Monoclonal antibody characterization of the C proteins of heterogeneous nuclear ribonucleoprotein complexes in vertebrate cells

The C proteins of heterogeneous nuclear ribonucleoprotein complexes interact with RNA sequences downstream of polyadenylation cleavage sites

The heterogeneous nuclear ribonucleoprotein C1 and C2 proteins were preferentially cross-linked by treatment with UV light in nuclear extracts to RNAs containing six different polyadenylation signals. The domain required for the interaction was located downstream of the poly(A) cleavage site, since deletion of this segment from several polyadenylation substrate RNAs greatly reduced cross-linking efficiency. In addition, RNAs containing only downstream sequences were efficiently cross-linked to C proteins, while fully processed, polyadenylated RNAs were not. Analysis of mutated variants of the simian virus 40 late polyadenylation signal showed that uridylate-rich sequences located in the region between 30 and 55 nucleotides downstream of the cleavage site were required for efficient cross-linking of C proteins. This downstream domain of the simian virus 40 late poly(A) addition signal has been shown to influence the efficiency of the polyadenylation reaction. However, there was not a strict correlation between cross-linking of C proteins and the efficiency of polyadenylation.

Influenza A virus M2 protein: monoclonal antibody restriction of virus growth and detection of M2 in virions

The influenza A virus M2 protein is an integral membrane protein of 97 amino acids that is expressed at the surface of infected cells with an extracellular N-terminal domain of 18 to 23 amino acid residues, an internal hydrophobic domain of approximately 19 residues, and a C-terminal cytoplasmic domain of 54 residues. To gain an understanding of the M2 protein function in the influenza virus replicative pathway, we produced and characterized a monoclonal antibody to M2. The antibody-binding site was located to the extracellular N terminus of M2 as shown by the loss of recognition after proteolysis at the infected-cell surface, which removes 18 N-terminal residues, and by the finding that the antibody recognizes M2 in cell surface fluorescence. The epitope was further defined to involve residues 11 and 14 by comparing the predicted amino acid sequences of M2 from several avian and human strains and the ability of the M2 protein to be recognized by the antibody. The M2-specific monoclonal antibody was used in a sensitive immunoblot assay to show that M2 protein could be detected in virion preparations. Quantitation of the amount of M2 associated with virions by two unrelated methods indicated that in the virion preparations used there are 14 to 68 molecules of M2 per virion. The monoclonal antibody, when included in a plaque assay overlay, considerably showed the growth of some influenza virus strains. This plaque size reduction is a specific effect for the M2 antibody as determined by an analysis of recombinants with defined genome composition and by the observation that competition by an N-terminal peptide prevents the antibody restriction of virus growth.

Classification and purification of proteins of heterogeneous nuclear ribonucleoprotein particles by RNA-binding specificities

Several proteins of heterogeneous nuclear ribonucleoprotein (hnRNP) particles display very high binding affinities for different ribonucleotide homopolymers. The specificity of some of these proteins at high salt concentrations and in the presence of heparin allows for their rapid one-step purification from HeLa nucleoplasm. We show that the hnRNP C proteins are poly(U)-binding proteins and compare their specificity to that of the previously described cytoplasmic poly(A)-binding protein. These findings provide a useful tool for the classification and purification of hnRNP proteins from various tissues and organisms and indicate that different hnRNP proteins have different RNA-binding specificities.

Intron sequences and the length of the downstream second exon affect the binding of hnRNP C proteins in an in vitro splicing reaction

The proteins that are in direct contact with the pre-mRNA in an in vitro splicing reaction were analyzed by UV cross-linking experiments. Six major proteins (120, 55, 44, 42, 39 and 38 KD) and three minor polypeptides (84, 72 and 63 KD) were detected. The predominant proteins 44, 42 KD belong to the class of hnRNP C proteins since they were immunoprecipitated by monoclonal antibodies directed against hnRNP C proteins. The cross-linked proteins were not detected in the absence of Mg2+, ATP or when RNA lacking introns were used as substrates in the splicing reactions. The effect of exon sequences on the binding efficiency for the photocrosslinked proteins was investigated. Transcripts containing a second exon of 24 nucleotides for the beta-globin or 107 nucleotides for the mouse insulin, yielded a reduced amount of cross-linked proteins when compared with "full length" pre-mRNAs. Sequences within the first exon of the beta-globin pre-mRNA did not affect the binding efficiency of these proteins. The reduced binding efficiency of the cross-linked proteins for the truncated beta-globin or mouse insulin pre-mRNAs correlated with the lower efficiency for in vitro splicing. Substitutions with unrelated sequences in the beta-globin second exon restore the binding of the cross-linked proteins indicating that the length of the second exon and not specific sequences are relevant for the binding efficiency of these proteins. The SP6/mouse insulin oligonucleotides cross-linked to the hnRNP C proteins were isolated and sequenced. A 17-mer was located in the second exon (134 nucleotides downstream from the 3' splice site) and a 14-mer in the intron region (25 nucleotides downstream the 5' splice site). The beta-globin oligonucleotides cross-linked to the hnRNP C proteins were a 13-mer in the second exon (28 nucleotides downstream the 3' splice site) and an 8-mer in the first exon (81 nucleotides downstream the 5' end of the pre-mRNA). Our results indicate that the hnRNP C proteins interact with those oligonucleotides located in different regions of the pre-mRNA. The binding efficiency of those proteins, however, depends on the length of the second exon and the presence of intron sequences (secondary and/or tertiary pre-mRNA structure).

Classification and purification of proteins of heterogeneous nuclear ribonucleoprotein particles by RNA-binding specificities

Several proteins of heterogeneous nuclear ribonucleoprotein (hnRNP) particles display very high binding affinities for different ribonucleotide homopolymers. The specificity of some of these proteins at high salt concentrations and in the presence of heparin allows for their rapid one-step purification from HeLa nucleoplasm. We show that the hnRNP C proteins are poly(U)-binding proteins and compare their specificity to that of the previously described cytoplasmic poly(A)-binding protein. These findings provide a useful tool for the classification and purification of hnRNP proteins from various tissues and organisms and indicate that different hnRNP proteins have different RNA-binding specificities.

Autoantibodies to the core proteins of hnRNPs

A novel autoantibody reacting the the core polypeptides of hnRNP particles has been detected in the serum of a patient with systemic lupus erythematosus (SLE) and Sjögren's syndrome manifestations. Immunoblot analysis, using either rat liver or HeLa nuclear extracts as the antigen source, demonstrated that the autoantibody interacts with a specific subgroup of the core polypeptides of hnRNP particles, namely A2, B1 and B2, but not with A1, C1 and C2.

The distribution of two hnRNP-associated proteins defined by a monoclonal antibody is altered in heat-shocked HeLa cells

A monoclonal antibody obtained after mice were immunized with hnRNP purified from HeLa cells recognizes two polypeptides of Mr 35,000 and 37,000. By immunocytofluorescence, these antigens can be visualized only in cells previously heat shocked at 45 degrees C for 5 or 10 min, although they are present at the same level in unstressed and stressed cells. The signal, which is mostly concentrated in the interchromatin space, where hnRNP fibrils are located, does not accumulate with time and disappears 4 to 5 h after heat shock. Discrimination between the two types of hnRNP substructures, the 30-50 S monoparticles and the nuclear matrix fibrils, based on differential sensitivity to salt or ribonuclease treatment, showed that in unstressed cells the antigens behave as monoparticle proteins. In contrast, in heat-shocked cells, most 35-37K antigens behave as nuclear matrix proteins. Thus, heat shock seems to induce a rapid and reversible switch of these two antigens from hnRNP monoparticles to the nuclear matrix. The data demonstrate that heat shock, which was previously shown not to alter the overall RNA: protein packaging ratio of hnRNP, induces subtle modifications of their substructure. Such modifications might be of importance since heat shock is known for instance to affect pre-mRNA processing.

Immunopurification of heterogeneous nuclear ribonucleoprotein particles reveals an assortment of RNA-binding proteins

Heterogeneous nuclear RNA-ribonucleoprotein (hnRNP) particles can be efficiently purified by a specific, rapid, and mild procedure using monoclonal antibodies to hnRNP proteins. We report here on the detailed analysis of the protein composition of immunopurified hnRNP particles from human HeLa cells. By two-dimensional gel electrophoresis, immunopurified hnRNP particles contain at least 24 polypeptides in the range of 34,000-120,000 daltons. The abundant 30,000-40,000 dalton proteins, A, B, and C, described previously, are a subset of these polypeptides. The protein compositions of hnRNP particles found in the nucleoplasm fraction and in the chromatin-nucleolar fraction are very similar. Upon addition of the polyanion heparin, most of the major proteins remain associated in heparin-resistant particles, and only several, mostly minor, proteins dissociate. This provides an aid in the classification of the proteins and an additional criterion for the definition of hnRNP particle components. Chromatography on single-stranded DNA (ssDNA)-agarose in a heparin- and moderate or high salt (higher than 300 mM NaCl)-resistant manner suggests that most, if not all, of these proteins are single-stranded nucleic acid-binding proteins. We describe a general method for the large-scale purification of hnRNP proteins by affinity chromatography on ssDNA columns and its use for the production of new monoclonal antibodies to hnRNP proteins.

A factor, U2AF, is required for U2 snRNP binding and splicing complex assembly

Pre-mRNA splicing complex assembly is mediated by two specific pre-mRNA-snRNP interactions: U1 snRNP binds to the 5' splice site and U2 snRNP binds to the branch point. Here we show that unlike a purified U1 snRNP, which can bind to a 5' splice site, a partially purified U2 snRNP cannot interact with its target pre-mRNA sequence. We identify a previously uncharacterized activity, U2AF, that is required for the U2 snRNP-branch point interaction and splicing complex formation. Using RNA substrate exclusion and competition assays, we demonstrate that U2AF binds to the 3' splice site region prior to the U2 snRNP-branch point interaction. This provides an explanation for the necessity of the 3' splice site region in U2 snRNP binding and, hence, the first step of splicing.

Changes in heterogeneous nuclear RNP core polypeptide complements during the cell cycle

Mammalian heterogeneous nuclear RNP (hnRNP) subcomplexes are shown to be comprised of 14-17 basic A and B core group polypeptides (chrp) when subjected to two-dimensional immunoblot analysis. These proteins are normally confined to the nucleus but are distributed throughout the cell during mitosis. However, not all of the 17 protein spots are observed for all stages of the cell cycle. HeLa cell populations have been synchronized and the basic hnRNP core protein complement examined during S, G2, mitosis, and G1. During cell division several distinct chrp polypeptide species at 35 and 37 kD appear, while another of 37 kD and a chrp of 38 kD are diminished. These altered chrp complements are not due to any effects induced by thymidine treatment but appear to be physiological changes in the chrp polypeptide modification state. The new charge isomers found during mitosis are not the result of selective phosphorylation of the chrp polypeptides. However the nature of the modifications has yet to be determined. The mitosis-specific modified forms of the chrp polypeptides are found in the cytoplasmic fraction derived from mitotic cell populations. When this fraction is centrifuged upon sucrose density gradients the modified chrp polypeptides sediment from 30-200S in a distribution similar to that of hnRNP complexes isolated from the nuclei of randomly dividing cell populations. RNase digestion experiments indicate that the general substructure of the RNA/protein complexes in mitotic cell cytoplasm is similar to that of nuclear hnRNP isolated from unsynchronized cells or tissue.

Adenovirus proteins associated with mRNA and hnRNA in infected HeLa cells

The proteins that interact with cytoplasmic and nuclear polyadenylated RNA in adenovirus type 5 (Ad5) infection of HeLa cells were examined by UV-induced RNA-protein cross-linking in intact cells. The Ad5 100-kilodalton late nonvirion protein (100K protein) was cross-linked to both host and viral polyadenylated cytoplasmic RNA (mRNA). The cross-linking of the 100K protein to mRNA appears to correlate with productive infection, because the protein is not cross-linked to mRNA in abortive infection of wild-type Ad5 in monkey cells (CV-1) even though normal amounts of it are produced. However, when CV-1 cells are infected with Ad5 hr404, and Ad5 mutant which overcomes the host restriction to wild-type Ad5 infection in these cells, the 100K protein is cross-linked to mRNA. To identify and obtain antibodies to RNA-contacting proteins, a mouse was immunized with oligo(dT)-selected cross-linked RNA-protein complexes from Ad5-infected cells and the serum was used for immunoblotting experiments. It was found that in addition to the 100K protein, the Ad5 72K DNA-binding protein is also associated with RNA in the infected cells. The 72K DNA-binding protein is cross-linked to polyadenylated nuclear RNA sequences. These findings indicate that adenovirus proteins interact with RNAs in the infected cell and suggest possible mechanisms for the effects of the virus on mRNA metabolism.

The small nuclear ribonucleoprotein SS-B/La binds RNA with a conserved protease-resistant domain of 28 kilodaltons

SS-B/La is a nuclear protein of 48 kilodaltons with two structural domains of Mr 28,000 and Mr 23,000 generated by proteolytic cleavage. UV irradiation was used to cross-link preexisting intracellular La-RNA complexes. Subsequent protease digestion and diagonal gel electrophoresis showed that the RNA-binding site resided in the nonphosphorylated, methionine-rich 28-kilodalton domain.

Primary structure of human nuclear ribonucleoprotein particle C proteins: conservation of sequence and domain structures in heterogeneous nuclear RNA, mRNA, and pre-rRNA-binding proteins

In the eucaryotic nucleus, heterogeneous nuclear RNAs exist in a complex with a specific set of proteins to form heterogeneous nuclear ribonucleoprotein particles (hnRNPs). The C proteins, C1 and C2, are major constituents of hnRNPs and appear to play a role in RNA splicing as suggested by antibody inhibition and immunodepletion experiments. With the use of a previously described partial cDNA clone as a hybridization probe, full-length cDNAs for the human C proteins were isolated. All of the cDNAs isolated hybridized to two poly(A)+ RNAs of 1.9 and 1.4 kilobases (kb). DNA sequencing of a cDNA clone for the 1.9-kb mRNA (pHC12) revealed a single open reading frame of 290 amino acids coding for a protein of 31,931 daltons and two polyadenylation signals, AAUAAA, approximately 400 base pairs apart in the 3' untranslated region of the mRNA. DNA sequencing of a clone corresponding to the 1.4-kb mRNA (pHC5) indicated that the sequence of this mRNA is identical to that of the 1.9-kb mRNA up to the first polyadenylation signal which it uses. Both mRNAs therefore have the same coding capacity and are probably transcribed from a single gene. Translation in vitro of the 1.9-kb mRNA selected by hybridization with a 3'-end subfragment of pHC12 demonstrated that it by itself can direct the synthesis of both C1 and C2. The difference between the C1 and C2 proteins which results in their electrophoretic separation is not known, but most likely one of them is generated from the other posttranslationally. Since several hnRNP proteins appeared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis as multiple antigenically related polypeptides, this raises the possibility that some of these other groups of hnRNP proteins are also each produced from a single mRNA. The predicted amino acid sequence of the protein indicates that it is composed of two distinct domains: an amino terminus that contains what we have recently described as a RNP consensus sequence, which is the putative RNA-binding site, and a carboxy terminus that is very negatively charged, contains no aromatic amino acids or prolines, and contains a putative nucleoside triphosphate-binding fold, as well as a phosphorylation site for casein kinase type II. The RNP consensus sequence was also found in the yeast poly(A)-binding protein (PABP), the heterogeneous nuclear RNA-binding proteins A1 and A2, and the pre-rRNA binding protein C23. All of these proteins are also composed of at least two distinct domains: an amino terminus, which possesses one or more RNP consensus sequences, and a carboxy terminus, which is unique to each protein, being very acidic in the C proteins and rich in glycine in A1, and C23 and rich in proline in the poly(A)-binding protein. These findings suggest that the amino terminus of these proteins possesses a highly conserved RNA-binding domain, whereas the carboxy terminus contains a region essential to the unique function and interactions of each of the RNA-binding proteins.

The small nuclear ribonucleoprotein SS-B/La binds RNA with a conserved protease-resistant domain of 28 kilodaltons

SS-B/La is a nuclear protein of 48 kilodaltons with two structural domains of Mr 28,000 and Mr 23,000 generated by proteolytic cleavage. UV irradiation was used to cross-link preexisting intracellular La-RNA complexes. Subsequent protease digestion and diagonal gel electrophoresis showed that the RNA-binding site resided in the nonphosphorylated, methionine-rich 28-kilodalton domain.

Primary structure of human nuclear ribonucleoprotein particle C proteins: conservation of sequence and domain structures in heterogeneous nuclear RNA, mRNA, and pre-rRNA-binding proteins

In the eucaryotic nucleus, heterogeneous nuclear RNAs exist in a complex with a specific set of proteins to form heterogeneous nuclear ribonucleoprotein particles (hnRNPs). The C proteins, C1 and C2, are major constituents of hnRNPs and appear to play a role in RNA splicing as suggested by antibody inhibition and immunodepletion experiments. With the use of a previously described partial cDNA clone as a hybridization probe, full-length cDNAs for the human C proteins were isolated. All of the cDNAs isolated hybridized to two poly(A)+ RNAs of 1.9 and 1.4 kilobases (kb). DNA sequencing of a cDNA clone for the 1.9-kb mRNA (pHC12) revealed a single open reading frame of 290 amino acids coding for a protein of 31,931 daltons and two polyadenylation signals, AAUAAA, approximately 400 base pairs apart in the 3' untranslated region of the mRNA. DNA sequencing of a clone corresponding to the 1.4-kb mRNA (pHC5) indicated that the sequence of this mRNA is identical to that of the 1.9-kb mRNA up to the first polyadenylation signal which it uses. Both mRNAs therefore have the same coding capacity and are probably transcribed from a single gene. Translation in vitro of the 1.9-kb mRNA selected by hybridization with a 3'-end subfragment of pHC12 demonstrated that it by itself can direct the synthesis of both C1 and C2. The difference between the C1 and C2 proteins which results in their electrophoretic separation is not known, but most likely one of them is generated from the other posttranslationally. Since several hnRNP proteins appeared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis as multiple antigenically related polypeptides, this raises the possibility that some of these other groups of hnRNP proteins are also each produced from a single mRNA. The predicted amino acid sequence of the protein indicates that it is composed of two distinct domains: an amino terminus that contains what we have recently described as a RNP consensus sequence, which is the putative RNA-binding site, and a carboxy terminus that is very negatively charged, contains no aromatic amino acids or prolines, and contains a putative nucleoside triphosphate-binding fold, as well as a phosphorylation site for casein kinase type II. The RNP consensus sequence was also found in the yeast poly(A)-binding protein (PABP), the heterogeneous nuclear RNA-binding proteins A1 and A2, and the pre-rRNA binding protein C23. All of these proteins are also composed of at least two distinct domains: an amino terminus, which possesses one or more RNP consensus sequences, and a carboxy terminus, which is unique to each protein, being very acidic in the C proteins and rich in glycine in A1, and C23 and rich in proline in the poly(A)-binding protein. These findings suggest that the amino terminus of these proteins possesses a highly conserved RNA-binding domain, whereas the carboxy terminus contains a region essential to the unique function and interactions of each of the RNA-binding proteins.

Association of the adenovirus DNA-binding protein with RNA both in vitro and in vivo

The multifunctional DNA-binding protein (DBP) encoded by human adenovirus binds RNA. The association of purified DBP with RNA in vitro was demonstrated by using either a gel filtration or a filter binding assay. This association is sensitive to ionic strength and exhibits no apparent sequence specificity. DBP also interacts with RNA in vivo; it can be crosslinked to polyadenylylated RNA by UV-irradiation of intact cells during the late phase of adenovirus infections. The 46-kDa carboxyl-terminal domain of DBP binds RNA in vitro and was found to be associated with polyadenylylated RNA in vivo. This is the same domain that interacts with DNA. However, the differences in sensitivity of DBP to trypsin when bound to RNA versus DNA suggest that RNA and DNA either bind at different sites within this domain or induce different conformational changes within the protein.

A cDNA clone of the hnRNP C proteins and its homology with the single-stranded DNA binding protein UP2

A cDNA clone which expresses a protein that cross-reacts immunologically with the human C1 and C2 hnRNP core proteins has been isolated. The clone was selected by a sensitive immunochemical assay employing an avidin-biotin complex for detection, and identified as a clone for the hnRNP C proteins by a highly sensitive antibody select assay that is described here. The clone contains 677 nucleotides, and, as shown by northern blotting, is derived from a 1.5 Kb poly(A)+ mRNA. There are regions of strong homology between the human and mouse genes, weak homology is seen with chicken DNA, and very little, if any, homology can be detected with Drosophila, Artemia, sea urchin, or yeast DNAs. Two peptides (a total of 24 amino acids) of the calf thymus single-stranded DNA binding protein UP2 show perfect homology with the deduced amino acid sequence of the clone, suggesting that UP2 is related to the hnRNP C proteins. There is also a region that has a sequence very similar to two regions of the single-stranded DNA binding protein UP1 that contain proposed DNA binding sites.

Molecular cloning of cDNA for the nuclear ribonucleoprotein particle C proteins: a conserved gene family

The C proteins, C1 and C2, are major constituents of the heterogeneous nuclear RNA (hnRNA) ribonucleoprotein (hnRNP) complex in vertebrates. C1 and C2 are antigenically related phosphoproteins that are in contact with hnRNA in intact cells and bind to RNA tightly in vitro. A cDNA clone for the C proteins was isolated by immunological screening of a human lambda gt11 expression vector cDNA library with monoclonal antibodies. The lacZ-cDNA fusion protein is recognized by two different anti-C protein monoclonal antibodies. HeLa cell mRNA that was hybrid-selected with the cDNA clone (1.1 kilobases long) was translated in vitro and yielded both the C1 and C2 proteins (41 and 43 kDa, respectively). RNA blot analysis showed strong hybridization to two polyadenylylated transcripts, of about 1.4 kb and 1.9 kb, in human cells. Genomic DNA blot analysis showed multiple hybridizing restriction fragments in human and mouse, and homologous DNA sequences are found across eukaryotes from human to yeast. These findings suggest that the sequences encoding the hnRNP C proteins are members of a conserved gene family and they open the way for detailed molecular and genetic studies of these proteins.

Heterogeneous nuclear ribonucleoproteins: role in RNA splicing

Splicing in vitro of a messenger RNA (mRNA) precursor (pre-mRNA) is inhibited by a monoclonal antibody to the C proteins (anti-C) of the heterogeneous nuclear RNA (hnRNA)-ribonucleoprotein (hnRNP) particles. This antibody, 4F4, inhibits an early step of the reaction: cleavage at the 3' end of the upstream exon and the formation of the intron lariat. In contrast, boiled 4F4, or a different monoclonal antibody (designated 2B12) to the C proteins, or antibodies to other hnRNP proteins (120 and 68 kilodaltons) and nonimmune mouse antibodies have no inhibitory effect. The 4F4 antibody does not prevent the adenosine triphosphate-dependent formation of a 60S splicing complex (spliceosome). Furthermore, the 60S splicing complex contains C proteins, and it can be immunoprecipitated with 4F4. Depletion of C proteins from the splicing extract by immunoadsorption with either of the two monoclonal antibodies to the C proteins (4F4 or 2B12) results in the loss of splicing activity, whereas mock-depletion with nonimmune mouse antibodies bodies has no effect. A 60S splicing complex does not form in a C protein-depleted nuclear extract. These results indicate an essential role for proteins of the hnRNP complex in the splicing of mRNA precursors.

Interaction of mRNA with proteins in vesicular stomatitis virus-infected cells

The interaction of mRNA with proteins in vesicular stomatitis virus (VSV)-infected cells was studied by photochemical cross-linking in intact cells. The major [35S]methionine-labeled proteins which became cross-linked by UV light to mRNA in uninfected and in VSV-infected HeLa cells were similar and had apparent mobilities in sodium dodecyl sulfate-polyacrylamide gel electrophoresis corresponding to 135, 93, 72, 68, 53, 50, 43, and 36 kilodaltons. The proteins which were cross-linked in vivo specifically to the five mRNAs of VSV were labeled through radioactive nucleotides incorporated only into VSV mRNAs under conditions (5 micrograms of actinomycin D per ml) in which only VSV mRNAs are labeled. The same major mRNP proteins that became cross-linked to host mRNAs also became cross-linked to VSV mRNAs, although several quantitative differences were detected. Photochemical cross-linking and immunoblotting of cross-linked mRNPs with VSV antiserum demonstrated that in addition to host proteins VSV mRNAs also became cross-linked to the VSV-encoded N protein. The poly(A) segment of both host and VSV mRNAs was associated in vivo selectively with the 72-kilodalton polypeptide. The major proteins of mRNA-ribonucleoprotein complexes are therefore ubiquitous and common to different mRNAs. Furthermore, since the major messenger ribonucleoproteins interact also with VSV mRNAs even though these mRNAs are transcribed in the cytoplasm, it appears that nuclear transcription and nucleocytoplasmic transport are not necessary for mRNA to interact with these proteins.