Immunohistochemical localization of La nuclear antigen in brain. Selective concentration of the La protein in neuronal nucleoli

La involvement in tRNA and other RNA processing events including differences among yeast and other eukaryotes

The conserved nuclear RNA-binding factor known as La protein arose in an ancient eukaryote, phylogenetically associated with another eukaryotic hallmark, synthesis of tRNA by RNA polymerase III (RNAP III). Because 3'-oligo(U) is the sequence-specific signal for transcription termination by RNAP III as well as the high affinity binding site for La, the latter is linked to the intranuclear posttranscriptional processing of eukaryotic precursor-tRNAs. The pre-tRNA processing pathway must accommodate a variety of substrates that are destined for both common steps as well as tRNA-specific events. The order of intranuclear pre-tRNA processing steps is mediated in part by three activities derived from interaction with La protein: 3'-end protection from untimely decay by 3' exonucleases, nuclear retention and chaperone activity that helps prevent pre-tRNA misfolding and mischanneling into offline pathways. A focus of this perspective will be on differences between yeast and mammals in the subcellular partitioning of pre-tRNA intermediates and differential interactions with La. We review how this is most relevant to pre-tRNA splicing which occurs in the cytoplasm of yeasts but in nuclei of higher eukaryotes. Also divergent is La architecture, comprised of three RNA-binding domains in organisms in all examined branches of the eukaryal tree except yeast, which have lost the C-terminal RNA recognition motif-2α (RRM2α) domain. We also review emerging data that suggest mammalian La interacts with nuclear pre-tRNA splicing intermediates and may impact this branch of the tRNA maturation pathway. Finally, because La is involved in intranuclear tRNA biogenesis we review relevant aspects of tRNA-associated neurodegenerative diseases. This article is part of a Special Issue entitled: SI: Regulation of tRNA synthesis and modification in physiological conditions and disease edited by Dr. Boguta Magdalena.

All small nuclear RNAs (snRNAs) of the [U4/U6.U5] Tri-snRNP localize to nucleoli; Identification of the nucleolar localization element of U6 snRNA

Previously, we showed that spliceosomal U6 small nuclear RNA (snRNA) transiently passes through the nucleolus. Herein, we report that all individual snRNAs of the [U4/U6.U5] tri-snRNP localize to nucleoli, demonstrated by fluorescence microscopy of nucleolar preparations after injection of fluorescein-labeled snRNA into Xenopus oocyte nuclei. Nucleolar localization of U6 is independent from [U4/U6] snRNP formation since sites of direct interaction of U6 snRNA with U4 snRNA are not nucleolar localization elements. Among all regions in U6, the only one required for nucleolar localization is its 3' end, which associates with the La protein and subsequently during maturation of U6 is bound by Lsm proteins. This 3'-nucleolar localization element of U6 is both essential and sufficient for nucleolar localization and also required for localization to Cajal bodies. Conversion of the 3' hydroxyl of U6 snRNA to a 3' phosphate prevents association with the La protein but does not affect U6 localization to nucleoli or Cajal bodies.

The La protein

Ubiquitous in eukaryotic cells, the La protein associates with the 3' termini of many newly synthesized small RNAs. RNAs bound by the La protein include all nascent transcripts made by RNA polymerase III as well as certain small RNAs synthesized by other RNA polymerases. Recent genetic and biochemical analyses have revealed that binding by the La protein protects the 3' ends of these RNAs from exonucleases. This La-mediated stabilization is required for the normal pathway of pre-tRNA maturation, facilitates assembly of small RNAs into functional RNA-protein complexes, and contributes to nuclear retention of certain small RNAs. Studies of mutant La proteins have given some insights into how the La protein specifically recognizes its RNA targets. However, many questions remain regarding the molecular mechanisms by which La protein binding influences multiple steps in small RNA biogenesis. This review focuses on the roles of the La protein in small RNA biogenesis and also discusses data that implicate the La protein in the translation of specific mRNAs.

Detection of immunoglobulin G antibodies in melanoma sera reactive with intracellular proteins

Immunoglobulin G (IgG) antibodies reactive with intracellular components of transformed cells were detected in 26/35 sera from patients with melanoma using immunofluorescence and/or Western blotting. By extracting cellular proteins with either sodium dodecyl sulphate or moderate concentrations of salt (400 mM NaCl), the protein antigens were partially characterized by immunoblotting procedures. Although considerable heterogeneity in the molecular weights of the protein antigens was observed, two common groups were delineated. The anti-Pol antibodies reacted with 30 kd cytoplasmic protein and the anti-Ca antibodies recognized acidic high molecular weight (75-95 kd) proteins. These antigens were detected in all transformed cell lines tested, but were not restricted to them. Anti-Ca and anti-Pol antibodies were not found in sera from patients with other solid tumors or in systemic lupus erythematosus.

SS-B (La) nuclear antigen. Organization in structural domains of the protein moiety

Stable degradation products, obtained by digestion with endogenous and V8 proteases of calf thymus SS-B (La) antigenic protein, have been studied. The most characteristic fragments have molecular masses of 47, 30, 23 and 17 kDa. The 47-kDa and 30-kDa fragments are complex and are constituted of a number of species of different isoelectric points, as has been described for the SS-B protein molecule from other sources. Degradation products from the entire SS-B nuclear antigen still contain the 30-kDa SS-B fragment, suggesting that the 30-kDa region of the SS-B protein molecule is firmly attached to the RNA moiety. A model is presented that implies the presence of two hinge regions sensitive to proteases and three structural domains that correspond to segments of 30 kDa, 17 kDa and 5-6 kDa.

Autoantibodies in the cerebrospinal fluid of patients with systemic lupus erythematosus

Autoantibodies may play an important role in the pathogenesis of central nervous system (CNS) disease in systemic lupus erythematosus (SLE). We obtained cerebrospinal fluid (CSF) and, in some cases, sera from 19 SLE patients with CNS lupus and from 12 SLE patients without CNS lupus. Autoantibodies to saline soluble cellular antigens were detected in the CSF of lupus patients and reflected those present in the serum. These antibodies were distinct from the previously described antineuronal antibodies. Analysis of the fine specificities of the anti-saline soluble cellular antigen antibodies revealed that the antiribosomal P protein antibody was present in 4 of 4 patients with lupus psychosis and was enriched in the CSF of 1 patient. Sera containing antiribosomal P protein showed prominent cytoplasmic staining of human cortical neurons, as well as an epithelial cell substrate. These observations, together with the increase in intrathecal IgG synthesis detected in 71% of patients tested, suggest that several populations of antibodies may contribute to the enhanced immunologic activity in the CSF of CNS lupus patients.