The Bacterial Ro60 Protein and Its Noncoding Y RNA Regulators

Ro60 ribonucleoproteins (RNPs), composed of the ring-shaped Ro 60-kDa (Ro60) protein and noncoding RNAs called Y RNAs, are present in all three domains of life. Ro60 was first described as an autoantigen in patients with rheumatic disease, and Ro60 orthologs have been identified in 3% to 5% of bacterial genomes, spanning the majority of phyla. Their functions have been characterized primarily in Deinococcus radiodurans, the first sequenced bacterium with a recognizable ortholog. In D. radiodurans, the Ro60 ortholog enhances the ability of 3'-to-5' exoribonucleases to degrade structured RNA during several forms of environmental stress. Y RNAs are regulators that inhibit or allow the interactions of Ro60 with other proteins and RNAs. Studies of Ro60 RNPs in other bacteria hint at additional functions, since the most conserved Y RNA contains a domain that is a close tRNA mimic and Ro60 RNPs are often encoded adjacent to components of RNA repair systems.

The association of a La module with the PABP-interacting motif PAM2 is a recurrent evolutionary process that led to the neofunctionalization of La-related proteins

La-related proteins (LARPs) are largely uncharacterized factors, well conserved throughout evolution. Recent reports on the function of human LARP4 and LARP6 suggest that these proteins fulfill key functions in mRNA metabolism and/or translation. We report here a detailed evolutionary history of the LARP4 and 6 families in eukaryotes. Genes coding for LARP4 and 6 were duplicated in the common ancestor of the vertebrate lineage, but one LARP6 gene was subsequently lost in the common ancestor of the eutherian lineage. The LARP6 gene was also independently duplicated several times in the vascular plant lineage. We observed that vertebrate LARP4 and plant LARP6 duplication events were correlated with the acquisition of a PABP-interacting motif 2 (PAM2) and with a significant reorganization of their RNA-binding modules. Using isothermal titration calorimetry (ITC) and immunoprecipitation methods, we show that the two plant PAM2-containing LARP6s (LARP6b and c) can, indeed, interact with the major plant poly(A)-binding protein (PAB2), while the third plant LARP6 (LARP6a) is unable to do so. We also analyzed the RNA-binding properties and the subcellular localizations of the two types of plant LARP6 proteins and found that they display nonredundant characteristics. As a whole, our results support a model in which the acquisition by LARP4 and LARP6 of a PAM2 allowed their targeting to mRNA 3' UTRs and led to their neofunctionalization.

Identification of protein domains required for makorin-2-mediated neurogenesis inhibition in Xenopus embryos

Makorin-2, consisting of four highly conserved C(3)H zinc fingers, a Cys-His motif and a C(3)HC(4) RING zinc finger domain, is a putative ribonucleoprotein. We have previously reported that Xenopus makorin-2 (mkrn2) is a neurogenesis inhibitor acting upstream of glycogen synthase kinase-3beta (GSK-3beta) in the phosphatidylinositol 3-kinase/Akt pathway. In an effort to identify the functional domains required for its anti-neurogenic activity, we designed and constructed a series of N- and C-terminal truncation mutants of mkrn2. Concurred with the full-length mkrn2, we showed that overexpression of one of the truncation mutants mkrn2(s)-7, which consists of only the third C(3)H zinc finger, Cys-His motif and C(3)HC(4) RING zinc finger, is essential and sufficient to produce the phenotypical dorso-posterior deficiencies and small-head/short-tail phenotype in tadpoles. In animal cap explant assay, we further demonstrated that mkrn2(s)-7 not only inhibits activin and retinoic acid-induced animal cap neuralization and the expression of a pan-neural marker neural cell adhesion molecule, but also induces GSK-3beta expression. These results collectively suggest that the third C(3)H zinc finger, Cys-His motif and C(3)HC(4) RING zinc finger are indispensable for the anti-neurogenic activity of mkrn2.

Sequence and transcriptional study of HNRPK pseudogenes, and expression and molecular modeling analysis of hnRNP K isoforms

The heterogeneous nuclear ribonucleoproteins (hnRNPs) comprise a large family of proteins that play important roles in telomere biogenesis, DNA repair, cellular signaling, and the regulation of expression at both the transcriptional and translational levels. One of the most extensively studied hnRNP family members, hnRNP K, has been implicated in a variety of processes, including chromatin remodeling, transcription, splicing, and translation events. In this study, we analyzed processed HNRPK pseudogenes (HNRPK psi1-psi4) and coding sequences. HNRPK pseudogenes are apparently nonfunctional, and psi1 might correspond to transcripts from an ancestral gene. Phylogenetic and sequence analyses suggest that HNRP genes arose by duplication, and that new structural and sequence features expanded the functions of hnRNPs. The expression analysis of hnRNP K isoforms showed that isoform a is expressed in normal testis and in non-small cell lung cancer (NCI-H1155 NSCLC cell line), although the shorter isoform (isoform b) is expressed in different tumor cell lines (IM9 B-lymphoblastoid, Hs578T human breast cancer epithelial, T98G human glioma cell lines). Using molecular modeling, we obtained KH1 and KH3 models, which pointed to important residues for DNA-protein binding and no structural differences between isoforms a and b. To our knowledge, this is the first phylogenetic study including vertebrate HNRP genes and HNRPK pseudogenes, and the first report comparing the KH1 and KH3 domains of isoforms a and b of the hnRNP K protein. New investigations in tumor samples must be done to validate the differential expression observed here. The results shown are important because the hnRNP K protein might represent a new target for pharmacologic intervention in virus replication and cancer.

Ro-associated Y RNAs in metazoans: evolution and diversification

The Y genes encode small noncoding RNAs whose functions remain elusive, whose numbers vary between species, and whose major property is to be bound by the Ro60 protein (or its ortholog in other species). To better understand the evolution of the Y gene family, we performed a homology search in 27 different genomes along with a structural search using Y RNA specific motifs. These searches confirmed that Y RNAs are well conserved in the animal kingdom and resulted in the detection of several new Y RNA genes, including the first Y RNAs in insects and a second Y RNA detected in Caenorhabditis elegans. Unexpectedly, Y5 genes were retrieved almost as frequently as Y1 and Y3 genes, and, consequently are not the result of a relatively recent apparition as is generally believed. Investigation of the organization of the Y genes demonstrated that the synteny was conserved among species. Interestingly, it revealed the presence of six putative "fossil" Y genes, all of which were Y4 and Y5 related. Sequence analysis led to inference of the ancestral sequences for all Y RNAs. In addition, the evolution of existing Y RNAs was deduced for many families, orders and classes. Moreover, a consensus sequence and secondary structure for each Y species was determined. Further evolutionary insight was obtained from the analysis of several thousand Y retropseudogenes among various species. Taken together, these results confirm the rich and diversified evolution history of Y RNAs.

Molecular characterization and phylogeny of U2AF35 homologs in plants

U2AF (U2 small nuclear ribonucleoprotein auxiliary factor) is an essential splicing factor with critical roles in recognition of the 3'-splice site. In animals, the U2AF small subunit (U2AF35) can bind to the 3'-AG intron border and promote U2 small nuclear RNP binding to the branch-point sequences of introns through interaction with the U2AF large subunit. Two copies of U2AF35-encoding genes were identified in Arabidopsis (Arabidopsis thaliana; atU2AF35a and atU2AF35b). Both are expressed in all tissues inspected, with atU2AF35a expressed at a higher level than atU2AF35b in most tissues. Differences in the expression patterns of atU2AF35a and atU2AF35b in roots were revealed by a promoter::beta-glucuronidase assay, with atU2AF35b expressed strongly in whole young roots and root tips and atU2AF35a limited to root vascular regions. Altered expression levels of atU2AF35a or atU2AF35b cause pleiotropic phenotypes (including flowering time, leaf morphology, and flower and silique shape). Novel slicing isoforms were generated from FCA pre-mRNA by splicing of noncanonical introns in plants with altered expression levels of atU2AF35. U2AF35 homologs were also identified from maize (Zea mays) and other plants with large-scale expressed sequence tag projects. A C-terminal motif (named SERE) is highly conserved in all seed plant protein homologs, suggesting it may have an important function specific to higher plants.

Phosphorylated and nonphosphorylated epitopes of the La/SSB autoantigen: comparison of their antigenic and conformational characteristics

La/SSB phosphoprotein is the target antigen of autoantibodies in sera of patients with Sjögren's syndrome (SS) and systemic lupus erythematosus (SLE). Among other structural and function motifs, four phosphorylation sites are encompassed in the primary sequence of La/SSB. Two of them (Thr-362 and Ser-366) are located within GSGKGKVQFQGKKTKFASDD (346-368) and one (Thr-302) within VTWEVLEGEVEKEALKKI (301-318), which are main B-cell epitopes of La/SSB. With the aim to investigate how phosphorylation, one of the most common posttranslational protein modifications, affects the antigenic and conformational characteristics of the La/SSB epitopes, we synthesized and studied the phosphorylated epitopes La/SSB(346-368)-P, La/SSB(359-368)-P, and La/SSB(301-318)-P with respect to their nonphosphorylated counterparts. Anti-La/SSB positive sera from SS and SLE patients are better recognized by the phosphorylated epitopes compared to their nonphosphorylated counterparts. Conformational analysis by (1)H nuclear magnetic resonance spectroscopy and molecular dynamics showed that the phosphorylated epitopes adopt different structural characteristics from those of the corresponding nonphosphorylated epitopes. It is concluded that phosphorylation can create neoepitopes with altered functions, compared to the nonphosphorylated epitopes, which might be seen from the immune system as "foreign."

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.

The 72/74-kDa polypeptides of the 70-110 S large heterogeneous nuclear ribonucleoprotein complex (LH-nRNP) represent a discrete subset of the hnRNP M protein family

Pre-mRNA processing in eukaryotes is thought to take place on a multitude of nuclear ribonucleoprotein (RNP) complexes, the most abundant of them being the heterogeneous nuclear (hn) RNP complexes. The identification in mammalian nuclear extracts of a novel, less-abundant 70-110 S heterogeneous RNP, named large heterogeneous nuclear RNP (LH-nRNP), has previously been reported by Aidinis, Sekeris and Guialis (1995) Nucleic Acids Res. 23, 2742-2753. The structural composition of the LH-nRNP complex has been determined following the production of polyclonal antibodies against the major protein constituents of the complex, the pair of the 72/74-kDa polypeptides. In the present study evidence is shown to prove that the 72/74-kDa proteins are members of the hnRNP M protein family, hereafter referred to as 72/74(M) polypeptides. The extensive application of two-dimensional gel electrophoresis, combined with specific immunoprecipitation and immunoblotting assays, has allowed the assignment of the 72/74(M) proteins to a subset of the hnRNP M family, characteristic of the presence of the LH-nRNP complex and distinct from the hnRNP-associated M1-M4 components. Moreover, the immunoselection of the LH-nRNP complex from [(32)P]orthophosphate-labelled HeLa cells, with the parallel application of UV irradiation, has permitted the identification of the 72/74(M) polypeptides as the sole protein constituents of the complex in direct contact with the RNA. It is proposed that LH-nRNP constitutes a discrete subset of hnRNP complexes, having a possible role in establishing specific interactions between hnRNP and nuclear-matrix protein components.

Protein-protein interaction among hnRNPs shuttling between nucleus and cytoplasm

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are involved in several RNA-related biological processes such as transcription, pre-mRNA processing, mature mRNA transport to the cytoplasm, and translation. About 20 major hnRNPs from A1 to U are known. Among them, hnRNP A, D, E, I, and K are known to shuttle between the nucleus and the cytoplasm. hnRNP E2 has been seen to stabilize alpha-globin mRNA and to enhance polioviral mRNA translation. hnRNP K modulates transcription and translation of some mRNAs. hnRNP I and its homologue hnRNP L have been suggested to enhance translation of some IRES-dependent mRNAs. In order to better understand the molecular mechanisms of the biological functions of hnRNPs, we investigated protein-protein interactions of six hnRNPs (hnRNP A1, C1, E2, I, K, and L) using the yeast two-hybrid system and in vitro co-precipitation assays. All of the hnRNPs tested exerted homomeric interactions, and hnRNP E2, I, K, and L interacted with each other. In the case of hnRNP E2 and hnRNP K, the N-terminal half of the proteins containing two KH (K homologous) domains were required for protein-protein interaction, and the second quarter of hnRNP I and hnRNP L containing RRM2 (RNA recognition motif 2) was essential for protein-protein interaction. hnRNP A1 and C1 did not form complexes with other hnRNPs in our assay systems. This suggests that the hnRNPs could fall into two groups: one group, including hnRNP A1 and C1, involved in hnRNP core complex formation and another group, including hnRNP E2, I, K, and L, involved in a variety of RNA-related biological processes. Different combinations of the proteins of the second group may facilitate different biological processes in conjunction with other factors.

Recognition of subsets of the mammalian A/B-type core heterogeneous nuclear ribonucleoprotein polypeptides by novel autoantibodies

The structurally related A/B-type core heterogeneous nuclear ribonucleoprotein (hnRNP) polypeptides of 34-39 kDa (A1, A2, B1 and B2) belong to a family of RNA-binding proteins that are major components of 40 S hnRNP complexes. By two-dimensional gel electrophoresis and peptide mapping analysis we compared each member of the A/B-type core proteins in the human and rat liver cells. This comparison revealed the unique presence in rat cells of major protein species, referred to as mBx polypeptides, that appeared as three charge isoforms at a position corresponding to the minor HeLa B1b protein spot. In addition, clear differences in the ratios of the A1 polypeptide to the A1b isoform were observed. The detection, in sera of patients with rheumatic autoimmune diseases, of two novel autoantibody specificities, one recognizing solely B2 protein and the second both the B2 and mBx polypeptides, helped to identify mBx proteins as new A/B-type hnRNP components, immunologically related to B2 protein. A common immunoreactive V8 protease peptide of approx. 17 kDa has been identified in B2 and mBx hnRNP polypeptides. mBx protein species are identified in cells of murine origin, and have a ubiquitous tissue distribution and developmental appearance.

Tyrosine phosphorylation of a M(r) 38,000 A/B-type hnRNP protein selectively modulates its RNA binding

The M(r) 38,000 RNA-binding protein (P38) is the major component of translationally repressed messenger ribonucleoproteins in cryptobiotic gastrulae of the brine shrimp Artemia. Partial elucidation of the amino acid sequence of P38 reveals that it is homologous to A/B-type hnRNP proteins. This was confirmed by immunodetection with antibodies specific for A/B-type hnRNP proteins from Drosophila melanogaster. P38 can be phosphorylated in vitro by a src-related protein tyrosine kinase on multiple tyrosine residues located predominantly in the glycine-rich domain. Tyrosine phosphorylated P38 can be efficiently dephosphorylated by a specific protein tyrosine phosphatase (1B-like) and by protein phosphatase 2A activated by the phosphotyrosyl phosphatase activator. Tyrosine phosphorylation of P38 slightly influences its subsequent phosphorylation by casein kinase II. The latter phosphorylation site is located in the glycine-rich domain of P38. Two-dimensional gel electrophoresis resolves P38 into multiple isoforms which shift to more acidic pI values after phosphorylation by protein tyrosine kinase or casein kinase II. From nitrocellulose filter binding and UV cross-linking analysis, evidence was obtained that tyrosine phosphorylation of P38 impairs its binding to poly(A) but not to poly(U). This demonstrates the involvement of tyrosine residues in polynucleotide-specific RNA binding that can be regulated by phosphorylation/dephosphorylation.

Function of conserved domains of hnRNP A1 and other hnRNP A/B proteins

hnRNP A1 is a pre-mRNA binding protein that antagonizes the alternative splicing activity of splicing factors SF2/ASF or SC35, causing activation of distal 5' splice sites. The structural requirements for hnRNP A1 function were determined by mutagenesis of recombinant human hnRNP A1. Two conserved Phe residues in the RNP-1 submotif of each of two RNA recognition motifs appear to be involved in specific RNA-protein interactions and are essential for modulating alternative splicing. These residues are not required for general pre-mRNA binding or RNA annealing activity. The C-terminal Gly-rich domain is necessary for alternative splicing activity, for stable RNA binding and for optimal RNA annealing activity. hnRNP A1B, which is an alternatively spliced isoform of hnRNP A1 with a longer Gly-rich domain, binds more strongly to pre-mRNA but has only limited alternative splicing activity. In contrast, hnRNP A2 and B1, which have 68% amino acid identity with hnRNP A1, bind more weakly to pre-mRNA and have stronger splice site switching activities than hnRNP A1. We propose that specific combinations of antagonistic hnRNP A/B and SR proteins are involved in regulating alternative splicing of distinct subsets of cellular premRNAs.

Interaction of cellular factors with intragenic cis-acting repressive sequences within the HIV genome

Expression of the human immunodeficiency virus (HIV) structural gene products is suppressed in the absence of the Rev protein. The block to expression reflects, in part, nuclear retention of those mRNAs which encode the structural proteins. The presence of intragenic cis-acting repressive sequences (CRS) and inefficient splicing of the primary viral transcript are thought to contribute to nuclear entrapment of viral RNA. To elucidate the mechanism for repression of HIV gene expression, the ability of a 270-bp segment of the pol gene shown previously to repress gene expression to interact with cellular factors was investigated. Incubation of RNA corresponding to the 270-bp CRS element with nuclear extract prepared from human T-cells revealed a strong and specific interaction with several cellular factors. Covalent cross-linking of the RNA-protein complex demonstrated the presence of at least three proteins, the predominant one having a molecular weight of approximately 42 kDa. A monoclonal antibody raised against hnRNP C, a component of the splicing machinery, recognized the CRS-protein complex, suggesting that hnRNP C or a closely related gene product interacts with CRS-containing RNA. Consistent with this conclusion, addition of RNA corresponding to a beta-globin intron sequence in the binding reaction completely blocked formation of the CRS-protein complex. These findings raise the possibility that the CRS elements elicit nuclear entrapment of viral RNA through formation of RNA-protein complexes that are not accessible to nuclear export pathways.

Diversity of a ribonucleoprotein family in tobacco chloroplasts: two new chloroplast ribonucleoproteins and a phylogenetic tree of ten chloroplast RNA-binding domains

Two new ribonucleoproteins (RNPs) have been identified from a tobacco chloroplast lysate. These two proteins (cp29A and cp29B) are nuclear-encoded and have a less affinity to single-stranded DNA as compared with three other chloroplast RNPs (cp28, cp31 and cp33) previously isolated. DNA sequencing revealed that both contain two consensus sequence-type homologous RNA-binding domains (CS-RBDs) and a very acidic amino-terminal domain but shorter than that of cp28, cp31 and cp33. Comparison of cp29A and cp29B showed a 19 amino acid insertion in the region separating the two CS-RBDs in cp29B. This insertion results in three tandem repeats of a glycine-rich sequence of 10 amino acids, which is a novel feature in RNPs. The two proteins are encoded by different single nuclear genes and no alternatively spliced transcripts could be identified. We constructed a phylogenetic tree for the ten chloroplast CS-RBDs. These results suggest that there is a sizable RNP family in chloroplasts and the diversity was mainly generated through a series of gene duplications rather than through alternative pre-mRNA splicing. The gene for cp29B contains three introns. The first and second introns interrupt the first CS-RBD and the third intron does the second CS-RBD. The position of the first intron site is the same as that in the human hnRNP A1 protein gene.

U3, U8 and U13 comprise a new class of mammalian snRNPs localized in the cell nucleolus

Using anti-(U3)RNP autoantibodies, we have isolated and characterized two additional small nucleolar RNAs from HeLa cells, which are less abundant than U3 RNA. Both RNAs possess a trimethylguanosine cap as judged by precipitation with anti-TMG antibody, but are not precipitated by either anti-Sm or anti-La antibodies. In addition, both RNAs are not precipitable by anti-Th serum, which recognizes another nucleolar RNP autoantigen. Sequence analysis revealed that one of these RNAs, 136 nucleotides long, is the human U8 homolog; while the other, 105 nucleotides long, represents a novel species which we designate U13. Both RNAs share with U3 two conserved sequences (boxes C and D). The role of one or both of these boxes in binding the common 34 kd antigenic protein, otherwise known as fibrillarin, is discussed. Fractionation of HeLa cells revealed that U8 and U13, like U3, reside in the nucleolus. In glycerol gradients both RNAs cosediment with larger structures possibly representing ribosomal precursors. We propose that U3, U8 and U13 comprise a new subset of mammalian snRNPs whose roles in ribosome biogenesis are discussed.

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.

Identification of bovine brain calcium binding proteins

Three peaks of calcium binding activity have been identified by the Chelex-100 calcium binding assay of the fractions from DEAE cellulose chromatography of 100,000 X g supernatant of bovine brain. These calcium binding activity peaks have been subjected to extensive purification and three novel calcium binding proteins (Mr 27,000, Mr 48,000 and Mr 63,000) and two previously characterized proteins (calcineurin and calmodulin) have been identified as components of calcium binding activity peaks. Analysis of the calcium binding properties of the novel proteins by equilibrium dialysis suggests these proteins may be intracellular calcium receptors.

Oligopeptide mapping of NP proteins of influenza A viruses

Oligopeptide mapping of nucleoproteins (NP proteins) of 54 strains of influenza A virus showed the presence of both common and individual oligopeptides. Using the distribution of variable oligopeptides as the criterion, NP proteins were subdivided into four groups (NP0, NP1, NP2 and NP3). The NP0 group is composed of H0N1 influenza viruses and the majority of animal influenza viruses. The NP1 group contains H1N1 (except A/California/78) and H2N2 influenza viruses isolated from man as well as H1N1 influenza viruses recently isolated from animals. A/New Jersey/76 (Hsw 1N1) influenza virus also belongs to the NP1 group. The NP2 group consists of H3N2 influenza viruses isolated from man and animals. A/California/78 (H1N1) influenza virus also falls into the NP2 group. The NP3 group contains NP proteins of four animal influenza viruses with antigenic formulas Hsw1N2, Hsw1Nav2, Hav6Nav5 and Heq2Neq2.