Structure and function of La and Ro RNPs

The NZB/W F1 mouse model for Sjögren's syndrome: A historical perspective and lessons learned

Sjögren's syndrome (SS) is a chronic rheumatic autoimmune disorder affecting multiple organ systems. The clinical findings in SS patients show considerable heterogeneity and overlap with other autoimmune diseases. In addition, the autoimmune response in SS initiates several years before the appearance of clinical symptoms. Thus, understanding the pathogenic mechanisms involved in the disease process have been a challenge. Several animal model systems of SS-like disease have been developed to overcome these issues. The New Zealand Black (NZB) x New Zealand White (NZW) F1 (NZB/W F1) mouse represents the first spontaneous mouse model of SS. In this review, we provide a historical perspective and detailed description of this mouse model focusing on exocrine gland histopathology, autoantibody populations, and glandular dysfunction. Considering that NZB/W F1 mice also develop a systemic lupus erythematosus (SLE)-like disease, this mouse model mimics the clinical presentation of polyautoimmunity seen in a sizable subset of SS patients. It is plausible that such patients will require distinct therapeutic interventions necessary to treat both SLE and SS. Therefore, the NZB/W F1 mouse is a powerful tool to decipher pathogenic mechanisms involved in SS related polyautoimmunity and develop appropriate therapeutic strategies.

Emerging players in the initiation of eukaryotic DNA replication

Faithful duplication of the genome in eukaryotes requires ordered assembly of a multi-protein complex called the pre-replicative complex (pre-RC) prior to S phase; transition to the pre-initiation complex (pre-IC) at the beginning of DNA replication; coordinated progression of the replisome during S phase; and well-controlled regulation of replication licensing to prevent re-replication. These events are achieved by the formation of distinct protein complexes that form in a cell cycle-dependent manner. Several components of the pre-RC and pre-IC are highly conserved across all examined eukaryotic species. Many of these proteins, in addition to their bona fide roles in DNA replication are also required for other cell cycle events including heterochromatin organization, chromosome segregation and centrosome biology. As the complexity of the genome increases dramatically from yeast to human, additional proteins have been identified in higher eukaryotes that dictate replication initiation, progression and licensing. In this review, we discuss the newly discovered components and their roles in cell cycle progression.

A fraction of the transcription factor TAF15 participates in interactions with a subset of the spliceosomal U1 snRNP complex

RNA/ssDNA-binding proteins comprise an emerging class of multifunctional proteins with an anticipated role in coupling transcription with RNA processing. We focused here on the highly related transcription factors of the TET sub-class: TLS/FUS, EWS and in particular the least studied member TAF15. An extensive array of immunoprecipitation studies on differentially extracted HeLa nuclei revealed the specific association of TAF15 with the spliceosomal U1 snRNP complex, as deduced by the co-precipitating U1 snRNA, U1-70K and Sm proteins. Additionally, application of anti-U1 RNP autoantibodies identified TAF15 in the immunoprecipitates. Minor fractions of nuclear TAF15 and U1 snRNP were involved in this association. Pull-down assays using recombinant TAF15 and U1 snRNP-specific proteins (U1-70K, U1A and U1C) provided in vitro evidence for a direct protein-protein interaction between TAF15 and U1C, which required the N-terminal domain of TAF15. The ability of TAF15 to directly contact RNA, most likely RNA pol II transcripts, was supported by in vivo UV cross-linking studies in the presence of α-amanitin. By all findings, the existence of a functionally discrete subset of U1 snRNP in association with TAF15 was suggested and provided further support for the involvement of U1 snRNP components in early steps of coordinated gene expression.

Ty3 requires yeast La homologous protein for wild-type frequencies of transposition

The Saccharomyces cerevisiae retrovirus-like element Ty3 inserts specifically into the initiation sites of genes transcribed by RNA polymerase III (pol III). A strain with a disruption of LHP1, which encodes the homologue of autoantigen La protein, was recovered in a screen for mutants defective in Ty3 transposition. Transposition into a target composed of divergent tRNA genes was decreased eightfold. In lhp1 mutants, Ty3 polyproteins were produced at wild-type levels, assembled into virus-like particles (VLPs) and processed efficiently. The amount of cDNA associated with these particles was about half the amount in a wild-type control at early times, but approached the wild-type level after 48 h of induction. Ty3 integration was examined at two genomic tRNA gene families and two plasmid-borne tRNA promoters. Integration was significantly decreased at one of the tRNA gene families, but was only slightly decreased at the second tRNA gene family. These findings suggest that Lhp1p contributes to Ty3 cDNA synthesis, but might also act at a target-specific step, such as integration.

Identification of human autoantigen La/SS-B as BC1/BC200 RNA-binding protein

Rodent BC1 RNA and primate BC200 RNA are small cytoplasmic non-messenger RNAs that are phylogenetically unrelated. Nevertheless, the two RNAs exhibit a large degree of parallelism. In addition to some sequence similarities in their 3' domains, they are prevalently expressed in a similar subset of neurons and belong to a small group of transcripts with a somatodendritic location. Both RNAs are complexed with proteins as ribonucleoprotein particles (RNPs). Their similarities may even extend to analogous functional roles, for example, in the regulation of decentralized dendritic translation. To shed further light on the physiological role(s) of the BC1/BC200 RNPs, we began to analyze protein components that specifically bind to these RNAs. Ultraviolet-crosslinking experiments and affinity purification techniques revealed that the human autoantigen La/SS-B is associated with BC1/BC200 RNA in vitro and in vivo. As with other RNA polymerase III transcripts, La protein binds with high affinity to the 3' end of BC200 RNA. Our results suggest that an additional function of La may be control of dendritic translation by providing a link between the 5' Alu domain of BC200 RNP and the ribosome via the La protein dimer. The fact that La binds both BC1 and BC200 RNAs further supports the notion that the RNAs are functional analogs despite the fact that they arose from two separate retroposition events in two different mammalian lineages.

The human autoantigen La/SS-B accelerates herpes simplex virus type 1 replication in transfected mouse 3T3 cells

Permanently transfected mouse cell lines which expressed different levels of the human autoantigen La/SS-B were infected with different strains of herpes simplex virus type 1, including the strains ANG, HSZP, 17syn+ and HFEM. During infection the localization of the human La protein was followed using an anti-La MoAb, which recognized only the human La protein but did not cross-react with either the endogenous mouse La protein or any viral encoded protein. After infection La protein was transported from the nucleus to the cytoplasm. The time course of translocation was dependent on the amount of human La protein expressed in the respective cell line. Moreover, acceleration of viral replication was dependent on the level of expression of human La protein, suggesting that La protein is a cellular factor that facilitates virus replication.

Analysis of expression of the gene encoding for the nuclear autoantigen La/SS-B using reporter gene constructs

In earlier studies mRNA isoforms encoding for the nuclear autoantigen La were identified. In an alternative La mRNA form the exon 1 was replaced with the exon 1'. Moreover, exon 1' La mRNAs were found to start at different 5'-regions. In dependence on the 5'-start the exon 1' La mRNAs encoded for up to three open reading frames upstream of the La frame, which starts in the exon 2. The exon 1' was located in the intron about 70 nts downstream of the exon 1. The exon 1' La mRNA was proposed to be the result of a promoter switch in combination with an alternative splicing mechanism. The commonly used technique to study the expression of a eucaryotic gene is to fuse a reportergene immediately downstream of the proposed regulatory elements. Due to (i) the short distance between exon 1 and exon 1', (ii) the varying 5'-starts of the exon 1' La mRNAs, and (iii) the upstream open reading frames in the exon 1' La mRNAs this technique appeared to be difficult to apply to the La gene. In order to overcome these problems a luciferase reportergene construct was cloned which started about 2500 nts upstream of the exon 1 and contained the exon 1, the intron including the exon 1', and a portion of the exon 2. Luciferase was fused into the exon 2. This construct was used to prepare 5'-deletion mutants. The constructs were transiently transfected into HeLa cells. RNAs were isolated from the transiently transfected cells and analyzed using the 5'-Rapid Amplification of cDNA End technique. The PCR products were subcloned and sequenced. This analysis showed that exon 1 and exon 1' transcripts were correctly transcribed and spliced from the La luciferase fusion construct. Moreover, the 5'-start of the respective transcript allowed to identify those genomic regions in the La gene that were most likely being involved in determining the respective transcription initiation site. In parallel to the estimation of the 5'-start of the transcripts, the luciferase activity was measured. Thereby we detected a cryptic promoter element in the intron between the exon 1 and exon 2.

Transfection analysis of expression of mRNA isoforms encoding the nuclear autoantigen La/SS-B

Transcription of the gene encoding for the nuclear autoantigen La resulted in La mRNA isoforms. A promoter switching combined with an alternative splicing pathway replaced the exon 1 with the exon 1'. The exon 1' contained GC-rich regions and an oligo(U) tail of 23 uridine residues. Moreover, it encoded for three open reading frames upstream of the La protein reading frame. Despite this unusual structure, when exon 1' La mRNAs were expressed in transfected cells, both exon 1 and 1' La mRNAs were translated to La protein, whereas the upstream open reading frames of the exon 1' were not translated. In addition to full-length exon 1' La mRNAs 5'-shortened exon 1' La mRNAs were detected. The exon 1' 5'-starts varied in dependence on the analyzed tissues. Like the full-length exon 1' La mRNA a 5'-shortened exon 1' construct starting downstream of the oligo(U) tail but upstream of the open reading frames 2 and 3 was also well translated when transfected in mouse cells. Thus all La mRNA forms represent functional La mRNAs.

The nuclear autoantigen La/SS-associated antigen B: one gene, three functional mRNAs

Transcription of the gene encoding for the nuclear autoantigen La resulted in three mRNA forms. A promoter switching combined with an alternative splicing pathway replaced exon 1 with either exon 1' or exon 1'. The exon 1' donor splice site was located 4 nts downstream of the exon 1' donor splice site. All three La mRNA forms were expressed in all the tissues analysed including peripheral blood lymphocytes, liver, fetal spleen, cultured primary endothelial cells, and mouse LTA cell lines permanently transfected with the human La gene. Both the exons 1' and 1' had unusual structures. They contained GC-rich regions and an oligo(U)-tail of 23 uridine residues. Moreover, they encoded for three open reading frames upstream of the La protein reading frame. In spite of this unusual structure, when exon 1' or exon 1' La mRNAs were expressed in transfected mouse LTA cells, both La mRNAs were translated to nuclear La protein, indicating that all La mRNA forms are functional mRNAs.

Characterization of the autoantigen La (SS-B) as a dsRNA unwinding enzyme

During the analysis of the La (SS-B) autoantigen for catalytic activities an ATP-dependent double-stranded RNA unwinding activity was detected. Both native and recombinant La proteins from different species displayed this activity, which could be inhibited by monospecific anti-La antibodies. La protein was able to melt dsRNA substrates with either two 3'-overhangs or a single 3'- and a 5'-overhang. Double-stranded RNAs with two 5'-overhangs were not unwound, indicating that at least one 3'-overhang is required for unwinding. Sequence elements of the La protein that might be involved in dsRNA unwinding, such as an evolutionarily conserved putative ATP-binding motif and an element that is homologous to the double-stranded RNA binding protein kinase PKR, are discussed.

Antibodies to EYRKK vesicular stomatitis virus-related peptide account only for a minority of anti-Ro60kD antibodies

Previous studies demonstrated a possible antigenic relation between the carboxyl terminal portion of anti-Ro60kD autoantigen and a nucleocapsid protein (N) of vesicular stomatitis virus (VSV). In order to investigate whether anti-Ro60kD autoantibodies react with the VSV homologous region of the Ro60kD protein we synthesized, according to Merrifield's method, the EYRKKMDI octapeptide (8p) sharing a common sequence with the N protein of VSV. Sera from 61 patients with autoimmune rheumatic diseases (34 systemic lupus erythematosus (SLE), 21 Sjörgren's syndrome (SS) and six rheumatoid arthritis (RA)) as well as 59 from normal blood donors were tested for the presence of anti-Ro60kD autoantibodies by ELISA and immunoblot (IB) and anti-8p antibodies by ELISA. Antibodies to 8p were found in 9/31 of anti-Ro60kD IB-positive sera, 5/30 of anti-Ro60kD-negative sera and 2/59 of normal control sera. The concordance between the anti-8p ELISA and the anti-Ro60kD IB was very poor (chi 2 = 0.71, P = 0.4) in contrast to the anti-Ro60kD ELISA and the anti-Ro IB (chi 2 = 27.6, P = 10(-7)). Subsequent affinity purification of the anti-8p antibodies from a strong positive anti-8p and anti-Ro60kD SLE serum yielded 95% depletion of the anti-8p activity and 37% reduction of the anti-Ro60kD activity. Inhibition assays with the affinity-purified anti-8p antibodies demonstrated that the octapeptide gave 94.5% inhibition of the anti-Ro60kD activity, while Ro60kD protein led to 42.3% inhibition of the anti-8p. Preincubation of the serum with the octapeptide produced 4% inhibition of anti-Ro60kD ELISA. These results indicate that the anti-8p antibodies account only for a minority of the anti-Ro60kD autoantibodies.

Developmental characterization of a Drosophila RNA-binding protein homologous to the human systemic lupus erythematosus-associated La/SS-B autoantigen

Patients with humoral autoimmune diseases such as systemic lupus erythematosus and Sjögren's syndrome contain antibodies in their sera directed against certain normal cellular components such as the La/SS-B autoantigen, an RNA-binding protein believed to function as a putative processor of RNA polymerase III precursor transcripts. We have identified cDNA clones from the fruit fly Drosophila melanogaster that encode a protein displaying significant sequence homology with human La/SS-B. The fly protein (which we refer to as D-La) contains a putative ribonucleoprotein 1 (RNP1) and RNP2 RNA-binding domain. D-La also possesses a leucine zipper motif, suggesting that it may interact with itself or other proteins. Using gel retardation analysis, we show that D-La can bind RNA; in addition, we demonstrate the first reported DNA-binding activity associated with a La protein. Northern (RNA) blot analysis revealed a single 1,600-nucleotide transcript expressed throughout embryonic, larval, pupal, and adult development. Surprisingly, whole-mount in situ hybridization experiments revealed that D-La transcripts are not present in all ovarian tissues. In addition, early expression throughout the embryo is followed by a restricted pattern of mesodermal expression that is later confined to the visceral mesoderm, gonads, gut, and salivary glands. These results suggest that D-La may play a more specialized role during fly development as opposed to a rather general role inferred by its homology to La proteins from other organisms.

Developmental characterization of a Drosophila RNA-binding protein homologous to the human systemic lupus erythematosus-associated La/SS-B autoantigen

Patients with humoral autoimmune diseases such as systemic lupus erythematosus and Sjögren's syndrome contain antibodies in their sera directed against certain normal cellular components such as the La/SS-B autoantigen, an RNA-binding protein believed to function as a putative processor of RNA polymerase III precursor transcripts. We have identified cDNA clones from the fruit fly Drosophila melanogaster that encode a protein displaying significant sequence homology with human La/SS-B. The fly protein (which we refer to as D-La) contains a putative ribonucleoprotein 1 (RNP1) and RNP2 RNA-binding domain. D-La also possesses a leucine zipper motif, suggesting that it may interact with itself or other proteins. Using gel retardation analysis, we show that D-La can bind RNA; in addition, we demonstrate the first reported DNA-binding activity associated with a La protein. Northern (RNA) blot analysis revealed a single 1,600-nucleotide transcript expressed throughout embryonic, larval, pupal, and adult development. Surprisingly, whole-mount in situ hybridization experiments revealed that D-La transcripts are not present in all ovarian tissues. In addition, early expression throughout the embryo is followed by a restricted pattern of mesodermal expression that is later confined to the visceral mesoderm, gonads, gut, and salivary glands. These results suggest that D-La may play a more specialized role during fly development as opposed to a rather general role inferred by its homology to La proteins from other organisms.

The La antigen inhibits the activation of the interferon-inducible protein kinase PKR by sequestering and unwinding double-stranded RNA

The La (SS-B) autoimmune antigen is an RNA-binding protein that is present in both nucleus and cytoplasm of eukaryotic cells. The spectrum of RNAs that interact with the La antigen includes species which also bind to the interferon-inducible protein kinase PKR. We have investigated whether the La antigen can regulate the activity of PKR and have observed that both the autophosphorylation of the protein kinase that accompanies its activation by dsRNA and the dsRNA-dependent phosphorylation of the alpha subunit of polypeptide chain initiation factor eIF-2 by PKR are inhibited in the presence of recombinant La antigen. This inhibition is partially relieved at higher concentrations of dsRNA. Once activated by dsRNA the protein kinase activity of PKR is insensitive to the La antigen. We have demonstrated by a filter binding assay that La is a dsRNA binding protein. Furthermore, when recombinant La is incubated with a 900 bp synthetic dsRNA or with naturally occurring reovirus dsRNA it converts these substrates to single-stranded forms. We conclude that the La antigen inhibits the dsRNA-dependent activation of PKR by binding and unwinding dsRNA and that it may therefore play a role in the regulation of this protein kinase in interferon-treated or virus-infected cells.

Common structural features of the Ro RNP associated hY1 and hY5 RNAs

The secondary structures of human hY1 and hY5 RNAs were determined using both chemical modification techniques and enzymatic structure probing. The results indicate that both for hY1 and for hY5 RNA the secondary structure largely corresponds to the structure predicted by sequence alignment and computerized energy-minimization. However, some important deviations were observed. In the case of hY1 RNA, two regions forming a predicted helix appeared to be single-stranded. Furthermore, the pyrimidine-rich region of hY1 RNA appeared to be very resistant to reagents under native conditions, although it was accessible to chemical reagents under semi-denaturing conditions. This may point to yet unidentified tertiary interactions for this region of hY1 RNA. In the case of hY5 RNA, two neighbouring internal loops in the predicted structure appeared to form one large internal loop.

Analysis of the intracellular localization and assembly of Ro ribonucleoprotein particles by microinjection into Xenopus laevis oocytes

Xenopus laevis oocytes have been used to determine the intracellular localization of components of Ro ribonucleoprotein particles (Ro RNPs) and to study the assembly of these RNA-protein complexes. Microinjection of the protein components of human Ro RNPs, i.e., La, Ro60, and Ro52, in X. laevis oocytes showed that all three proteins are able to enter the nucleus, albeit with different efficiencies. In contrast, the RNA components of human Ro RNPs (the Y RNAs) accumulate in the X. laevis cytoplasm upon injection. Localization studies performed at low temperatures indicated that both nuclear import of Ro RNP proteins and nuclear export of Y RNAs are mediated by active transport mechanisms. Immunoprecipitation experiments using monospecific anti-La and anti-Ro60 antibodies showed that the X. laevis La and Ro60 homologues were cross-reactive with the respective antibodies and that both X. laevis proteins were able to interact with human Y1 RNA. Further analyses indicated that: (a) association of X. laevis La and Ro60 with Y RNAs most likely takes place in the nucleus; (b) once formed, Ro RNPs are rapidly exported out of the nucleus; and (c) the association with La is lost during or shortly after nuclear export.

Ro RNP associated Y RNAs are highly conserved among mammals

Y RNAs are small cytoplasmic RNAs which are components of the Ro ribonucleoprotein complexes in higher eukaryotes. These complexes are frequently recognized by antibodies present in autoimmune sera. In this study we analysed the occurrence of Y RNAs in various mammalian and human cell lines and erythrocytes by means of hybridization with human Y RNA probes. Y RNAs homologous to their human counterparts, both in length and in sequence, were detected in all mammalian cells analysed. While hY1 and hY3 analogues were found in all cells, Y4 and Y5 RNA could not be detected in rodent cells. In addition, Y5 RNA was absent from bovine cells. Attempts to determine the sequence of rat Y RNAs by genomic cloning resulted in the isolation of a presumptive Y1 RNA pseudogene. Analysis of the hY RNA content of various human cell lines showed that all four human Y RNAs were present in all cell lines examined. However, the relative levels to which these RNAs were expressed showed marked differences.

A herpes simplex virus type 1 immediate-early gene product, IE63, regulates small nuclear ribonucleoprotein distribution

Herpes simplex virus 1 (HSV-1), a nuclear replicating DNA virus, has 73 identified genes of which only 4 contain introns. For this reason the virus probably makes only minimal use of the cellular RNA-splicing machinery. Antigens associated with the small nuclear ribonucleoprotein particles (snRNPs) that are subunits of splicing complexes have been reported to redistribute in the nucleus and become concentrated into the intranuclear structures, the interchromatin granules, after HSV-1 infection [Martin, T. E., Barghusen, S. C., Leser, G. P. & Spear, P. G. (1987) J. Cell Biol. 105, 2069-2082]. We observe this snRNP redistribution upon HSV-1 infection, in which the widespread snRNP staining pattern changes to a restricted punctate distribution with a concomitant loss of coiled bodies in HSV-1-infected cells. We show here that expression of the immediate-early (IE) subset of HSV-1 genes is necessary and sufficient for snRNP redistribution. Using a series of HSV-1 mutants in different IE genes, we have established that specifically the product of the viral IE63 (ICP27) gene is essential for this effect, and transfection experiments revealed that IE63 expression alone can cause the snRNP redistribution. Further, we show that the IE63 gene product colocalizes with the redistributed snRNP in the nucleus. The snRNP redistribution caused by HSV-1 infection resembles the effect seen after inhibition of transcription in uninfected cells. In HSV-1-infected cells, however, the snRNP redistribution is under the control of viral IE gene products and occurs during active virus gene transcription.

Ro ribonucleoprotein assembly in vitro. Identification of RNA-protein and protein-protein interactions

The human Y RNAs, small RNAs with an unknown function, are complexed with at least three proteins: the 60,000 M(r) Ro protein (Ro60), the 52,000 M(r) Ro protein (Ro52) and the La protein (La). In this study we examined the intermolecular interactions between the components of these so-called Ro ribonucleoprotein (Ro RNP) complexes. Incubation of 32P-labelled hY1 RNA in HeLa S100 extract allows the reconstitution of Ro RNP complexes, which were analysed by immunoprecipitation with monospecific antisera. By immunodepletion of HeLa S100 extracts for either Ro60, Ro52 or La, followed by supplementation with recombinant Ro60 or La, it was demonstrated that both Ro60 and La bind to hY1 RNA directly without being influenced by one of the other proteins. However, binding of Ro52 to hY1 RNA required the presence of Ro60, which strongly suggests that the association of Ro52 with Ro RNPs is mediated by protein-protein interactions between Ro60 and Ro52.

Refined definition of the 56K and other autoantigens in the 50-60 kDa region

Alteration of the acrylamide:bisacrylamide ratio in the SDS-polyacrylamide gel used for Western blotting strongly improved the unambiguous detection of antibodies against 50-60 kDa autoantigens present in autoimmune patient sera. The relative migration of Ro52, the 56K autoantigen and calreticulin increased with reduced acrylamide:bisacrylamide ratios in contrast to that of Ro60, La and Jo-1. These analyses indicated that these six autoantigens correspond to six distinct polypeptides. Further analyses using recombinant calreticulin showed that (i) the 56K autoantigen is neither identical nor related to calreticulin and (ii) calreticulin is not a Ro autoantigen. A series of experiments designed to better characterize the 56K autoantigen showed that (i) the antigen is not detectable in fixed cells, presumably due to masking of the epitopes; (ii) about equal amounts of the antigen were recovered in nuclear and cytoplasmic cell fractions after enucleation of the cells; (iii) the 56K autoantigen is not stably associated with either RNA or other proteins.

Analysis of protein--RNA interactions within Ro ribonucleoprotein complexes

The interactions between Ro and La proteins and hY RNAs have been analysed. The binding site for the 60 kDa Ro protein on hY RNAs is shown to be the terminal part of the base paired stem structure, which contains the most highly conserved sequence among hY RNAs. The bulged C-residue within this region plays an important role in the recognition by this protein. The same regions of hY RNAs are essential for the association of the 52 kDa Ro protein with the RNAs, strongly suggesting that the 60 kDa Ro protein is required for the 52 kDa Ro protein to bind, presumably via protein-protein interactions, to Ro RNPs. The binding site for the La protein on hY RNAs is shown to be the oligouridylate stretch near the 3'-end of the RNAs, which is also recognized when additional nucleotides flank this motif at the 3'-side. Additional sequence elements in hY3 and hY5, but not in hY1, are bound by the La protein as well. Deletion mutagenesis showed that the RNP motif, previously identified in many ribonucleoprotein (RNP) proteins and in some cases shown to be almost sufficient for the interaction with RNA, of both the 60 kDa Ro and the La protein are not sufficient for the interaction with hY RNAs. Substantial parts of these proteins flanking the RNP motif are needed as well. It is likely that they stabilize the correct conformation of the RNP motif for RNA binding.

Detection and occurrence of the 60- and 52-kD Ro (SS-A) antigens and of autoantibodies against these proteins

The simultaneous detection of anti-La, anti-60-kD Ro and anti-52-kD Ro antibodies by immunoblotting is greatly improved by changing the crosslinking level in the gel to an acrylamide/bisacrylamide ratio of 19:1. Using this method for the analysis of a number of systemic lupus erythematosus (SLE) and Sjögren's syndrome patient sera it was observed that antibody to the 52-kD Ro protein without anti-60-kD Ro antibody was restricted to Sjögren's syndrome patients (9/26), whereas antibody to the 60-kD Ro protein without contaminating anti-52-kD Ro antibody was only found in SLE patients (8/38). Moreover, in Sjögren's syndrome patient sera anti-Ro antibody was found only in combination with anti-La antibody (20/26), whereas in SLE patient sera anti-Ro antibody could be found without detectable anti-La specificity (4/38). Double immunofluorescence microscopy revealed that the 52-kD Ro and the 60-kD Ro proteins co-localize in the cytoplasm as well as in the nucleus, whereas immunoprecipitation of [32P]-labelled HeLa cell extract with monospecific anti-52-kD Ro and anti-60-kD Ro sera showed that both proteins are associated with the Ro RNAs. These data suggest the presence of both the 52-kD and the 60-kD Ro proteins in the same ribonucleoprotein complexes. To study the evolutionary conservation of the 52-kD Ro, the 60-kD Ro and the La proteins, extracts of cell lines derived from various mammalian species were analysed on Western blots using monospecific human antibodies. In contrast to the 60-kD Ro and the La antigens which are well conserved in evolution, the 52-kD Ro antigen could be detected in primate cells only by this immunological approach.

A 60-kDa protein from rabbit reticulocytes specifically recognizes the capped 5' end of beta-globin mRNA

The binding of proteins from rabbit reticulocyte lysate to in-vitro-generated beta-globin mRNA and its defined segments was investigated using ultraviolet-cross-linking experiments as well as gel-retardation assays. Under stringent conditions, only three proteins (72, 60 and 50 kDa) were found associated with full-length beta-globin mRNA at different positions. The 72-kDa protein is most likely the poly(A)-binding protein and binds, as expected, to the poly(A) tail, whereas the 50-kDa protein exhibits affinity for the trailer region of beta-globin mRNA. The binding region of the 60-kDa protein is located at the 5' end of beta-globin mRNA. The interaction of this protein is dependent on the presence of the 5' cap structure, as indicated by competition experiments using an uncapped beta-globin-mRNA leader segment. Further competition experiments with beta-globin mRNA, deleted in part in the leader region, suggest that, besides the cap structure, certain sequence elements are necessary for the interaction of the 60-kDa protein and the beta-globin mRNA leader.

The consensus workshops for the detection of autoantibodies to intracellular antigens in rheumatic diseases

In 1988 and in 1989 consensus workshops were organized in order to define the interlaboratory concordance in detecting autoantibody specificities in selected sera from patients with rheumatoid disorders and to determine the possible causes of discrepancies. In total 20 sera were tested for the presence of antibodies against nRNP, Sm, Ro (SS-A), La (SS-B), Scl-70, centromeric antigens, ribosomal RNP and Jo-1. The methods used for detection by the 28 European laboratories who participated included immunofluorescence, counter-immunoelectrophoresis, immunodiffusion, immunoblotting and ELISA. The results showed that only a combination of two or more techniques was able to detect all specificities with an adequate efficiency. Recommendations to improve the efficiency of autoantibody detection and to standardize laboratory protocols are given.

The use of adenovirus-infected HeLa cells for the detection of low titer autoantibodies

Following infection of HeLa cells with adenovirus type 5 the cellular La protein becomes predominantly associated with the virally encoded RNA polymerase III products VAI, and VAII, while most of the host RNA polymerase II (e.g. U1, U2, U4, U5 and mRNA) and RNA polymerase III transcription (e.g. U6 and pre-tRNAs) ceases. Other RNA polymerase III products such as the cellular Ro RNAs continue to be transcribed and assembled into ribonucleoprotein complexes containing the Ro (SS-A) antigens. Using a 32P-pulse chase-labeled, adenovirus-infected HeLa cellular extract as a source of antigen, anti-La (SS-B) and anti-Ro (SS-A) antibodies can be detected simultaneously using an immunoprecipitation assay. In the present study this method was found to be more sensitive in detecting anti-La antibodies then counter immunoelectrophoresis and immunoblotting. In studies of sera from patients suffering from rheumatic diseases the percentage positive for anti-La antibody was significantly elevated using this method, especially in patients with systemic lupus erythematosus.

Binding of Epstein-Barr virus small RNA EBER-1 to the double-stranded RNA-activated protein kinase DAI

Epstein-Barr virus encodes two small RNAs, EBER-1 and -2, that are abundantly expressed in latently infected cells. Recent evidence suggests a role for EBER-1 in regulation of translation since this RNA is able to prevent the inhibition of protein synthesis by double-stranded RNA in rabbit reticulocyte lysates. We show here that EBER-1 that has been synthesized in vitro forms a complex with the dsRNA-activated inhibitor of protein synthesis DAI, a protein kinase that specifically phosphorylates polypeptide chain initiation factor eIF-2. Gel retardation assays and UV crosslinking experiments indicate that complex formation is specific for EBER-1 and requires the presence of some secondary structure in the molecule. RNA competition studies show that EBER-1-DAI complex formation is not inhibited in the presence of other small RNA species, heparin or the synthetic double-stranded RNA, poly(I).poly(C). SDS gel analysis reveals the existence of two forms of the crosslinked complex, of 64-68kDa and 46-53kDa, both of which are recognized by anti-DAI antibodies in immunoprecipitation experiments. These data suggest that EBER-1 regulates protein synthesis through its ability to interact with DAI.

Anti-(U1) small nuclear RNA antibodies in anti-small nuclear ribonucleoprotein sera from patients with connective tissue diseases

Small nuclear ribonucleoprotein (snRNP) particles are a class of RNA-containing particles in the nucleus of eukaryotic cells. Sera from patients with connective tissue diseases often contain antibodies against the proteins present in these snRNPs. Antibodies against the RNA components of snRNPs, the U snRNAs, are thought to be rare. We tested 118 anti-snRNP sera for the presence of anti-snRNA antibodies and found them in 45 sera (38%). In all sera the antibodies (IgG and F(ab)2 fragments thereof) were exclusively directed against U1 snRNA. The anti-(U1) RNA antibodies were always accompanied by anti-(U1)RNP antibodies but were not found in sera which contain antibodies of the Sm serotype directed against all nucleoplasmic U snRNP particles. Like anti-RNP antibodies, anti-U1 RNA activity is confined to sera from patients with SLE or SLE overlap syndromes and is rarely found in patients with other connective tissue diseases. By analyzing binding to subfragments of U1 snRNA made in vitro, it was demonstrated that anti-(U1)RNA antibodies recognize epitopes distributed throughout the U1 RNA molecule. In most sera, however, either the second or the fourth hairpin loop is the main target of the antibody. The possible mechanisms that could lead to the production of this new type of autoantibody are discussed.