using this strategy, we have investigated a coregulated set of genes. We have shown that NMD regulates accumulation of ADR1 and GAL4 mRNAs, which encode transcription activators, and that Adr1 is probably a transcription activator of ATS1. This regulation is physiologically significant because overexpression of ADR1 causes a respiratory defect that mimics the defect seen in strains with an inactive NMD pathway. This strategy is significant because it allows us to classify the genes regulated by NMD into functionally related sets, an important step toward understanding the role NMD plays in the normal functioning of yeast cells.

Decapping the message: a beginning or an end

Removal of the mRNA 5′ cap is an important step in the regulation of mRNA stability. mRNAs are degraded by at least two distinct exonucleolytic decay pathways, one from the 5′ end, and the second from the 3′ end. Two major cellular decapping enzymes have been identified, and each primarily functions in one of the two decay pathways. The Dcp2 decapping enzyme utilizes capped mRNA as substrate and hydrolyses the cap to release m7GDP (N7-methyl GDP), while a scavenger decapping enzyme, DcpS, utilizes cap dinucleotides or capped oligonucleotides as substrate and releases m7GMP (N7-methyl GMP). In this review, we will highlight the function of different decapping enzymes and their role in mRNA turnover.

Cell and Molecular Biology of the Exosome: How to Make or Break an RNA

The identification and characterization of the exosome complex has shown that the exosome is a complex of 3' --> 5' exoribonucleases that plays a key role in the processing and degradation of a wide variety of RNA substrates. Advances in the understanding of exosome function have led to the identification of numerous cofactors that are required for a selective recruitment of the exosome to substrate RNAs, for their structural alterations to facilitate degradation, and to aid in their complete degradation/processing. Structural data obtained by two-hybrid interaction analyses and X-ray crystallography show that the core of the exosome adopts a doughnut-like structure and demonstrates that probably not all exosome subunits are active exoribonucleases. Despite all data obtained on the structure and function of the exosome during the last decade, there are still a lot of unanswered questions. What is the molecular mechanism by which cofactors select and target substrate RNAs to the exosome and modulate its function for correct processing or degradation? How can the exosome discriminate between processing or degradation of a specific substrate RNA? What is the precise structure of exosome subunits and how do they contribute to its function? Here we discuss studies that provide some insight to these questions and speculate on the mechanisms that control the exosome.

Crystal structure and functional analysis of Dcp2p from Schizosaccharomyces pombe

Decapping is a key step in both general and nonsense-mediated 5' --> 3' mRNA-decay pathways. Removal of the cap structure is catalyzed by the Dcp1-Dcp2 complex. The crystal structure of a C-terminally truncated Schizosaccharomyces pombe Dcp2p reveals two distinct domains: an all-helical N-terminal domain and a C-terminal domain that is a classic Nudix fold. The C-terminal domain of both Saccharomyces cerevisiae and S. pombe Dcp2p proteins is sufficient for decapping activity, although the N-terminal domain can affect the efficiency of Dcp2p function. The binding of Dcp2p to Dcp1p is mediated by a conserved surface on its N-terminal domain, and the N-terminal domain is required for Dcp1p to stimulate Dcp2p activity. The flexible nature of the N-terminal domain relative to the C-terminal domain suggests that Dcp1p binding to Dcp2p may regulate Dcp2p activity through conformational changes of the two domains.

Caenorhabditis elegans decapping proteins: localization and functional analysis of Dcp1, Dcp2, and DcpS during embryogenesis

Though posttranscriptional regulation is important for early embryogenesis, little is understood regarding control of mRNA decay during development. Previous work defined two major pathways by which normal transcripts are degraded in eukaryotes. However it is not known which pathways are key in mRNA decay during early patterning or whether developmental transcripts are turned over via specific pathways. Here we show that Caenorhabditis elegans Dcp2 is localized to distinct foci during embryogenesis, reminiscent of P-bodies, the sites of mRNA degradation in yeast and mammals. However the decapping enzyme of the 3' to 5' transcript decay system (DcpS) localizes throughout the cytoplasm, suggesting this degradation pathway is not highly organized. In addition we find that Dcp2 is localized to P-granules, showing that Dcp2 is stored and/or active in these structures. However RNAi of these decapping enzymes has no obvious effect on embryogenesis. In contrast we find that nuclear cap binding proteins (CBP-20 and 80), eIF4G, and PAB-1 are absolutely required for development. Together our data provides further evidence that pathways of general mRNA metabolism can be remarkably organized during development, with two different decapping enzymes localized in distinct cytoplasmic domains.

Dcp2 Decaps m2,2,7GpppN-capped RNAs, and its activity is sequence and context dependent

Hydrolysis of the mRNA cap plays a pivotal role in initiating and completing mRNA turnover. In nematodes, mRNA metabolism and cap-interacting proteins must deal with two populations of mRNAs, spliced leader trans-spliced mRNAs with a trimethylguanosine cap and non-trans-spliced mRNAs with a monomethylguanosine cap. We describe here the characterization of nematode Dcp1 and Dcp2 proteins. Dcp1 was inactive in vitro on both free cap and capped RNA and did not significantly enhance Dcp2 activity. Nematode Dcp2 is an RNA-decapping protein that does not bind cap and is not inhibited by cap analogs but is effectively inhibited by competing RNA irrespective of RNA sequence and cap. Nematode Dcp2 activity is influenced by both 5' end sequence and its context. The trans-spliced leader sequence on mRNAs reduces Dcp2 activity approximately 10-fold, suggesting that 5'-to-3' turnover of trans-spliced RNAs may be regulated. Nematode Dcp2 decaps both m(7)GpppG- and m(2,2,7)GpppG-capped RNAs. Surprisingly, both budding yeast and human Dcp2 are also active on m(2,2,7)GpppG-capped RNAs. Overall, the data suggest that Dcp2 activity can be influenced by both sequence and context and that Dcp2 may contribute to gene regulation in multiple RNA pathways, including monomethyl- and trimethylguanosine-capped RNAs.

A role for the eIF4E-binding protein 4E-T in P-body formation and mRNA decay

4E-transporter (4E-T) is one of several proteins that bind the mRNA 5′cap-binding protein, eukaryotic initiation factor 4E (eIF4E), through a conserved binding motif. We previously showed that 4E-T is a nucleocytoplasmic shuttling protein, which mediates the import of eIF4E into the nucleus. At steady state, 4E-T is predominantly cytoplasmic and is concentrated in bodies that conspicuously resemble the recently described processing bodies (P-bodies), which are believed to be sites of mRNA decay. In this paper, we demonstrate that 4E-T colocalizes with mRNA decapping factors in bona fide P-bodies. Moreover, 4E-T controls mRNA half-life, because its depletion from cells using short interfering RNA increases mRNA stability. The 4E-T binding partner, eIF4E, also is localized in P-bodies. 4E-T interaction with eIF4E represses translation, which is believed to be a prerequisite for targeting of mRNAs to P-bodies. Collectively, these data suggest that 4E-T interaction with eIF4E is a priming event in inducing messenger ribonucleoprotein rearrangement and transition from translation to decay.

Scavenger decapping activity facilitates 5' to 3' mRNA decay

mRNA degradation occurs through distinct pathways, one primarily from the 5' end of the mRNA and the second from the 3' end. Decay from the 3' end generates the m7GpppN cap dinucleotide, which is subsequently hydrolyzed to m7Gp and ppN in Saccharomyces cerevisiae by a scavenger decapping activity termed Dcs1p. Although Dcs1p functions in the last step of mRNA turnover, we demonstrate that its activity modulates earlier steps of mRNA decay. Disruption of the DCS1 gene manifests a threefold increase of the TIF51A mRNA half-life. Interestingly, the hydrolytic activity of Dcs1p was essential for the altered mRNA turnover, as Dcs1p, but not a catalytically inactive Dcs1p mutant, complemented the increased mRNA stability. Mechanistic analysis revealed that 5' to 3' exoribonucleolytic activity was impeded in the dcs1Delta strain, resulting in the accumulation of uncapped mRNA. These data define a new role for the Dcs1p scavenger decapping enzyme and demonstrate a novel mechanism whereby the final step in the 3' mRNA decay pathway can influence 5' to 3' exoribonucleolytic activity.

The role of SMG-1 in nonsense-mediated mRNA decay

SMG-1, a member of the PIKK (phosphoinositide 3-kinase related kinases) family, plays a critical role in the mRNA quality control system termed nonsense-mediated mRNA decay (NMD). NMD protects the cells from the accumulation of aberrant mRNAs with premature termination codons (PTCs) that encode nonfunctional or potentially harmful truncated proteins. SMG-1 directly phosphorylates Upf1, another key component of NMD, and this phosphorylation occurs upon recognition of PTC on post-spliced mRNA during the initial round of translation. At present, a variety of tools are available that can specifically suppress NMD, and it is possible to examine the contribution of NMD in a variety of physiological and pathological conditions.

Mutations in the Saccharomyces cerevisiae LSM1 gene that affect mRNA decapping and 3' end protection

The decapping of eukaryotic mRNAs is a key step in their degradation. The heteroheptameric Lsm1p-7p complex is a general activator of decapping and also functions in protecting the 3' ends of deadenylated mRNAs from a 3'-trimming reaction. Lsm1p is the unique member of the Lsm1p-7p complex, distinguishing that complex from the functionally different Lsm2p-8p complex. To understand the function of Lsm1p, we constructed a series of deletion and point mutations of the LSM1 gene and examined their effects on phenotype. These studies revealed the following: (i) Mutations affecting the predicted RNA-binding and inter-subunit interaction residues of Lsm1p led to impairment of mRNA decay, suggesting that the integrity of the Lsm1p-7p complex and the ability of the Lsm1p-7p complex to interact with mRNA are important for mRNA decay function; (ii) mutations affecting the predicted RNA contact residues did not affect the localization of the Lsm1p-7p complex to the P-bodies; (iii) mRNA 3'-end protection could be indicative of the binding of the Lsm1p-7p complex to the mRNA prior to activation of decapping, since all the mutants defective in mRNA 3' end protection were also blocked in mRNA decay; and (iv) in addition to the Sm domain, the C-terminal domain of Lsm1p is also important for mRNA decay function.

Messenger RNA turnover in eukaryotes: pathways and enzymes

The control of mRNA degradation is an important component of the regulation of gene expression since the steady-state concentration of mRNA is determined both by the rates of synthesis and of decay. Two general pathways of mRNA decay have been described in eukaryotes. Both pathways share the exonucleolytic removal of the poly(A) tail (deadenylation) as the first step. In one pathway, deadenylation is followed by the hydrolysis of the cap and processive degradation of the mRNA body by a 5' exonuclease. In the second pathway, the mRNA body is degraded by a complex of 3' exonucleases before the remaining cap structure is hydrolyzed. This review discusses the proteins involved in the catalysis and control of both decay pathways.

The yeast EDC1 mRNA undergoes deadenylation-independent decapping stimulated by Not2p, Not4p, and Not5p

A major mechanism of eukaryotic mRNA degradation initiates with deadenylation followed by decapping and 5' to 3' degradation. We demonstrate that the yeast EDC1 mRNA, which encodes a protein that enhances decapping, has unique properties and is both protected from deadenylation and undergoes deadenylation-independent decapping. The 3' UTR of the EDC1 mRNA is sufficient for both protection from deadenylation and deadenylation-independent decapping and an extended poly(U) tract within the 3' UTR is required. These observations highlight the diverse forms of decapping regulation and identify a feedback loop that can compensate for decreases in activity of the decapping enzyme. Surprisingly, the decapping of the EDC1 mRNA is slowed by the loss of Not2p, Not4p, and Not5p, which interact with the Ccr4p/Pop2p deadenylase complex. This indicates that the Not proteins can affect decapping, which suggests a possible link between the mRNA deadenylation and decapping machinery.

Nuclear pre-mRNA decapping and 5' degradation in yeast require the Lsm2-8p complex

Previous analyses have identified related cytoplasmic Lsm1-7p and nuclear Lsm2-8p complexes. Here we report that mature heat shock and MET mRNAs that are trapped in the nucleus due to a block in mRNA export were strongly stabilized in strains lacking Lsm6p or the nucleus-specific Lsm8p protein but not by the absence of the cytoplasmic Lsm1p. These nucleus-restricted mRNAs remain polyadenylated until their degradation, indicating that nuclear mRNA degradation does not involve the incremental deadenylation that is a key feature of cytoplasmic turnover. Lsm8p can be UV cross-linked to nuclear poly(A)(+) RNA, indicating that an Lsm2-8p complex interacts directly with nucleus-restricted mRNA. Analysis of pre-mRNAs that contain intronic snoRNAs indicates that their 5' degradation is specifically inhibited in strains lacking any of the Lsm2-8p proteins but Lsm1p. Nucleus-restricted mRNAs and pre-mRNA degradation intermediates that accumulate in lsm mutants remain 5' capped. We conclude that the Lsm2-8p complex normally targets nuclear RNA substrates for decapping.

Identification of Edc3p as an enhancer of mRNA decapping in Saccharomyces cerevisiae

The major pathway of mRNA decay in yeast initiates with deadenylation, followed by mRNA decapping and 5'-3' exonuclease digestion. An in silico approach was used to identify new proteins involved in the mRNA decay pathway. One such protein, Edc3p, was identified as a conserved protein of unknown function having extensive two-hybrid interactions with several proteins involved in mRNA decapping and 5'-3' degradation including Dcp1p, Dcp2p, Dhh1p, Lsm1p, and the 5'-3' exonuclease, Xrn1p. We show that Edc3p can stimulate mRNA decapping of both unstable and stable mRNAs in yeast when the decapping enzyme is compromised by temperature-sensitive alleles of either the DCP1 or the DCP2 genes. In these cases, deletion of EDC3 caused a synergistic mRNA-decapping defect at the permissive temperatures. The edc3Delta had no effect when combined with the lsm1Delta, dhh1Delta, or pat1Delta mutations, which appear to affect an early step in the decapping pathway. This suggests that Edc3p specifically affects the function of the decapping enzyme per se. Consistent with a functional role in decapping, GFP-tagged Edc3p localizes to cytoplasmic foci involved in mRNA decapping referred to as P-bodies. These results identify Edc3p as a new protein involved in the decapping reaction.

Eukaryotic mRNA decapping

Eukaryotic mRNAs are primarily degraded by removal of the 3' poly(A) tail, followed either by cleavage of the 5' cap structure (decapping) and 5'->3' exonucleolytic digestion, or by 3' to 5' degradation. mRNA decapping represents a critical step in turnover because this permits the degradation of the mRNA and is a site of numerous control inputs. Recent analyses suggest decapping of an mRNA consists of four central and related events. These include removal, or inactivation, of the poly(A) tail as an inhibitor of decapping, exit from active translation, assembly of a decapping complex on the mRNA, and sequestration of the mRNA into discrete cytoplasmic foci where decapping can occur. Each of these steps is a demonstrated, or potential, site for the regulation of mRNA decay. We discuss the decapping process in the light of these central properties, which also suggest fundamental aspects of cytoplasmic mRNA physiology that connect decapping, translation, and storage of mRNA.

'Cap-tabolism'

distinctive feature of eukaryotic mRNA and small nuclear RNA (snRNA) that are transcribed by RNA polymerase II (Pol II) is the presence of a cap structure at their 5' end. This essential modification serves as an inviting 'landing pad' for factors that are involved in various cellular processes such as pre-mRNA splicing, nucleocytoplasmic RNA export and localization, and translation initiation. Because of the important functions mediated by the mRNA cap, this structure needs to be modified and/or degraded in a tightly controlled manner. Several cellular and viral systems implicated in cap metabolism have been uncovered recently; their analyses provide interesting new information on cell structure and function.

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.

Positive and negative regulation of poly(A) nuclease

PAN, a yeast poly(A) nuclease, plays an important nuclear role in the posttranscriptional maturation of mRNA poly(A) tails. The activity of this enzyme is dependent on its Pan2p and Pan3p subunits, as well as the presence of poly(A)-binding protein (Pab1p). We have identified and characterized the associated network of factors controlling the maturation of mRNA poly(A) tails in yeast and defined its relevant protein-protein interactions. Pan3p, a positive regulator of PAN activity, interacts with Pab1p, thus providing substrate specificity for this nuclease. Pab1p also regulates poly(A) tail trimming by interacting with Pbp1p, a factor that appears to negatively regulate PAN. Pan3p and Pbp1p both interact with themselves and with the C terminus of Pab1p. However, the domains required for Pan3p and Pbp1p binding on Pab1p are distinct. Single amino acid changes that disrupt Pan3p interaction with Pab1p have been identified and define a binding pocket in helices 2 and 3 of Pab1p's carboxy terminus. The importance of these amino acids for Pab1p-Pan3p interaction, and poly(A) tail regulation, is underscored by experiments demonstrating that strains harboring substitutions in these residues accumulate mRNAs with long poly(A) tails in vivo.

Xenopus U8 snoRNA binding protein is a conserved nuclear decapping enzyme

U8 snoRNP is required for accumulation of mature 5.8S and 28S rRNA in vertebrates. We are identifying proteins that bind U8 RNA with high specificity to understand how U8 functions in ribosome biogenesis. Here, we characterize a Xenopus 29 kDa protein (X29), which we previously showed binds U8 RNA with high affinity. X29 and putative homologs in other vertebrates contain a NUDIX domain found in MutT and other nucleotide diphosphatases. Recombinant X29 protein has diphosphatase activity that removes m(7)G and m(227)G caps from U8 and other RNAs in vitro; the putative 29 kDa human homolog also displays this decapping activity. X29 is primarily nucleolar in Xenopus tissue culture cells. We propose that X29 is a member of a conserved family of nuclear decapping proteins that function in regulating the level of U8 snoRNA and other nuclear RNAs with methylated caps.

Loss of translational control in yeast compromised for the major mRNA decay pathway

The cytoplasmic fate of mRNAs is dictated by the relative activities of the intimately connected mRNA decay and translation initiation pathways. In this study, we have found that yeast strains compromised for stages downstream of deadenylation in the major mRNA decay pathway are incapable of inhibiting global translation initiation in response to stress. In the past, the paradigm of the eIF2alpha kinase-dependent amino acid starvation pathway in yeast has been used to evaluate this highly conserved stress response in all eukaryotic cells. Using a similar approach we have found that even though the mRNA decay mutants maintain high levels of general translation, they exhibit many of the hallmarks of amino acid starvation, including increased eIF2alpha phosphorylation and activated GCN4 mRNA translation. Therefore, these mutants appear translationally oblivious to decreased ternary complex abundance, and we propose that this is due to higher rates of mRNA recruitment to the 40S ribosomal subunit.

Poly(A)-binding-protein-mediated regulation of hDcp2 decapping in vitro

Regulation of mRNA decapping is a critical determinant for gene expression. We demonstrate that the poly(A) tail-mediated regulation of mRNA decapping observed in humans can be recapitulated in vitro by the cytoplasmic poly(A)-binding protein PABP through a direct and specific binding to the 5' end of capped mRNA. The specific association of PABP with the cap occurred only within the context of the RNA whereby a cap attached to an RNA moiety served as the high-affinity substrate but not the cap structure or RNA alone. Binding of PABP to the RNA 5' end required the presence of the cap and was accentuated by the N7 methyl moiety of the cap. Interestingly, conditions that enhanced hDcp2 decapping activity reduced the affinity of PABP for cap association and consequently its ability to inhibit decapping, suggestive of a regulated association of PABP with the cap. These observations reveal a novel direct involvement of human PABP in the stabilization of mRNA by protecting the 5' end from decapping.

Cytoplasmic foci are sites of mRNA decay in human cells

Understanding gene expression control requires defining the molecular and cellular basis of mRNA turnover. We have previously shown that the human decapping factors hDcp2 and hDcp1a are concentrated in specific cytoplasmic structures. Here, we show that hCcr4, hDcp1b, hLsm, and rck/p54 proteins related to 5'-3' mRNA decay also localize to these structures, whereas DcpS, which is involved in cap nucleotide catabolism, is nuclear. Functional analysis using fluorescence resonance energy transfer revealed that hDcp1a and hDcp2 interact in vivo in these structures that were shown to differ from the previously described stress granules. Our data indicate that these new structures are dynamic, as they disappear when mRNA breakdown is abolished by treatment with inhibitors. Accumulation of poly(A)(+) RNA in these structures, after RNAi-mediated inactivation of the Xrn1 exonuclease, demonstrates that they represent active mRNA decay sites. The occurrence of 5'-3' mRNA decay in specific subcellular locations in human cells suggests that the cytoplasm of eukaryotic cells may be more organized than previously anticipated.

The enzymes and control of eukaryotic mRNA turnover

The degradation of eukaryotic mRNAs plays important roles in the modulation of gene expression, quality control of mRNA biogenesis and antiviral defenses. In the past five years, many of the enzymes involved in this process have been identified and mechanisms that modulate their activities have begun to be identified. In this review, we describe the enzymes of mRNA degradation and their properties. We highlight that there are a variety of enzymes with different specificities, suggesting that individual nucleases act on distinct subpopulations of transcripts within the cell. In several cases, translation factors that bind mRNA inhibit these nucleases. In addition, recent work has begun to identify distinct mRNP complexes that recruit the nucleases to transcripts through different mRNA-interacting proteins. These properties and complexes suggest multiple mechanisms by which mRNA degradation could be regulated.

Leaky termination at premature stop codons antagonizes nonsense-mediated mRNA decay in S. cerevisiae

The Nonsense-Mediated mRNA Decay (NMD) pathway mediates the rapid degradation of mRNAs that contain premature stop mutations in eukaryotic organisms. It was recently shown that mutations in three yeast genes that encode proteins involved in the NMD process, UPF1, UPF2, and UPF3, also reduce the efficiency of translation termination. In the current study, we compared the efficiency of translation termination in a upf1Delta strain and a [PSI(+)] strain using a collection of translation termination reporter constructs. The [PSI(+)] state is caused by a prion form of the polypeptide chain release factor eRF3 that limits its availability to participate in translation termination. In contrast, the mechanism by which Upf1p influences translation termination is poorly understood. The efficiency of translation termination is primarily determined by a tetranucleotide termination signal consisting of the stop codon and the first nucleotide immediately 3' of the stop codon. We found that the upf1Delta mutation, like the [PSI(+)] state, decreases the efficiency of translation termination over a broad range of tetranucleotide termination signals in a unique, context-dependent manner. These results suggest that Upf1p may associate with the termination complex prior to polypeptide chain release. We also found that the increase in readthrough observed in a [PSI(+)]/upf1Delta strain was larger than the readthrough observed in strains carrying either defect alone, indicating that the upf1Delta mutation and the [PSI(+)] state influence the termination process in distinct ways. Finally, our analysis revealed that the mRNA destabilization associated with NMD could be separated into two distinct forms that correlated with the extent the premature stop codon was suppressed. The minor component of NMD was a 25% decrease in mRNA levels observed when readthrough was >/=0.5%, while the major component was represented by a larger decrease in mRNA abundance that was observed only when readthrough was </=0.5%. This low threshold for the onset of the major component of NMD indicates that mRNA surveillance is an ongoing process that occurs throughout the lifetime of an mRNA.

Inhibition of 5' to 3' mRNA degradation under stress conditions in Saccharomyces cerevisiae: from GCN4 to MET16

After deadenylation, most cytoplasmic mRNAs are decapped and digested by 5' to 3' exonucleases in Saccharomyces cerevisiae. Capped and deadenylated mRNAs are degraded to a lesser extent by 3' to 5' exonucleases. We have used a method, based on the electroporation of in vitro synthetised mRNAs, to study the relative importance of these two exonucleolytic pathways under stress conditions. We show that derepression of GCN4 upon amino acid starvation specifically limits the 5'-to-3'-degradation pathway. Because adenosine 3'-5' biphosphate (pAp), which is produced by Met16p, inhibits this degradation pathway to a comparable extent, we were prompted to analyse the role of Met16p in this phenomenon. We show that the inhibitory effects of amino acid limitation on 5' to 3' mRNA degradation are absent in a met16 mutant. We therefore conclude that the GCN4 dependence of MET16 expression is responsible for the decrease in 5' to 3' digestion under stress conditions and that cells use pAp as a signal to limit 5' to 3' RNA degradation under stress conditions. Because 3' to 5' mRNA degradation is unaffected, the relative importance of this pathway in the decay of certain RNAs may be increased under stress conditions.

Genome-wide analysis of mRNAs regulated by the nonsense-mediated and 5' to 3' mRNA decay pathways in yeast

Transcripts regulated by the yeast nonsense-mediated and 5' to 3' mRNA decay pathways were identified by expression profiling of wild-type, upf1Delta, nmd2Delta, upf3Delta, dcp1Delta, and xrn1Delta cells. This analysis revealed that inactivation of Upf1p, Nmd2p, or Upf3p has identical effects on global RNA accumulation; inactivation of Dcp1p or Xrn1p exhibits both common and unique effects on global RNA accumulation but causes upregulation of only a small fraction of transcripts; and the majority of transcripts upregulated in upf/nmd strains are also upregulated to similar extents in dcp1Delta and xrn1Delta strains. Our results define the core transcripts regulated by NMD, identify several novel structural classes of NMD substrates, demonstrate that nonsense-containing mRNAs are primarily degraded by the 5' to 3' decay pathway even in the absence of functional NMD, and indicate that 3' to 5' decay, not 5' to 3' decay, may be the major mRNA decay activity in yeast cells.

Crystal structure of Dcp1p and its functional implications in mRNA decapping

A major pathway of eukaryotic mRNA turnover begins with deadenylation, followed by decapping and 5'-->3' exonucleolytic degradation. A critical step in this pathway is decapping, which is carried out by an enzyme composed of Dcp1p and Dcp2p. The crystal structure of Dcp1p shows that it markedly resembles the EVH1 family of protein domains. Comparison of the proline-rich sequence (PRS)-binding sites in this family of proteins with Dcp1p indicates that it belongs to a novel class of EVH1 domains. Mapping of the sequence conservation on the molecular surface of Dcp1p reveals two prominent sites. One of these is required for the function of the Dcp1p-Dcp2p complex, and the other, corresponding to the PRS-binding site of EVH1 domains, is probably a binding site for decapping regulatory proteins. Moreover, a conserved hydrophobic patch is shown to be critical for decapping.

Identification of Edc3p as an Enhancer of mRNA Decapping in Saccharomyces cerevisiae

The major pathway of mRNA decay in yeast initiates with deadenylation, followed by mRNA decapping and 5′-3′ exonuclease digestion. An in silico approach was used to identify new proteins involved in the mRNA decay pathway. One such protein, Edc3p, was identified as a conserved protein of unknown function having extensive two-hybrid interactions with several proteins involved in mRNA decapping and 5′-3′ degradation including Dcp1p, Dcp2p, Dhh1p, Lsm1p, and the 5′-3′ exonuclease, Xrn1p. We show that Edc3p can stimulate mRNA decapping of both unstable and stable mRNAs in yeast when the decapping enzyme is compromised by temperature-sensitive alleles of either the DCP1 or the DCP2 genes. In these cases, deletion of EDC3 caused a synergistic mRNA-decapping defect at the permissive temperatures. The edc3Δ had no effect when combined with the lsm1Δ, dhh1Δ, or pat1Δ mutations, which appear to affect an early step in the decapping pathway. This suggests that Edc3p specifically affects the function of the decapping enzyme per se. Consistent with a functional role in decapping, GFP-tagged Edc3p localizes to cytoplasmic foci involved in mRNA decapping referred to as P-bodies. These results identify Edc3p as a new protein involved in the decapping reaction.

Premature termination codons enhance mRNA decapping in human cells

Nonsense-mediated mRNA decay (NMD) is a eukaryotic surveillance process that promotes selective degradation of imperfect messages containing premature translation termination codons (PTCs). In yeast, PTCs trigger both deadenylylation-independent mRNA decapping, thereby allowing their rapid degradation by a 5' to 3' exonuclease, and to a smaller extent accelerated deadenylylation. It is not clear to what extent this decay pathway is conserved in higher eukaryotes. We used a transcriptional pulse strategy relying on a tetracycline-regulated promoter to study the decay of a PTC- containing beta-globin mRNA in human cells. We show that a PTC destabilizes the mRNA and decreases its half-life from >16 h to 3 h. The deadenylylation rate is increased, but not sufficiently to account for the decreased half-life on its own. Using a circularization RT-PCR (cRT-PCR) strategy, we could detect decapped degradation intermediates and measure simultaneously their poly(A) tail length. This allowed us to show that a PTC enhances the rate of mRNA decapping and that decapped products have been deadenylylated to a certain extent. Thus the major feature of the NMD pathway, enhanced decapping, is conserved from yeast to man even though the kinetic details might differ between various mRNAs and/or species.

Glucocorticoid receptor-JNK interaction mediates inhibition of the JNK pathway by glucocorticoids

Inhibition of the c-Jun N-terminal kinase (JNK) pathway by glucocorticoids (GCs) results in AP-1 repression. GC antagonism of AP-1 relies mainly on the transrepression function of the GC receptor (GR) and mediates essential physiological and pharmacological actions. Here we show that GCs induce the disassembly of JNK from mitogen-activated protein kinase kinase 7 (MKK7) by promoting its association with GR. Moreover, we have characterized a hormone-regulated JNK docking site in the GR ligand-binding domain that mediates GR-JNK interaction. The binding of GR to JNK is required for inhibition of JNK activation and induction of inactive JNK nuclear transfer by GCs. The dissociation of these two hormone actions shows that JNK nuclear transfer is dispensable for the downregulation of JNK activation by GCs. Nonetheless, nuclear accumulation of inactive JNK may still be relevant for enhancing the repression of AP-1 activity by GCs. In this regard, chromatin immunoprecipitation assays show that GC-induced GR-JNK association correlates with an increase in the loading of inactive JNK on the AP-1-bound response elements of the c-jun gene.

p38 Mitogen-activated protein kinase stabilizes mRNAs that contain cyclooxygenase-2 and tumor necrosis factor AU-rich elements by inhibiting deadenylation

AU-rich elements (AREs) in 3'-untranslated regions of mRNAs confer instability. They target mRNAs for rapid deadenylation and degradation and may enhance decapping. The p38 MAPK pathway stabilizes many otherwise unstable ARE-containing mRNAs encoding proteins involved in inflammation; however, the mRNA decay step(s) regulated by the signaling pathway are unknown. To investigate whether it regulates deadenylation or the decay of the mRNA body, we used a tetracycline-regulated beta-globin mRNA reporter system to transcribe pulses of mRNA of uniform length. We measured on Northern gels the migration of reporter mRNAs isolated from cells transfected only with reporter plasmid or co-transfected with an active mutant of MAPK kinase-6, and treated either with or without the p38 MAPK inhibitor SB 203580. Differences in migration were shown by RNase H mapping with oligo(dT) to be due to poly(A) shortening. Insertion of an ARE into the beta-globin reporter mRNA promoted rapid deadenylation and decay of hypo-adenylated reporter mRNA. p38 MAPK activation inhibited the deadenylation of reporter mRNAs containing either the cyclooxygenase-2 or tumor necrosis factor AREs. The regulation of deadenylation by p38 MAPK was found to be specific because deadenylation of the beta-globin reporter mRNA either lacking an ARE or containing the c-Myc 3'-untranslated region (which is not p38 MAPK-responsive) was unaffected by p38 MAPK. It was concluded that the p38 MAPK pathway predominantly regulates deadenylation, rather than decay of the mRNA body, and this provides an explanation for why p38 MAPK regulates mRNA stability in some situations and translation in others.

DcpS can act in the 5'-3' mRNA decay pathway in addition to the 3'-5' pathway

Eukaryotic mRNA degradation proceeds through two main pathways, both involving mRNA cap breakdown. In the 3'-5' mRNA decay pathway, mRNA body degradation generates free m7GpppN that is hydrolyzed by DcpS generating m7GMP. In the 5'-3' pathway, the recently identified human Dcp2 decapping enzyme cleaves the cap of deadenylated mRNAs to produce m7GDP and 5'-phosphorylated mRNA. We investigated mRNA decay in human cell extracts by using a new assay for decapping. We observed that 5'-phosphorylated intermediates resulting from decapping appear after incubation of a substrate RNA in human cell extracts, indicating the presence of an active 5'-3' mRNA decay pathway. Surprisingly, however, the cognate m7GDP product was not detected, whereas abundant amounts of m7GMP were generated. Additional experiments revealed that m7GDP is, unexpectedly, efficiently converted to m7GMP in extracts from various organisms. The factor necessary and sufficient for this reaction was identified as DcpS in both yeast and human. m7GMP is thus a general, pathway-independent, by-product of eukaryotic mRNA decay. m7GDP breakdown should prevent misincorporation of methylated nucleotides in nucleic acids and could generate a unique indicator allowing the cell to monitor mRNA decay.

Functional characterization of the mammalian mRNA decapping enzyme hDcp2

Regulation of decapping is a critical determinant of mRNA stability. We recently identified hDcp2 as a human decapping enzyme with intrinsic decapping activity. This activity is specific to N(7)-methylated guanosine containing RNA. The hDcp2 enzyme does not function on the cap structure alone and is not sensitive to competition by cap analog, suggesting that hDcp2 requires the RNA for cap recognition. We now demonstrate that hDcp2 is an RNA-binding protein and its recognition and hydrolysis of the cap substrate is dependent on an initial interaction with the RNA moiety. A biochemical characterization of hDcp2 revealed that a 163 amino acid region containing two evolutionarily conserved regions, the Nudix fold hydrolase domain and the adjacent Box B region contained methyl-cap-specific hydrolysis activity. Maximum decapping activity for wild-type as well as truncation mutants of hDcp2 required Mn(2+) as a divalent cation. The demonstration that hDcp2 is an RNA-binding protein with an RNA-dependent decapping activity will now provide new approaches to identify specific mRNAs that are regulated by this decapping enzyme as well as provide novel avenues to control mRNA decapping and turnover by influencing the RNA-binding property of hDcp2.

Degradation of normal mRNA in the nucleus of Saccharomyces cerevisiae

A nuclear mRNA degradation (DRN) system was identified from analysis of mRNA turnover rates in nup116-Delta strains of Saccharomyces cerevisiae lacking the ability to export all RNAs, including poly(A) mRNAs, at the restrictive temperature. Northern blotting, in situ hybridization, and blocking transcription with thiolutin in nup116-delta strains revealed a rapid degradation of mRNAs in the nucleus that was suppressed by the rrp6-delta, rai1-delta, and cbc1-delta deletions, but not by the upf1-delta deletion, suggesting that DRN requires Rrp6p, a 3'-to-5' nuclear exonuclease, the Rat1p, a 5'-to-3' nuclear exonuclease, and Cbc1p, a component of CBC, the nuclear cap binding complex, which may direct the mRNAs to the site of degradation. We propose that certain normal mRNAs retained in the nucleus are degraded by the DRN system, similar to degradation of transcripts with 3' end formation defects in certain mutants.

Interaction between Ski7p and Upf1p is required for nonsense-mediated 3'-to-5' mRNA decay in yeast

Aberrant mRNAs containing premature termination codons (PTC-mRNAs) are degraded by a conserved surveillance system, referred to as the nonsense- mediated decay (NMD) pathway. Although NMD is reported to operate on the decapping and 5'-to-3' exonucleolytic decay of PTC-mRNAs without affecting deadenylation, a role for an opposite 3'-to-5' decay pathway remains largely unexplored. In this study, we have characterized the 3'-to-5' directed mRNA degradation in the yeast NMD pathway. PTC-mRNAs are stabilized in yeast cells lacking the components of 3'-to-5' mRNA-decay machinery. The 3'-to-5' directed degradation of PTC-mRNAs proceeds more rapidly than that of the PTC-free transcript, in a manner dependent on the cytoplasmic exosome and Upf proteins. Moreover, Upf1p, but not Upf2p, interacts physically with an N-terminal domain of Ski7p, although the interaction requires Upf2p. The efficiency of 3'-to-5' directed degradation of PTC-mRNAs is impaired by overexpression of Ski7p N-domain fragments that contain a sequence of the Upf1p-interaction region. These data suggest that the activation of 3'-to-5' directed NMD is mediated through the interaction between Upf1p and the Ski7p N domain.

The Upf-dependent decay of wild-type PPR1 mRNA depends on its 5'-UTR and first 92 ORF nucleotides

mRNAs containing premature translation termination codons (nonsense mRNAs) are targeted for deadenylation-independent degradation in a mechanism that depends on Upf1p, Upf2p and Upf3p. This decay pathway is often called nonsense- mediated mRNA decay (NMD). Nonsense mRNAs are decapped by Dcp1p and then degraded 5' to 3' by Xrn1p. In the yeast Saccharomyces cerevisiae, a significant number of wild-type mRNAs accumulate in upf mutants. Wild-type PPR1 mRNA is one of these mRNAs. Here we show that PPR1 mRNA degradation depends on the Upf proteins, Dcp1p, Xrn1p and Hrp1p. We have mapped an Upf1p-dependent destabilizing element to a region located within the 5'-UTR and the first 92 bases of the PPR1 ORF. This element targets PPR1 mRNA for Upf-dependent decay by a novel mechanism.

Genetic background affects relative nonsense mRNA accumulation in wild-type and upf mutant yeast strains

The Saccharomyces cerevisiae nonsense-mediated mRNA decay (NMD) pathway targets mRNAs with premature stop codons and some wild-type mRNAs for accelerated decay. Upf1p, Upf2p and Upf3p are required for NMD. NMD-targeted mRNAs are degraded rapidly in wild-type cells and stabilized in upf1, upf2 or upf3 mutants. We report here that the relative CYH2 pre-mRNA/mRNA accumulation is enhanced in cells derived from a W303 background, compared with a variety of commonly used strains. The enhanced CYH2 pre-mRNA accumulation phenotype results from a larger difference in mRNA half-lives in the W303 strains than two previously used strains. This phenotype can be selected in crosses and is also seen in upf2 and upf3 mutants. These results suggest there are genes that influence the efficiency of NMD and that yeast strains derived from the W303 background may be useful for measurement of abundance and half-lives of low abundance, short-lived NMD substrates.

Decapping and decay of messenger RNA occur in cytoplasmic processing bodies

A major pathway of eukaryotic messenger RNA (mRNA) turnover begins with deadenylation, followed by decapping and 5' to 3' exonucleolytic decay. We provide evidence that mRNA decapping and 5' to 3' degradation occur in discrete cytoplasmic foci in yeast, which we call processing bodies (P bodies). First, proteins that activate or catalyze decapping are concentrated in P bodies. Second, inhibiting mRNA turnover before decapping leads to loss of P bodies; however, inhibiting turnover at, or after, decapping, increases the abundance and size of P bodies. Finally, mRNA degradation intermediates are localized to P bodies. These results define the flux of mRNAs between polysomes and P bodies as a critical aspect of cytoplasmic mRNA metabolism and a possible site for regulation of mRNA degradation.

An NMD pathway in yeast involving accelerated deadenylation and exosome-mediated 3'-->5' degradation

Eukaryotic mRNAs containing premature termination codons are subjected to accelerated turnover, known as nonsense-mediated decay (NMD). Recognition of translation termination events as premature requires a surveillance complex, which includes the RNA helicase Upf1p. In Saccharomyces cerevisiae, NMD provokes rapid decapping followed by 5'-->3' exonucleolytic decay. Here we report an alternative, decapping-independent NMD pathway involving deadenylation and subsequent 3'-->5' exonucleolytic decay. Accelerated turnover via this pathway required Upf1p and was blocked by the translation inhibitor cycloheximide. Degradation of the deadenylated mRNA required the Rrp4p and Ski7p components of the cytoplasmic exosome complex, as well as the putative RNA helicase Ski2p. We conclude that recognition of NMD substrates by the Upf surveillance complex can target mRNAs to rapid deadenylation and exosome-mediated degradation.

Looking at mRNA decay pathways through the window of molecular evolution

In eukaryotes, mRNAs are monitored for errors in gene expression by RNA surveillance where untranslatable mRNAs are selectively degraded by the nonsense-mediated mRNA decay (NMD) pathway. Depending on the organism, three to seven genes are required for NMD. Besides RNA surveillance, the genes required for NMD serve a second purpose by controlling the overall abundance of a substantial fraction of the transcriptome.

Autographa californica nucleopolyhedrovirus orf69 encodes an RNA cap (nucleoside-2'-O)-methyltransferase

The AcNPV orf69 gene encodes a protein that contains an S-adenosylmethionine (AdoMet)-dependent methyltransferase signature motif. More significantly, ORF69 shows high conservation at residues diagnostic for (nucleoside 2'-O)-methyltransferase activity. To analyze the function of this protein, which was renamed MTase1, it was overexpressed in Escherichia coli and purified to homogeneity. Photo cross-linking experiments showed that MTase1 bound AdoMet, and functional assays demonstrated cap 0-dependent methyltransferase activity. In vivo expression assays in insect cells showed that MTase1 was synthesized during the late phase of infection and that its expression was dependent on viral DNA replication. Primer extension analysis identified a late promoter motif, ATAAG, at the transcription start site. A mutant virus was constructed by inserting the lacZ gene into the coding region of mtase1. Immunoblot analysis confirmed that MTase1 was not synthesized in these cells, and single-step growth curves revealed that the rate of virus replication in tissue culture was not affected by the absence of MTase1.

SMG-5, required for C.elegans nonsense-mediated mRNA decay, associates with SMG-2 and protein phosphatase 2A

mRNAs that contain premature stop codons are degraded selectively and rapidly in eukaryotes, a phenomenon termed 'nonsense-mediated mRNA decay' (NMD). We report here molecular analysis of smg-5, which encodes a novel protein required for NMD in Caenorhabditis elegans. Using a combination of immunoprecipitation and yeast two-hybrid assays, we identified a series of protein-protein interactions involving SMG-5. SMG-5 interacts with at least four proteins: (i) SMG-7, a previously identified protein required for NMD; (ii) SMG-2, a phosphorylated protein required for NMD in worms, yeasts and mammals; (iii) PR65, the structural subunit of protein phosphatase 2A (PP2A); and (iv) PP2A(C), the catalytic subunit of PP2A. Previous work demonstrated that both SMG-5 and SMG-7 are required for efficient dephosphorylation of SMG-2. Our results suggest that PP2A is the SMG-2 phosphatase, and the role of SMG-5 is to direct PP2A to its SMG-2 substrate. We discuss cycles of SMG-2 phosphorylation and their roles in NMD.

Nonsense-containing mRNAs that accumulate in the absence of a functional nonsense-mediated mRNA decay pathway are destabilized rapidly upon its restitution

Nonsense-mediated mRNA decay (NMD) is a conserved proofreading mechanism that protects eukaryotic cells from the potentially deleterious effects of truncated proteins. Studies of Saccharomyces cerevisiae imply that NMD is a predominantly cytoplasmic decay pathway, while studies of mammalian systems suggest that decay of most substrate mRNAs may occur while they are still associated with the nucleus, possibly during a round of translation that occurs during their export to the cytoplasm. Complete entry of the latter mRNAs into the cytoplasm appears to render them immune to further NMD; i.e., they escape further susceptibility to this decay pathway. To determine if yeast cytoplasmic nonsense-containing mRNAs that evade decay are subsequently immune to NMD, we examined the consequences of placing each of the three UPF/NMD genes under the control of a galactose-inducible promoter. The decay kinetics of ADE2 and PGK1 nonsense-containing mRNAs were then analyzed when expression of UPF1, NMD2, or UPF3 was either repressed or subsequently induced. Results from these experiments demonstrated that activation of NMD caused rapid and immediate degradation of both substrate transcripts, with half-lives of both stable mRNA populations shortened to approximately 7 min. These findings make it unlikely that yeast nonsense-containing mRNAs can escape degradation by NMD and indicate that such mRNAs are available to this decay pathway at each round of translation.

A truncated form of KlLsm4p and the absence of factors involved in mRNA decapping trigger apoptosis in yeast

The LSM4 gene of Saccharomyces cerevisiae codes for an essential protein involved in pre-mRNA splicing and also in mRNA decapping, a crucial step for mRNA degradation. We previously demonstrated that the first 72 amino acids of the Kluyveromyces lactis Lsm4p (KlLsm4p), which contain the Sm-like domains, can restore cell viability in both K. lactis and S. cerevisiae cells not expressing the endogenous protein. However, the absence of the carboxy-terminal region resulted in a remarkable loss of viability in stationary phase cells (). Herein, we demonstrate that S. cerevisiae cells expressing the truncated LSM4 protein of K. lactis showed the phenotypic markers of yeast apoptosis such as chromatin condensation, DNA fragmentation, and accumulation of reactive oxygen species. The study of deletion mutants revealed that apoptotic markers were clearly evident also in strains lacking genes involved in mRNA decapping, such as LSM1, DCP1, and DCP2, whereas a slight effect was observed in strains lacking the genes DHH1 and PAT1. This is the first time that a connection between mRNA stability and apoptosis is reported in yeast, pointing to mRNA decapping as the crucial step responsible of the observed apoptotic phenotypes.

Human Dcp2: a catalytically active mRNA decapping enzyme located in specific cytoplasmic structures

We have cloned cDNAs for the human homologues of the yeast Dcp1 and Dcp2 factors involved in the major (5'-3') and NMD mRNA decay pathways. While yeast Dcp1 has been reported to be the decapping enzyme, we show that recombinant human Dcp2 (hDcp2) is enzymatically active. Dcp2 activity appears evolutionarily conserved. Mutational and biochemical analyses indicate that the hDcp2 MutT/Nudix domain mediates this activity. hDcp2 generates m7GDP and 5'-phosphorylated mRNAs that are 5'-3' exonuclease substrates. Corresponding decay intermediates are present in human cells showing the relevance of this activity. hDcp1 and hDcp2 co-localize in cell cytoplasm, consistent with a role in mRNA decay. Interestingly, these two proteins show a non-uniform distribution, accumulating in specific foci.

Identification of a human decapping complex associated with hUpf proteins in nonsense-mediated decay

Decapping is a key step in general and regulated mRNA decay. In Saccharomyces cerevisiae it constitutes a rate-limiting step in the nonsense-mediated decay pathway that rids cells of mRNAs containing premature termination codons. Here two human decapping enzymes are identified, hDcp1a and hDcp2, as well as a homolog of hDcp1a, termed hDcp1b. Transiently expressed hDcp1a and hDcp2 proteins localize primarily to the cytoplasm and form a complex in human cell extracts. hDcp1a and hDcp2 copurify with decapping activity, an activity sensitive to mutation of critical hDcp residues. Importantly, coimmunoprecipitation assays demonstrate that hDcp1a and hDcp2 interact with the nonsense-mediated decay factor hUpf1, both in the presence and in the absence of the other hUpf proteins, hUpf2, hUpf3a, and hUpf3b. These data suggest that a human decapping complex may be recruited to mRNAs containing premature termination codons by the hUpf proteins.