Characterization of 3′hExo, a 3′ Exonuclease Specifically Interacting with the 3′ End of Histone mRNA

Abstract

The 3′ end of mammalian histone mRNAs consisting of a conserved stem-loop and a terminal ACCCA interacts with a recently identified human 3′ exonuclease designated 3′hExo. The sequence-specific interaction suggests that 3′hExo may participate in the degradation of histone mRNAs. ERI-1, a Caenorhabditis elegans homologue of 3′hExo, has been implicated in degradation of small interfering RNAs. We introduced a number of mutations to 3′hExo to identify residues required for RNA binding and catalysis. To assure that the introduced mutations specifically target one of these two activities of 3′hExo rather than cause global structural defects, the mutant proteins were tested in parallel for the ability both to bind the stem-loop RNA and to degrade RNA substrates. Our analysis confirms that 3′hExo is a member of the DEDDh family of 3′ exonucleases. Specific binding to the RNA requires the SAP domain and two lysines located immediately to its C terminus. 3′hExo binds with the highest affinity to the wild-type 3′ end of histone mRNA, and any changes to this sequence reduce efficiency of binding. 3′hExo has only residual, if any, 3′ exonuclease activity on DNA substrates and localizes mostly to the cytoplasm, suggesting that in vivo it performs exclusively RNA-specific functions. Efficient degradation of RNA substrates by 3′hExo requires 2′ and 3′ hydroxyl groups at the last nucleotide. 3′hExo removes 3′ overhangs of small interfering RNAs, whereas the double-stranded region is resistant to the enzymatic activity. The 3′ end of mammalian histone mRNAs consisting of a conserved stem-loop and a terminal ACCCA interacts with a recently identified human 3′ exonuclease designated 3′hExo. The sequence-specific interaction suggests that 3′hExo may participate in the degradation of histone mRNAs. ERI-1, a Caenorhabditis elegans homologue of 3′hExo, has been implicated in degradation of small interfering RNAs. We introduced a number of mutations to 3′hExo to identify residues required for RNA binding and catalysis. To assure that the introduced mutations specifically target one of these two activities of 3′hExo rather than cause global structural defects, the mutant proteins were tested in parallel for the ability both to bind the stem-loop RNA and to degrade RNA substrates. Our analysis confirms that 3′hExo is a member of the DEDDh family of 3′ exonucleases. Specific binding to the RNA requires the SAP domain and two lysines located immediately to its C terminus. 3′hExo binds with the highest affinity to the wild-type 3′ end of histone mRNA, and any changes to this sequence reduce efficiency of binding. 3′hExo has only residual, if any, 3′ exonuclease activity on DNA substrates and localizes mostly to the cytoplasm, suggesting that in vivo it performs exclusively RNA-specific functions. Efficient degradation of RNA substrates by 3′hExo requires 2′ and 3′ hydroxyl groups at the last nucleotide. 3′hExo removes 3′ overhangs of small interfering RNAs, whereas the double-stranded region is resistant to the enzymatic activity. Mammalian histone mRNAs end with a highly conserved and unique stem-loop structure followed by a single-stranded ACCCA sequence (1Marzluff W.F. Duronio R.J. Curr. Opin. Cell Biol. 2002; 14: 692-699Crossref PubMed Scopus (204) Google Scholar). The 3′ end of histone mRNAs is specifically recognized by two proteins, the stem-loop-binding protein (SLBP) and 3′hExo (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). Binding of SLBP requires the nucleotides at the 5′ side of the stem-loop, whereas binding of 3′hExo requires the single-stranded ACCCA at the 3′ end. In addition, binding of each protein requires specific nucleotides in the stem and the loop. SLBP and 3′hExo can bind the 3′ end of histone mRNA either individually or simultaneously, forming a ternary complex (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). SLBP stimulates binding of 3′hExo to the stem-loop and allows 3′hExo to form a complex with suboptimal RNA targets, raising the possibility that the two proteins make direct contact with each other in the ternary complex (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). Binding of SLBP to the stem-loop in histone mRNA precursors is required for formation of the correct 3′ end of histone mRNAs in the nucleus (3Dominski Z. Marzluff W.F. Gene. 1999; 239: 1-14Crossref PubMed Scopus (187) Google Scholar). SLBP bound to the stem-loop accompanies mature histone mRNA to the cytoplasm, where it stimulates histone mRNA translation (4Sanchez R. Marzluff W.F. Mol. Cell. Biol. 2002; 22: 7093-7104Crossref PubMed Scopus (111) Google Scholar). The half-life of histone mRNAs is greatly reduced in response to completion or inhibition of DNA replication, resulting in rapid disappearance of histone mRNAs from the cytoplasm and cessation of histone production (5Osley M.A. Annu. Rev. Biochem. 1991; 60: 827-861Crossref PubMed Scopus (452) Google Scholar). The stem-loop structure is both necessary and sufficient for the selective degradation of histone mRNA and confers the same type of regulation on other mRNAs when introduced at their 3′ end (6Pandey N.B. Marzluff W.F. Mol. Cell. Biol. 1987; 7: 4557-4559Crossref PubMed Scopus (185) Google Scholar). Sequence-specific and tight binding of 3′hExo to the 3′ end of histone mRNAs simultaneously with SLBP makes this protein a primary candidate for an exonuclease that participates in the rapid degradation of histone mRNAs. However, it has been recently suggested that 3′hExo (7Buhler M. Mohn F. Stalder L. Muhlemann O. Mol. Cell. 2005; 18: 307-317Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar) and its putative Caenorhabditis elegans homologue, ERI-1 (8Kennedy S. Wang D. Ruvkun G. Nature. 2004; 427: 645-649Crossref PubMed Scopus (468) Google Scholar), might play a role in down-regulation of RNA interference by degrading small interfering RNAs (siRNAs). 2The abbreviations used are: siRNA, small interfering RNA; SL, stem-loop. Crystallographic studies of the 3′ exonuclease domain of 3′hExo in complex with rAMP demonstrated that 3′hExo is a member of a DEDD superfamily of 3′ exonucleases (9Cheng Y. Patel D.J. J. Mol. Biol. 2004; 343: 305-312Crossref PubMed Scopus (40) Google Scholar) that includes both RNases and DNases (10Zuo Y. Deutscher M.P. Nucleic Acids Res. 2001; 29: 1017-1026Crossref PubMed Scopus (399) Google Scholar). Among the most prominent members of the family are RNase T, poly(A)-specific ribonuclease, the exosome component PM-Scl 100-kDa autoantigen, and the proofreading subunits of DNA polymerases (10Zuo Y. Deutscher M.P. Nucleic Acids Res. 2001; 29: 1017-1026Crossref PubMed Scopus (399) Google Scholar). Interestingly, based on data base searches 3′hExo and ERI-1 appear to be part of a group of closely related but uncharacterized putative exonucleases present in a number of eukaryotes. To get some insight into the molecular function of 3′hExo and its homologues, we carried out experiments to further define the specificity of this enzyme and the regions required for its activity. Expression and Mutagenesis of 3′hExo—The 35S-labeled 3′hExo was synthesized in rabbit reticulocytes using the transcription and translation-coupled system (TNT), as suggested by the manufacturer (Promega). SLBP and the wild-type and mutant forms of 3′hExo were overexpressed using the baculovirus expression system (Invitrogen). Briefly, the cDNAs for each protein were cloned into a pFastBac plasmid and used to generate the recombinant viruses. For preparative purification, a 200-ml culture of Sf9 insect cells was infected with each virus and the His-tagged proteins were purified by chromatography on nickel-agarose. Point mutations and internal deletions within the 3′hExo coding region were generated using the QuikChange site-directed mutagenesis kit (Stratagene). RNAs—The 31-nucleotide stem-loop (SL) RNA and its derivative at the 5′ end were synthesized by The sequence of these RNAs is in The of the and RNAs, both generated by were (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). mutant RNAs deletions or mutations at the 3′ end were generated by transcription using DNA The single-stranded RNA were from and used as substrates for 3′hExo in RNA substrates were at the 5′ end using and RNAs generated by transcription to were with to the 5′ The RNA was at the 3′ end with using RNA as suggested by the of the of the 5′ RNA was in of and at for The was with of and of The RNA was with and in of 3′hExo Binding was carried out using of 3′hExo SLBP and of 5′ RNA in a of In the binding 35S-labeled 3′hExo was bound to either the RNA or to RNA to a to the nucleotides of the were carried out as (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). 3′hExo exonuclease activity of 3′hExo was tested using or of protein as for each and of RNA substrates in a of of and In some was used to the enzymatic activity of 3′hExo. The were at for and the RNA was in and by 3′hExo the ACCCA at the 3′ that in the of 3′hExo can bind to the 3′ end of histone mRNAs both and simultaneously with SLBP (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). a of SLBP and 3′hExo bind to the target sequence consisting of a highly conserved followed by SLBP the 5′ side of the with an role by two located and nucleotides of the whereas 3′hExo binds to the 3′ side of the structure with an role by the ACCCA terminus. We the role of the ACCCA in the binding of 3′hExo in using a as (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). In this 35S-labeled 3′hExo was in the of with of wild-type RNA or its to a at the 3′ end. The were on and the of the protein with the RNA using and The wild-type RNA with the ACCCA bound of the 3′hExo whereas in the of RNA protein was on the of the ACCCA region in a of binding of 3′hExo to the RNA in the efficiency of binding was by the 3′ end with or nucleotides as in the RNA In to the of the 3′ single-stranded we the ACCCA sequence with Binding of 3′hExo to this RNA was reduced with the wild-type RNA by a of in with (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar), the ACCCA sequence at the end of histone mRNA is for 3′hExo binding. of the SAP and the in RNA interaction 3′hExo and the 3′ end of histone mRNA was 3′hExo any RNA binding domain The SAP domain has been as a an and a conserved of and L. Biochem. Full Text Full Text PDF PubMed Scopus Google Scholar). The SAP domain is present in a number of proteins in with other that these proteins with RNA or DNA L. Biochem. Full Text Full Text PDF PubMed Scopus Google Scholar). The role of the SAP domain is it has been suggested that it binding of proteins to double-stranded DNA L. Biochem. Full Text Full Text PDF PubMed Scopus Google Scholar). In 3′hExo, the SAP domain with the 3′ exonuclease domain is of of 3′hExo in other and is in elegans ERI-1 the only protein that both the SAP domain and the 3′ exonuclease domain (8Kennedy S. Wang D. Ruvkun G. Nature. 2004; 427: 645-649Crossref PubMed Scopus (468) Google Scholar). We synthesized a 35S-labeled mutant the SAP domain and its ability to bind or of the RNA at the 5′ end of mutant protein was on that of the SAP domain binding of 3′hExo to the RNA and To of the SAP domain in global of 3′hExo rather than specifically RNA we the protein in the baculovirus system and tested the ability of the mutant protein to degrade the 5′ RNA the located stem-loop, degradation of the RNA by 3′hExo, resulting in the formation of a with the is nucleotides than the RNA generated in a from the the ACCCA the stem-loop The mutant protein at both and the RNA to the stem and enzymatic activity with that of the wild-type 3′hExo. that of the SAP domain cause structural changes in 3′hExo and is with the that most of the DEDD family the SAP domain and enzymatic activity. To identify of the SAP domain for RNA we with The SAP domain of 3′hExo two and and two and that are conserved in ERI-1 but present in other SAP domain proteins suggesting that these residues may participate in sequence-specific RNA binding. proteins at or the wild-type efficiency to bind the RNA in the and on binding was when the two mutations were in one protein of is present in SAP reduced the ability of 3′hExo to bind the RNA only of two at and with a small on binding these experiments to identify residues of the SAP domain that are in RNA suggesting that the specificity are by other of the domain that of may be required to RNA binding ability of 3′hExo. the structure of 3′hExo in complex with the RNA was and it demonstrated that the SAP domain is for binding to the RNA and that a role is by at and D. by these we this with was sufficient to binding of 3′hExo to the RNA and whereas of in to the structure is in RNA We the and with the mutant two lysines are located in the that the SAP domain from the 3′ exonuclease The 35S-labeled protein when tested in the with of the RNA The protein in a of enzymatic that the global of the exonuclease and We the and the exonuclease domain within these and or reduced or binding to the RNA However, these mutations a on the enzymatic activity of 3′hExo suggesting that may the structure of the protein rather than specifically RNA binding. of 3′hExo for the 3′ exonuclease domain of 3′hExo is located and on the sequence of this we 3′hExo as a member of the DEDD family of 3′ exonucleases (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). members of this family are by the of the residues required for (10Zuo Y. Deutscher M.P. Nucleic Acids Res. 2001; 29: 1017-1026Crossref PubMed Scopus (399) Google Scholar). The DEDD family has been in two on the of either a conserved or studies of the exonuclease domain of 3′hExo with rAMP in the of 3′hExo as a member of the DEDDh and identified the residues of the and (9Cheng Y. Patel D.J. J. Mol. Biol. 2004; 343: 305-312Crossref PubMed Scopus (40) Google Scholar) The residues two and with at a for of RNA substrates in the (9Cheng Y. Patel D.J. J. Mol. Biol. 2004; 343: 305-312Crossref PubMed Scopus (40) Google Scholar). To that the at and are required for the exonuclease activity of 3′hExo, we each of the two residues with an was used to identify residues in two other 3′ exonucleases of the DEDD family Y. Deutscher M.P. J. Biol. 2002; Full Text Full Text PDF PubMed Scopus Google J. J. Biol. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar). The two mutant proteins, and were in the baculovirus system and tested for the ability to degrade the 5′ The wild-type 3′hExo the RNA into the whereas the two mutant proteins at this protein were the of in and for and 3′hExo and its as as elegans ERI-1 conserved located in 3′hExo at of this to the enzymatic activity of 3′hExo, and at the mutant protein a activity to that of the wild-type protein and protein the mutant was than the wild-type 3′hExo and of a with the exonuclease activity of 3′hExo on the of this the of 3′hExo and elegans ERI-1 been as members of a rather than the DEDD (8Kennedy S. Wang D. Ruvkun G. Nature. 2004; 427: 645-649Crossref PubMed Scopus (468) Google with 3′ as substrates for 3′hExo. the ability of of the 3′hExo to degrade RNA generated by or of the 5′ of the RNA by 3′hExo is in The RNA substrates are in and the ability of of the 3′hExo to degrade of the RNA at the 3′ end by The RNA was in the of with 3′hExo with of or simultaneously with both The were in a of and as for The RNA is in the ability of or of the of 3′hExo to degrade of the 5′ with a 2′ The were carried out in a of the ability of of the 3′hExo to degrade of the 5′ with a The were carried out in a of with either or the ability of of the of 3′hExo to degrade of the 5′ RNA in the of We the mutant proteins of mutations within the conserved residues ability to bind the 31-nucleotide RNA in the The RNA was to the but was at the 5′ end and In the of both 3′hExo and SLBP form a complex with the the interaction of the RNA with 3′hExo is as as with SLBP and ternary complex with the RNA is in the of SLBP and 3′hExo The same ability to form a and a ternary complex in the of was for the protein and and protein that these two 3′hExo ability to with the RNA and the protein was to with the RNA or to form a ternary complex SLBP and suggesting that of to global changes in the 3′hExo structure that both binding and of the of the with the was in the suggesting protein and further the that of in protein We tested the ability of the mutant in the baculovirus system to form the and ternary In with the of the and this mutant was to with the RNA and to form the ternary complex and SLBP forms a complex with the RNA in the of (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar), these 3′hExo the of a complex SLBP degradation of the RNA by 3′hExo in the of of a ternary complex the two proteins (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). the protein a complex with the RNA in the of and with SLBP in forming a ternary complex and for Efficient of RNA by 3′hExo is to the of the 3′ end of the we tested a number of RNAs groups at the 3′ terminal nucleotide. We generated a of degradation by the 5′ RNA with either a or a 3′ hydroxyl J. 2001; PubMed Scopus Google Scholar). The of were resistant to 3′hExo, whereas the RNA present in the same was to the by the that the of the the 3′hExo activity and the RNA by was to the and To further that a group at the 3′ end enzymatic we to the 3′ end of the an RNA the 3′ end and with a 3′ RNA was resistant to of 3′hExo and by this of the enzyme only of that removes the terminal We tested 3′hExo can degrade a 5′ RNA with a 2′ We used an consisting of at the 3′ end by a 2′ The wild-type enzyme at this whereas the mutant protein was and In the of of the wild-type 3′hExo, the degradation of the was and only a of the was to a as by using The same of 3′hExo the of the RNA to the that the of 2′ at the last RNA The for the of the RNA is The of two at the 3′ end of this RNA may that 3′hExo at is in degrading RNAs when it a of We of an with degradation of the stem-loop RNA (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). In to the wild-type 3′hExo and the mutant protein we tested the this mutant protein with the wild-type 3′hExo reduced activity on both the and the substrates We tested an RNA to the but with a with the was resistant to the wild-type 3′hExo, and in the of the highest of the enzyme and only of the was degradation was in the of the enzyme these that the of the at the 2′ and 3′ of the last greatly the ability of 3′hExo to degrade RNA substrates and a a Our studies to that 3′hExo is on substrates consisting of and suggested that 3′hExo is an RNA-specific exonuclease (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). We tested a number of single-stranded DNA substrates and a 3′ exonuclease activity with some only in the of enzyme and studies are required to 3′hExo can function as 3′hExo the from for proteins in RNA interference suggested that ERI-1, a elegans homologue of 3′hExo, may be in degrading (8Kennedy S. Wang D. Ruvkun G. Nature. 2004; 427: 645-649Crossref PubMed Scopus (468) Google Scholar). both 3′hExo and ERI-1, when in rabbit were to degrade double-stranded RNAs and to be on single-stranded RNA substrates (8Kennedy S. Wang D. Ruvkun G. Nature. 2004; 427: 645-649Crossref PubMed Scopus (468) Google Scholar). To the ability of the 3′hExo to degrade we carried out a number of experiments using two single-stranded RNAs, and a double-stranded region on each side by 3′ each at the 5′ end was by of 3′hExo in the of and and degradation was in the of the enzyme and To formation of the double-stranded RNA degradation by 3′hExo, of the enzyme were with of the 5′ either with of the same or the The RNA used in the or in the of to of the same RNA was by 3′hExo degradation However, the of the the RNA the activity of 3′hExo, to of nucleotides in the of the RNA in of efficiency in formation of the double-stranded of an that 3′hExo, in to the can the of the double-stranded In the of of 3′hExo the RNA was that the enzyme at this is to 3′ overhangs and it can degrade the single-stranded RNA these The of the of 3′hExo, we cloned 3′hExo with protein and the of the protein in cells using The protein protein in the cytoplasm and the of the protein to these two was in cells and was of the of expression of the is with demonstrated the of the 3′hExo both in the and The highly conserved sequence at the 3′ end of mammalian histone mRNAs a stem and a followed by ACCCA is specifically recognized by two proteins, the stem-loop binding protein (SLBP) and a protein with 3′ exonuclease activity designated 3′hExo (2Dominski Z. Yang X. Kaygun H. Marzluff W.F. Mol. Cell. 2003; 12: 295-305Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). SLBP is to play in and of histone mRNAs (1Marzluff W.F. Duronio R.J. Curr. Opin. Cell Biol. 2002; 14: 692-699Crossref PubMed Scopus (204) Google Scholar), the role of 3′hExo we carried out a number of experiments to further this Crystallographic studies of the 3′ exonuclease domain of 3′hExo that 3′hExo to the DEDDh of exonucleases and that the residues in and (9Cheng Y. Patel D.J. J. Mol. Biol. 2004; 343: 305-312Crossref PubMed Scopus (40) Google Scholar). has been by analysis that of either or enzymatic activity of 3′hExo. The residues of the DEDDh identified in 3′hExo by the studies are highly conserved in of 3′hExo and in other putative exonucleases that with 3′hExo, ERI-1 and other of this protein in elegans Interestingly, these proteins a highly conserved in human that is part of the DEDDh of this to the enzymatic activity of 3′hExo but reduced its ability to degrade RNA substrates. with other 3′ exonucleases of the DEDD family that 3′hExo is to a proofreading domain of DNA than to RNases (9Cheng Y. Patel D.J. J. Mol. Biol. 2004; 343: 305-312Crossref PubMed Scopus (40) Google Scholar). one enzyme of the DEDD RNase T, is both and RNase (10Zuo Y. Deutscher M.P. Nucleic Acids Res. 2001; 29: 1017-1026Crossref PubMed Scopus (399) Google Y. Deutscher M.P. Nucleic Acids Res. 1999; PubMed Scopus Google Y. Deutscher M.P. J. Biol. 2002; Full Text Full Text PDF PubMed Scopus Google M. J. Biol. Full Text Full Text PDF PubMed Scopus Google Scholar). In RNase is on DNA than on RNA substrates Y. Deutscher M.P. Nucleic Acids Res. 1999; PubMed Scopus Google Scholar) and can in DNA in cells other DNases in nucleotides from DNA M. 1999; PubMed Google Scholar). suggests that in some of DNA may be a function of RNase in to its role in RNA We demonstrated that 3′hExo is an are if at for 3′hExo and to be by this enzyme Efficient degradation of RNA substrates by 3′hExo requires 2′ and 3′ hydroxyl groups at the last nucleotide. The activity of two other DEDD poly(A)-specific J. Biol. Full Text Full Text PDF PubMed Scopus Google Scholar) and RNase Y. Deutscher M.P. J. Biol. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar), has been to on the of the 3′ hydroxyl at the last of their substrates. The most of 3′hExo is its ability to specifically and bind the stem-loop at the 3′ end of histone mRNAs. Binding of 3′hExo to the stem-loop is by binding of the of the same RNA of 3′hExo interacts with the RNA and SLBP binding of 3′hExo to its RNA target structural studies of the and ternary of immediately the 3′ exonuclease domain a on RNA binding but at the same reduced activity of 3′hExo, suggesting that these mutations at global of the protein rather than target specifically in RNA binding. The on RNA binding ability of 3′hExo was by of the SAP domain or an of and located immediately to its C terminus. two mutations enzymatic activity of 3′hExo, suggesting that both the SAP domain and the two lysines are specifically required for interaction with the studies of the RNA complex demonstrated that the SAP domain is in binding to the RNA with the role by at of this with was sufficient to binding of 3′hExo to the The sequence-specific interaction of 3′hExo with the 3′ end of histone mRNAs either or with SLBP suggests that 3′hExo may play a role in the rapid of histone mRNA at the end of function is with the of 3′hExo in the cytoplasm, the of histone mRNA ERI-1 has been identified by for of elegans with RNA to the that one function of this exonuclease may be to degrade (8Kennedy S. Wang D. Ruvkun G. Nature. 2004; 427: 645-649Crossref PubMed Scopus (468) Google Scholar). However, as demonstrated ERI-1 in a complex with in to the response to double-stranded RNAs, is required for of S. Y. D. S. S. Ruvkun G. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). on these studies a for the molecular function of ERI-1 has been In this by the unique of 3′hExo, ERI-1 binds to in a group of RNAs and removes 3′ a structure for of double-stranded RNA by and in RNA interference S. Y. D. S. S. Ruvkun G. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). studies on molecular of ERI-1 and 3′hExo and to these two proteins are related in to sequence We Y. and D. Patel for their to We for with the