Antibodies against small nuclear ribonucleoproteins immunoprecipitate complexes containing ts110 Moloney murine sarcoma virus genomic and messenger RNAs

La autoantigen specifically recognizes a predicted stem-loop in hepatitis B virus RNA

We recently identified three nuclear proteins (p45, p39, and p26) that bind to a 91-nucleotide (nt) RNA element between nt 1243 and 1333 in hepatitis B virus (HBV) RNA, and we showed that these proteins and HBV RNA are regulated coordinately by gamma interferon and tumor necrosis factor alpha. Purification and sequence analysis of tryptic peptides obtained from p39 revealed sequence homology to the mouse La protein. Immunoprecipitation experiments showed that p45, p39, and p26 were recognized by anti-La-specific antiserum, indicating that p45 is the full-length La protein and that p39 and p26 are likely to be proteolytic La cleavage products. Furthermore, in competition experiments we found that all three La proteins bind, in a phosphorylation-dependent manner, to the same predicted stem-loop structure located between nt 1275 and 1291 of HBV, with Kds of approximately 1.0 nM. Collectively, these results support the notion that the La protein may contribute to HBV RNA stability, constitutively and in response to inflammatory cytokines.

Interaction of the La (SS-B) autoantigen with small ribosomal subunits

The La (SS-B) autoantigen is an evolutionarily conserved 47-kDa protein which binds to nascent RNA polymerase III transcripts and to a number of viral leader RNAs. The La protein plays a direct role in the termination of RNA polymerase III transcription and recent findings have suggested an additional role in several aspects of translation of (viral) messenger RNAs. Here, we show that La in the cytoplasm is associated with a subset of small ribosomal subunits, possibly by direct association with 18S ribosomal RNA. This association is likely to be related to the putative role of this protein in translation regulation.

Localization of antigens PwA33 and La on lampbrush chromosomes and on nucleoplasmic structures in the oocyte of the urodele Pleurodeles waltl: light and electron microscopic immunocytochemical studies

Monoclonal antibodies A33/22 and La11G7 have been used to study the distribution of the corresponding antigens, PwA33 and La, on the lampbrush chromosome loops and nucleoplasmic structures of P. waltl oocytes, using immunofluorescence, confocal laser scanning microscopy and immunogold labeling. The results obtained with these antibodies have been compared with those obtained with the Sm-antigen-specific monoclonal antibody Y12. All these monoclonal antibodies (mAbs) labeled the matrices of the majority of normal loops along their whole length. Nucleoplasmic RNP granules showed a strong staining with the mAbs La11G7 and Y12 throughout their mass, but with the mAb A33/22, they showed only a weak peripheral labeling in the form of patches on their surface. This patchy labeling was confirmed by confocal laser scanning microscopy. Electron microscopy revealed that this patchy labeling might be due to a hitherto undescribed type of submicroscopic granular structure, around 100 nm in either dimension, formed by 10-nm particles. Such granules were observed either attached to the RNP granules or free in the nucleoplasm, but rarely in relation with the normal loop matrices. These 100-nm granules may have a role in the movement of proteins and snRNPs inside the oocyte nuclei for storage, recycling, and/or degradation. Our results also suggest that all the microscopically visible free RNP granules of the nucleoplasm of P. waltl oocytes correspond to B snurposomes. The granules forming the B (globular) loops showed a labeling pattern similar to that of B snurposomes; their possible relationship is discussed.

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.

Regulation of RNA splicing in gag-deficient mutants of Moloney murine sarcoma virus MuSVts110

We investigated whether the MuSVts110 gag gene product (P58gag) can regulate the novel growth temperature dependence of MuSVts110 RNA splicing. MuSVts110 mutants with either frameshifts or deletions in the gag gene were tested for their ability to maintain the MuSVts110 splicing phenotype. Only small decreases in splicing efficiency and no changes in the thermosensitivity of viral RNA splicing were observed in MuSVts110 gag gene frameshift mutants. Deletions within the gag gene, however, variably decreased MuSVts110 splicing efficiency but had no effect on its thermosensitivity. Another class of MuSVts110 splicing mutants generated by treatment of MuSVts110-infected cells with NiCl2 was also examined. In these "nickel revertants," P58gag is made, but splicing of the viral transcript is nearly complete at all growth temperatures. The splicing of "tagged" viral RNA transcribed from a modified MuSVts110 DNA introduced into nickel revertant cells remained thermosensitive, arguing against trans effects of viral gene products on splicing efficiency. These experiments indicated that neither the MuSVts110 P58gag protein nor any other viral gene product acts in trans to regulate MuSVts110 splicing.

Characterization of the autoantigen La as a nucleic acid-dependent ATPase/dATPase with melting properties

The autoantigen La, a known transcription termination factor of RNA polymerase III, was purified to homogeneity from mouse 3T3 cells and calf thymus by different isolation procedures. The La protein from calf thymus was separated into RNA binding and nonbinding subclasses. The murine La protein and the RNA binding subclass of calf thymus La protein showed ATPase/dATPase activity in the presence of DNA-RNA or RNA-RNA hybrids. A novel monoclonal anti-La antibody (La11G7) and patients' anti-La antibodies immunoadsorbed to homogeneously purified La protein were able to inhibit the enzyme activity of La protein. La protein was able to melt a synthetic DNA-RNA hybrid in a reaction that required ATP hydrolysis. The RNA binding ability of the nonbinding subclass was restored by treatment with sialidase. This treatment also restored the protein's ATP-dependent melting activity.

Identification of La ribonucleoproteins as a component of interchromatin granules

Monoclonal antibodies raised against the La antigen were used to localize by preembedding immunoelectron microscopy, snRNPs containing this protein. The results demonstrate that La RNPs are localized in clusters of interchromatin granules, both in Triton X-100-extracted and DNase-digested nuclei. DNase-digested salt-extracted nuclei contained, in addition, labeled structures identified as perichromatin granules and fibers. A close association of labeled granules with the nucleoli was also observed. Digestion of nuclei with DNase yielded residual scaffolds of intermediate filaments and nuclear lamina devoid of interchromatin granules and La immunostaining. Release of the La antigen was tested in the presence of ATP and cytochalasin B. Only cytochalasin was effective suggesting a role for nuclear actin in anchorage of snRNPs.

The La antigen shuttles between the nucleus and the cytoplasm in CV-1 cells

Recently we established a monoclonal antibody against the La-protein (Bachmann et al., Proc. Natl. Acad. Sci. USA, 83, 7770, 1986). The antibody gives a nuclear speckled type staining and, in addition, a perinuclear cytoplasmic staining on cultured cells in immunofluorescence microscopy. After inhibition of RNA synthesis the La-protein is transported into the cytoplasm. After prolonged inhibition it returns into the nucleus forming large growing speckles. The transport into the nucleus apparently depends on glycosylation.

Exploitation of a thermosensitive splicing event to study pre-mRNA splicing in vivo

The spliced form of MuSVts110 viral RNA is approximately 20-fold more abundant at growth temperatures of 33 degrees C or lower than at 37 to 41 degrees C. This difference is due to changes in the efficiency of MuSVts110 RNA splicing rather than selective thermolability of the spliced species at 37 to 41 degrees C or general thermosensitivity of RNA splicing in MuSVts110-infected cells. Moreover, RNA transcribed from MuSVts110 DNA introduced into a variety of cell lines is spliced in a temperature-sensitive fashion, suggesting that the structure of the viral RNA controls the efficiency of the event. We exploited this novel splicing event to study the cleavage and ligation events during splicing in vivo. No spliced viral mRNA or splicing intermediates were observed in MuSVts110-infected cells (6m2 cells) at 39 degrees C. However, after a short (about 30-min) lag following a shift to 33 degrees C, viral pre-mRNA cleaved at the 5' splice site began to accumulate. Ligated exons were not detected until about 60 min following the initial detection of cleavage at the 5' splice site, suggesting that these two splicing reactions did not occur concurrently. Splicing of viral RNA in the MuSVts110 revertant 54-5A4, which lacks the sequence -AG/TGT- at the usual 3' splice site, was studied. Cleavage at the 5' splice site in the revertant viral RNA proceeded in a temperature-sensitive fashion. No novel cryptic 3' splice sites were activated; however, splicing at an alternate upstream 3' splice site used at low efficiency in normal MuSVts110 RNA was increased to a level close to that of 5'-splice-site cleavage in the revertant viral RNA. Increased splicing at this site in 54-5A4 viral RNA is probably driven by the unavailability of the usual 3' splice site for exon ligation. The thermosensitivity of this alternate splice event suggests that the sequences governing the thermodependence of MuSVts110 RNA splicing do not involve any particular 3' splice site or branch point sequence, but rather lie near the 5' end of the intron.

Exploitation of a thermosensitive splicing event to study pre-mRNA splicing in vivo

The spliced form of MuSVts110 viral RNA is approximately 20-fold more abundant at growth temperatures of 33 degrees C or lower than at 37 to 41 degrees C. This difference is due to changes in the efficiency of MuSVts110 RNA splicing rather than selective thermolability of the spliced species at 37 to 41 degrees C or general thermosensitivity of RNA splicing in MuSVts110-infected cells. Moreover, RNA transcribed from MuSVts110 DNA introduced into a variety of cell lines is spliced in a temperature-sensitive fashion, suggesting that the structure of the viral RNA controls the efficiency of the event. We exploited this novel splicing event to study the cleavage and ligation events during splicing in vivo. No spliced viral mRNA or splicing intermediates were observed in MuSVts110-infected cells (6m2 cells) at 39 degrees C. However, after a short (about 30-min) lag following a shift to 33 degrees C, viral pre-mRNA cleaved at the 5' splice site began to accumulate. Ligated exons were not detected until about 60 min following the initial detection of cleavage at the 5' splice site, suggesting that these two splicing reactions did not occur concurrently. Splicing of viral RNA in the MuSVts110 revertant 54-5A4, which lacks the sequence -AG/TGT- at the usual 3' splice site, was studied. Cleavage at the 5' splice site in the revertant viral RNA proceeded in a temperature-sensitive fashion. No novel cryptic 3' splice sites were activated; however, splicing at an alternate upstream 3' splice site used at low efficiency in normal MuSVts110 RNA was increased to a level close to that of 5'-splice-site cleavage in the revertant viral RNA. Increased splicing at this site in 54-5A4 viral RNA is probably driven by the unavailability of the usual 3' splice site for exon ligation. The thermosensitivity of this alternate splice event suggests that the sequences governing the thermodependence of MuSVts110 RNA splicing do not involve any particular 3' splice site or branch point sequence, but rather lie near the 5' end of the intron.