Human ribosomal protein L7 binds RNA with an alpha-helical arginine-rich and lysine-rich domain

Cryo-EM structure of RNA-induced tau fibrils reveals a small C-terminal core that may nucleate fibril formation

In neurodegenerative diseases including Alzheimer’s and amyotrophic lateral sclerosis, proteins that bind RNA are found in aggregated forms in autopsied brains. Evidence suggests that RNA aids nucleation of these pathological aggregates; however, the mechanism has not been investigated at the level of atomic structure. Here, we present the 3.4-Å resolution structure of fibrils of full-length recombinant tau protein in the presence of RNA, determined by electron cryomicroscopy (cryo-EM). The structure reveals the familiar in-register cross-β amyloid scaffold but with a small fibril core spanning residues Glu391 to Ala426, a region disordered in the fuzzy coat in all previously studied tau polymorphs. RNA is bound on the fibril surface to the positively charged residues Arg406 and His407 and runs parallel to the fibril axis. The fibrils dissolve when RNase is added, showing that RNA is necessary for fibril integrity. While this structure cannot exist simultaneously with the tau fibril structures extracted from patients’ brains, it could conceivably account for the nucleating effects of RNA cofactors followed by remodeling as fibrils mature.

Role of salt-bridging interactions in recognition of viral RNA by arginine-rich peptides

Interactions between RNA molecules and proteins are critical to many cellular processes and are implicated in various diseases. The RNA-peptide complexes are good model systems to probe the recognition mechanism of RNA by proteins. In this work, we report studies on the binding-unbinding process of a helical peptide from a viral RNA element using nonequilibrium molecular dynamics simulations. We explored the existence of various dissociation pathways with distinct free-energy profiles that reveal metastable states and distinct barriers to peptide dissociation. We also report the free-energy differences for each of the four pathways to be 96.47 ± 12.63, 96.1 ± 10.95, 91.83 ± 9.81, and 92 ± 11.32 kcal/mol. Based on the free-energy analysis, we further propose the preferred pathway and the mechanism of peptide dissociation. The preferred pathway is characterized by the formation of sequential hydrogen-bonding and salt-bridging interactions between several key arginine amino acids and the viral RNA nucleotides. Specifically, we identified one arginine amino acid (R8) of the peptide to play a significant role in the recognition mechanism of the peptide by the viral RNA molecule.

Identification of an RNA aptamer binding hTERT-derived peptide and inhibiting telomerase activity in MCF7 cells

Human telomerase reverse transcriptase is an essential rate-limiting component of telomerase complex. hTERT protein in association with other proteins and the human telomerase RNA (hTR) shows telomerase activity, essential for maintaining genomic integrity in proliferating cells. hTERT binds hTR through a decapeptide located in the RID2 (RNA interactive domain 2) domain of N-terminal region. Since hTERT is essential for telomerase activity, inhibitors of hTERT are of great interest as potential anti-cancer agent. We have selected RNA aptamers against a synthetic peptide from the RID2 domain of hTERT by employing in vitro selection protocol (SELEX). The selected RNAs could bind the free peptide, as CD spectra suggested conformational change in aptamer upon RID2 binding. Extracts of cultured breast cancer cells (MCF7) expressing this aptamer showed lower telomerase activity as estimated by TRAP assay. hTERT-binding RNA aptamers hold promise as probable anti-cancer therapeutic agent.

Characterization of the interaction between the HIV-1 Gag structural polyprotein and the cellular ribosomal protein L7 and its implication in viral nucleic acid remodeling

Background

In HIV-1 infected cells, the integrated viral DNA is transcribed by the host cell machinery to generate the full length HIV-1 RNA (FL RNA) that serves as mRNA encoding for the Gag and GagPol precursors. Virion formation is orchestrated by Gag, and the current view is that a specific interaction between newly made Gag molecules and FL RNA initiates the process. This in turn would cause FL RNA dimerization by the NC domain of Gag (GagNC). However the RNA chaperoning activity of unprocessed Gag is low as compared to the mature NC protein. This prompted us to search for GagNC co-factors.

Results

Here we report that RPL7, a major ribosomal protein involved in translation regulation, is a partner of Gag via its interaction with the NC domain. This interaction is mediated by the NC zinc fingers and the N- and C-termini of RPL7, respectively, but seems independent of RNA binding, Gag oligomerization and its interaction with the plasma membrane. Interestingly, RPL7 is shown for the first time to exhibit a potent DNA/RNA chaperone activity higher than that of Gag. In addition, Gag and RPL7 can function in concert to drive rapid nucleic acid hybridization.

Conclusions

Our results show that GagNC interacts with the ribosomal protein RPL7 endowed with nucleic acid chaperone activity, favoring the notion that RPL7 could be a Gag helper chaperoning factor possibly contributing to the start of Gag assembly.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-016-0287-4) contains supplementary material, which is available to authorized users.

Thalamic WNT3 Secretion Spatiotemporally Regulates the Neocortical Ribosome Signature and mRNA Translation to Specify Neocortical Cell Subtypes

Neocortical development requires tightly controlled spatiotemporal gene expression. However, the mechanisms regulating ribosomal complexes and the timed specificity of neocortical mRNA translation are poorly understood. We show that active mRNA translation complexes (polysomes) contain ribosomal protein subsets that undergo dynamic spatiotemporal rearrangements during mouse neocortical development. Ribosomal protein specificity within polysome complexes is regulated by the arrival of in-growing thalamic axons, which secrete the morphogen Wingless-related MMTV (mouse mammary tumor virus) integration site 3 (WNT3). Thalamic WNT3 release during midneurogenesis promotes a change in the levels of Ribosomal protein L7 in polysomes, thereby regulating neocortical translation machinery specificity. Furthermore, we present an RNA sequencing dataset analyzing mRNAs that dynamically associate with polysome complexes as neocortical development progresses, and thus may be regulated spatiotemporally at the level of translation. Thalamic WNT3 regulates neocortical translation of two such mRNAs, Foxp2 and Apc, to promote FOXP2 expression while inhibiting APC expression, thereby driving neocortical neuronal differentiation and suppressing oligodendrocyte maturation, respectively. This mechanism may enable targeted and rapid spatiotemporal control of ribosome composition and selective mRNA translation in complex developing systems like the neocortex.

SIGNIFICANCE STATEMENT

The neocortex is a highly complex circuit generating the most evolutionarily advanced complex cognitive and sensorimotor functions. An intricate progression of molecular and cellular steps during neocortical development determines its structure and function. Our goal is to study the steps regulating spatiotemporal specificity of mRNA translation that govern neocortical development. In this work, we show that the timed secretion of Wingless-related MMTV (mouse mammary tumor virus) integration site 3 (WNT3) by ingrowing axons from the thalamus regulates the combinatorial composition of ribosomal proteins in developing neocortex, which we term the "neocortical ribosome signature." Thalamic WNT3 further regulates the specificity of mRNA translation and development of neurons and oligodendrocytes in the neocortex. This study advances our overall understanding of WNT signaling and the spatiotemporal regulation of mRNA translation in highly complex developing systems.

The arginine-rich RNA-binding motif of HIV-1 Rev is intrinsically disordered and folds upon RRE binding

Arginine-rich motifs (ARMs) capable of binding diverse RNA structures play critical roles in transcription, translation, RNA trafficking, and RNA packaging. The regulatory HIV-1 protein Rev is essential for viral replication and belongs to the ARM family of RNA-binding proteins. During the early stages of the HIV-1 life cycle, incompletely spliced and full-length viral mRNAs are very inefficiently recognized by the splicing machinery of the host cell and are subject to degradation in the cell nucleus. These transcripts harbor the Rev Response Element (RRE), which orchestrates the interaction with the Rev ARM and the successive Rev-dependent mRNA export pathway. Based on established criteria for predicting intrinsic disorder, such as hydropathy, combined with significant net charge, the very basic primary sequences of ARMs are expected to adopt coil-like structures. Thus, we initiated this study to investigate the conformational changes of the Rev ARM associated with RNA binding. We used multidimensional NMR and circular dichroism spectroscopy to monitor the observed structural transitions, and described the conformational landscapes using statistical ensemble and molecular-dynamics simulations. The combined spectroscopic and simulated results imply that the Rev ARM is intrinsically disordered not only as an isolated peptide but also when it is embedded into an oligomerization-deficient Rev mutant. RRE recognition triggers a crucial coil-to-helix transition employing an induced-fit mechanism.

Amino acid signature enables proteins to recognize modified tRNA

Human tRNA^Lys3_UUU is the primer for HIV replication. The HIV-1 nucleocapsid protein, NCp7, facilitates htRNA^Lys3_UUU recruitment from the host cell by binding to and remodeling the tRNA structure. Human tRNA^Lys3_UUU is post-transcriptionally modified, but until recently, the importance of those modifications in tRNA recognition by NCp7 was unknown. Modifications such as the 5-methoxycarbonylmethyl-2-thiouridine at anticodon wobble position-34 and 2-methylthio-N⁶-threonylcarbamoyladenosine, adjacent to the anticodon at position-37, are important to the recognition of htRNA^Lys3_UUU by NCp7. Several short peptides selected from phage display libraries were found to also preferentially recognize these modifications. Evolutionary algorithms (Monte Carlo and self-consistent mean field) and assisted model building with energy refinement were used to optimize the peptide sequence in silico, while fluorescence assays were developed and conducted to verify the in silico results and elucidate a 15-amino acid signature sequence (R-W-Q/N-H-X₂-F-Pho-X-G/A-W-R-X₂-G, where X can be most amino acids, and Pho is hydrophobic) that recognized the tRNA’s fully modified anticodon stem and loop domain, hASL^Lys3_UUU. Peptides of this sequence specifically recognized and bound modified htRNA^Lys3_UUU with an affinity 10-fold higher than that of the starting sequence. Thus, this approach provides an effective means of predicting sequences of RNA binding peptides that have better binding properties. Such peptides can be used in cell and molecular biology as well as biochemistry to explore RNA binding proteins and to inhibit those protein functions.

Preparation of novel alkylated arginine derivatives suitable for click-cycloaddition chemistry and their incorporation into pseudosubstrate- and bisubstrate-based kinase inhibitors

Efficient strategies for the introduction of arginine residues featuring acetylene or azide moieties in their side chains are described. The substituents are introduced in a way that maintains the basicity of the guanidine moiety. The methodology can be used e.g. for non-invasive labeling of arginine-containing peptides. Its applicability is demonstrated by the introduction of 'click' handles into a Protein Kinase C (PKC) pseudosubstrate peptide, and the subsequent preparation and evaluation of a novel bisubstrate-based inhibitor based on such a peptide.

The selective estrogen receptor-alpha coactivator, RPL7, and sexual differentiation of the songbird brain

The brain and behavior of the Australian zebra finch (Taeniopygia guttata) are sexually dimorphic. Only males sing courtship songs and the regions of the brain involved in the learning and production of song are significantly larger in males than females. Therefore the zebra finch serves as an excellent model for studying the mechanisms that influence brain sexual differentiation, and the majority of past research on this system has focused on the actions of steroid hormones in the development of these sex differences. Coregulators, such as coactivators and corepressors, are proteins and RNA activators that work by enhancing or depressing the transcriptional activity of the nuclear steroid receptor with which they associate, and thereby modulating the development of sex-specific brain morphologies and behaviors. The actions of these proteins may help elucidate the hormonal mechanisms that underlie song nuclei development. Research described in this review focus on the role of estrogen receptor coactivators in the avian brain; more specifically we will focus on the role of RPL7 (ribosomal protein L7; also known as L7/SPA) on sexual differentiation of the zebra finch song system. Collectively, these studies provide information about the role of steroid receptor coactivators on development of the zebra finch song system as well as on sexual differentiation of brain.

The sexually dimorphic expression of L7/SPA, an estrogen receptor coactivator, in zebra finch telencephalon

Sex differences in the zebra finch (Taeniopygia guttata) brain are robust and include differences in morphology (song control nuclei in males are significantly larger) and behavior (only males sing courtship songs). In zebra finches, hormonal manipulations during development fail to reverse sex differences in song nuclei size and suggest that the classical model of sexual differentiation is incomplete for birds. Coactivators act to initiate transcriptional activity of steroid receptors, and may help explain why hormonal manipulations alone are not sufficient to demasculinize the male zebra finch brain. The present study investigated the expression and localization of L7/SPA (an estrogen receptor coactivator) mRNA and protein expression across the development of zebra finch song nuclei from males and females collected on P1 (song nuclei not yet formed), P10 (posthatch day 10, song nuclei formed), P30 (30 days posthatch, sexually immature but song nuclei formed and birds learning to sing), and adult birds (older than 65 days and sexually mature). Northern blot analysis showed a significant sex difference in P1 and adult L7/SPA mRNA expression while Western blot analysis also showed enhanced expression in the male brain at all age points. Both in situ hybridization and immunohistochemistry demonstrated that L7/SPA mRNA and protein were located in the song nuclei as well as expressed globally. Elevated coactivator expression may be a possible mechanism controlling the development of male song control nuclei, and coactivators such as L7/SPA may be important regulators of the masculinizing effects of estradiol on brain sexual differentiation.

Arginine-rich motifs present multiple interfaces for specific binding by RNA

A number of proteins containing arginine-rich motifs (ARMs) are known to bind RNA and are involved in regulating RNA processing in viruses and cells. Using automated selection methods we have generated a number of aptamers against ARM peptides from various natural proteins. Aptamers bind tightly to their cognate ARMs, with K(d) values in the nanomolar range, and frequently show no propensity to bind to other ARMs or even to single amino acid variants of the cognate ARM. However, at least some anti-ARM aptamers can cross-recognize a limited set of other ARMs, just as natural RNA-binding sites have been shown to exhibit so-called "chameleonism." We expand upon the number of examples of cross-recognition and, using mutational and circular dichroism (CD) analyses, demonstrate that there are multiple mechanisms by which RNA ligands can cross-recognize ARMs. These studies support a model in which individual arginine residues govern binding to an RNA ligand, and the inherent flexibility of the peptide backbone may make it possible for "semi-specific" recognition of a discrete set of RNAs by a discrete set of ARM peptides and proteins.

Anticodon domain methylated nucleosides of yeast tRNA(Phe) are significant recognition determinants in the binding of a phage display selected peptide

The contributions of the natural modified nucleosides to RNA identity in protein/RNA interactions are not understood. We had demonstrated that 15 amino acid long peptides could be selected from a random phage display library using the criterion of binding to a modified, rather than unmodified, anticodon domain of yeast tRNA(Phe) (ASL(Phe)). Affinity and specificity of the selected peptides for the modified ASL(Phe) have been characterized by fluorescence spectroscopy of the peptides' tryptophans. One of the peptides selected, peptide t(F)2, exhibited the highest specificity and most significant affinity for ASL(Phe) modified with 2'-O-methylated cytidine-32 and guanosine-34 (Cm(32) and Gm(34)) and 5-methylated cytidine-40 (m(5)C(40)) (K(d) = 1.3 +/- 0.4 microM) and a doubly modified ASL(Phe)-Gm(34),m(5)C(40) and native yeast tRNA(Phe) (K(d) congruent with 2.3 and 3.8 microM, respectively) in comparison to that for the unmodified ASL(Phe) (K(d) = 70.1 +/- 12.3 microM). Affinity was reduced when a modification altered the ASL loop structure, and binding was negated by modifications that disfavored hairpin formation. Peptide t(F)2's higher affinity for the ASL(Phe)-Cm(32),Gm(34),m(5)C(40) hairpin and fluorescence resonance energy transfer from its tryptophan to the hypermodified wybutosine-37 in the native tRNA(Phe) placed the peptide across the anticodon loop and onto the 3'-side of the stem. Inhibition of purified yeast phenylalanyl-tRNA synthetase (FRS) catalyzed aminoacylation of cognate yeast tRNA(Phe) corroborated the peptide's binding to the anticodon domain. The phage-selected peptide t(F)2 has three of the four amino acids crucial to G(34) recognition by the beta-structure of the anticodon-binding domain of Thermus thermophilus FRS and exhibited circular dichroism spectral properties characteristic of beta-structure. Thus, modifications as simple as methylations contribute identity elements that a selected peptide specifically recognizes in binding synthetic and native tRNA and in inhibiting tRNA aminoacylation.

tRNA-assisted overproduction of eukaryotic ribosomal proteins

Structural studies of eukaryotic ribosomes are complicated by the tendency of their constituent proteins to be expressed at very low levels in Escherichia coli. We find that this is mainly due to their exceptionally high content of AGA/AGG arginine codons, which are poorly utilized by the bacterial translational machinery. In fact, we could overcome this limitation by the combined use of a T7 RNA polymerase expression vector and a plasmid carrying the E. coli gene argU, which encodes the minor tRNA(Arg) species that reads AGA/AGG codons. In this system, five cytoplasmic ribosomal proteins from three different eukaryotic lineages (Saccharomyces cerevisiae S8, L13, and L14; Arabidopsis thaliana L13; and Homo sapiens L7) could be overexpressed to up to 50% of total bacterial protein and were purified to homogeneity in tens of milligrams amounts. The purification procedure simply involved metal affinity chromatography followed, in some cases, by an additional heparin chromatography step. Recombinant polypeptides bound RNA with high affinity (K(d) between 50 and 300 nM). This novel overexpression/purification strategy will allow the production of high amounts of most eukaryotic ribosomal proteins in a form suitable for structural and functional studies. Coupled with recently completed and ongoing whole-genome sequencing projects, it will facilitate the molecular characterization of the eukaryotic ribosome.

Ribosomal proteins in cell proliferation and apoptosis

Ribosomal proteins have the complex task of coordinating protein biosynthesis to maintain cell homeostasis and survival. Recent evidence suggests that a number of ribosomal proteins have secondary functions independent of their involvement in protein biosynthesis. A number of these proteins function as cell proliferation regulators and in some instances as inducers of cell death. Specifically, expression of human ribosomal protein L13a has been shown to induce apoptosis, presumably by arresting cell growth in the G2/M phase of the cell cycle. In addition, inhibition of expression of L13a induces apoptosis in target cells, suggesting that this protein is necessary for cell survival. Similar results have been obtained in the yeast Saccharomyces cerevisiae, where inactivation of the yeast homologues of L13a, rp22 and rp23, by homologous recombination results in severe growth retardation and death. In addition, a closely related ribosomal protein, L7, arrests cells in G1 and also induces apoptosis. Thus, it appears that a group of ribosomal proteins may function as cell cycle checkpoints and compose a new family of cell proliferation regulators.

Human ribosomal protein L7 carries two nucleic acid-binding domains with distinct specificities

Protein L7 is associated with the large subunit of eukaryotic ribosomes that can act as a co-regulator of nuclear receptor-mediated transcription. In this study we show that L7 carries in addition to the known N-terminal nucleic acid-binding domain (NBD 1) a second one (NBD 2) which maps to the 50 C-terminal amino acids of the protein. The amino acid sequence of this region does not contain any of the known nucleic acid binding motifs; thus, NBD 2 may represent a new class of nucleic acid-binding protein motifs. NBD 2 is conserved in all known eukaryotic L7 homologs, whereas NBD 1 is only present in mammalian L7. Binding studies show that NBD 2 is functionally different from NBD 1 in that it binds preferentially to 28S rRNA, suggesting that NBD 2 is involved in the attachment of protein L7 to the large ribosomal subunit. Potential functions of NBD 1 and NBD 2 in translational and nuclear receptor-mediated transcriptional control are discussed.

Autoreactive human T cell lines recognizing ribosomal protein L7

Sera of patients suffering from systemic lupus erythematosus (SLE) frequently contain oligoclonal IgG autoantibodies with high affinity for the ribosomal protein L7 (rpL7). The humoral autoimmune response to rpL7 apparently is driven by antigen and T cell dependent. In order to analyze the T cell response to rpL7 we cultured peripheral blood lymphocytes of healthy individuals and SLE patients in the presence of recombinant rpL7. After 10 days, the cytokine response to re-stimulation with rpL7 was examined using a spot-ELISA. Measuring IFN-gamma secretion, the T cells of two patients and four healthy donors showed a significant increase in the number of spots as compared to control cells. Secretion of IL-4 or IL-10 was not detected. From the antigen-stimulated primary cultures we established by limiting dilution cloning six rpL7-reactive, IFN-gamma-secreting T cell lines which show a CD3+CD4+CD8- phenotype. One line additionally was shown to be positive for HLA-DR and CD45R0, but negative for CD27 and CD31. The cell lines carry alphabeta TCR chains which differ from each other in sequence and specificity. rpL7 fragments rich in basic amino acids could be identified as epitopes recognized by the TCR of three cell lines. Recognition of rpL7 is HLA-DR6 restricted or respectively HLA-DP restricted in the two cell lines analyzed.

The beta subunit of eukaryotic translation initiation factor 2 binds mRNA through the lysine repeats and a region comprising the C2-C2 motif

Eukaryotic translation initiation factor 2 (eIF2) has been implicated in the selection of the AUG codon as the start site for eukaryotic translation initiation, since mutations in its three subunits in yeast that allow the recognition of a UUG codon by the anticodon of the initiator Met-tRNAMet have been identified. All such mutations in the beta subunit of eIF2 (eIF2beta) mapped to a region containing a putative zinc finger structure of the C2-C2 type, indicating that these sequences could be involved in RNA recognition. Another feature of eIF2beta that could mediate an interaction with RNA is located in the amino-terminal sequences and is composed of three repeats of seven lysine residues which are highly conserved in other species. We show here the ability of eIF2beta, purified from Escherichia coli as a fusion to glutathione S-transferase, to bind mRNA in vitro. Through a deletion analysis, mRNA binding was found to be dependent on the lysine repeats and a region encompassing the C2-C2 motif. Strong mRNA binding in vitro could be maintained by the presence of only one lysine or one arginine run but not one alanine run. We further show that only one run of lysine residues is sufficient for the in vivo function of eIF2beta, probably through charge interaction, since its replacement by arginines did not impair cell viability, whereas substitution for alanines resulted in inviable cells. mRNA binding, but not GTP-dependent initiator Met-tRNAMet binding, by the eIF2 complex was determined to be dependent on the presence of the lysine runs of the beta subunit.

Correlation between chlamydial infection and autoimmune response: molecular mimicry between RNA polymerase major sigma subunit from Chlamydia trachomatis and human L7

L7 is one of the ribosomal proteins frequently targeted by autoantibodies in rheumatic autoimmune diseases. A computer search revealed a region within the immunodominant epitope of L7 (peptide II) that is highly homologous to amino acid sequence 264-286 of the RNA polymerase major sigma factor of the eubacterium Chlamydia trachomatis. Anti-L7 autoantibodies affinity purified from the immunodominant epitope were able to recognize this sequence as they reacted with purified recombinant sigma factor. Immunofluorescence labeling experiments on C. trachomatis lysates revealed a punctate staining pattern of numerous spots when incubated with the affinity-purified anti-peptide II autoantibodies. Binding of autoantibodies to peptide II was inhibited by the homologous sigma peptide. This is the first demonstration of epitope mimicry between a human and a chlamydial protein on the level of B cells. Antibody screening revealed a significant correlation between the presence of anti-L7 autoantibodies and C. trachomatis infection in patients with systemic lupus erythematosus and mixed connective tissue disease. Our results suggest that molecular mimicry is involved in the initiation of anti-L7 autoantibody response and may represent a first glance into the immunopathology of Chlamydia with respect to systemic rheumatic diseases.

Mobilization of chromatin-bound Mcm proteins by micrococcal nuclease

Mcm (minichromosome maintenance) proteins are important components of the eukaryotic replication initiation apparatus. We investigate the binding of human Mcm proteins to HeLa cell chromatin using micrococcal nuclease as a tool. In previous work we prepared chromatin under low ionic strength conditions. The use of a low salt buffer was necessary to prevent the dissociation of Mcm proteins. Here we use chromatin prepared at more physiological salt concentrations (100 mM NaCl) following the procedure of Fujita et al. (J. Biol. Chem. 272, 10928-10935; 1997) who had shown that ATP stabilizes the interaction of Mcm proteins with chromatin. We show here that micrococcal nuclease released Mcm proteins early during the digestion process suggesting that Mcm proteins reside on chromatin sites which are more open to nuclease attack than bulk chromatin. Released Mcm proteins sedimented through glycerol gradients as a multiprotein complex comprising several of the six known human Mcm proteins.

Malaria circumsporozoite protein inhibits protein synthesis in mammalian cells

Native Plasmodium circumsporozoite (CS) protein, translocated by sporozoites into the cytosol of host cells, as well as recombinant CS constructs introduced into the cytoplasm by liposome fusion or transient transfection, all lead to inhibition of protein synthesis in mammalian cells. The following findings suggest that this inhibition of translation is caused by a binding of the CS protein to ribosomes. (i) The distribution of native CS protein translocated by sporozoites into the cytoplasm as well as microinjected recombinant CS protein suggests association with ribosomes. (ii) Recombinant CS protein binds to RNase-sensitive sites on rough microsomes. (iii) Synthetic peptides representing the conserved regions I and II-plus of the P.falciparum CS protein displace recombinant CS protein from rough microsomes with dissociation constants in the nanomolar range. (iv) Synthetic peptides representing region I from the P.falciparum CS protein and region II-plus from the P.falciparum, P.berghei or P.vivax CS protein inhibit in vitro translation. We propose that Plasmodium manipulates hepatocyte protein synthesis to meet the requirements of a rapidly developing schizont. Since macrophages appear to be particularly sensitive to the presence of CS protein in the cytosol, inhibition of translation may represent a novel immune evasion mechanism of Plasmodium.

L7 protein is a coregulator of vitamin D receptor-retinoid X receptor-mediated transactivation

The vitamin D receptor (VDR) heterodimerizes with the retinoid X receptor (RXR) and requires additional protein-protein interactions to regulate the expression of target genes. Using the yeast two-hybrid system, we identified the previously described protein L7, that specifically interacted with the VDR in the presence of vitamin D. Deletion analysis indicated, that the N-terminus of L7, which harbours a basic region leucine zipper like domain, mediated interaction with the VDR. Binding assays with purified GST-L7 demonstrated, that L7 specifically pulled down the VDR, that was either expressed in yeast or endogenously contained in the cell line U937. Interestingly, L7 inhibited ligand-dependent VDR-RXR heterodimerization, when constitutively expressed in yeast. We also demonstrate that L7 repressed binding of VDR-RXR heterodimers to a vitamin D response element. Surprisingly, L7 recruited RXR to the same response element in the presence of 9-cis retinoic acid. Ligand-dependent protein-protein interaction in the yeast two-hybrid system confirmed, that binding of L7 also was targeted at the RXR. Our data suggest, that protein L7 is a coregulator of VDR-RXR mediated transactivation of genes, that modulates transcriptional activity by interfering with binding of the receptors to genomic enhancer elements.

Specific interactions of the autoantigen L7 with multi-zinc finger protein ZNF7 and ribosomal protein S7

The eucaryotic protein L7, which associates with the large subunit of ribosomes, has been shown to be a major autoantigen in systemic autoimmune arthritis. The N terminus carries a sequence motif that is similar to the leucine zipper domain of eucaryotic transcription factors. This domain promotes the homodimerization of protein L7 through alpha-helical coiled-coil formation and binds to distinct mRNAs, thereby inhibiting their cell-free translation. Using a yeast two-hybrid selection, we have identified from a Jurkat T lymphoma cDNA library ribosomal protein S7 and the multi-zinc finger protein ZNF7 as proteins that interact with protein L7. A fragment of L7 carrying the leucine zipper-like domain is fully sufficient to mediate these interactions. Their potential biological significance is indicated by low apparent dissociation constants of S7-L7 (15 x 10(-9) M) and, respectively, ZNF7-L7 (2 x 10(-9) M) complexes and co-immunoprecipitation of proteins S7, ZNF7, and L7 from a cell lysate with an anti-L7 antibody. We also show that ZNF7-like L7 and S7 can exist in a ribosome-bound form. This study provides further evidence suggesting that L7 is involved in translational regulation through interactions with components of the translational apparatus.