The N-terminal regions of eukaryotic acidic phosphoproteins P1 and P2 are crucial for heterodimerization and assembly into the ribosomal GTPase-associated center

Anti-TCP1 Antibody Is a Potential Biomarker for Diagnosing Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a chronic inflammatory disease caused by autoantibodies. Serum samples from patients with SLE (n = 10) were compared with those from normal controls (NCs, n = 5) using 21K protein chip analysis to identify a biomarker for SLE, revealing 63 SLE-specific autoantibodies. The anti-chaperonin-containing t-complex polypeptide-1 (TCP1) antibody exhibited higher expression in patients with SLE than in NCs. To validate the specificity of the anti-TCP1 antibody in SLE, dot blot analysis was conducted using sera from patients with SLE (n = 100), rheumatoid arthritis (RA; n = 25), Behçet's disease (BD; n = 28), and systemic sclerosis (SSc; n = 30) and NCs (n = 50). The results confirmed the detection of anti-TCP1 antibodies in 79 of 100 patients with SLE, with substantially elevated expression compared to both NCs and patients with other autoimmune diseases. We performed an enzyme-linked immunosorbent assay to determine the relative amounts of anti-TCP1 antibodies; markedly elevated anti-TCP1 antibody levels were detected in the sera of patients with SLE (50.1 ± 17.3 arbitrary unit (AU), n = 251) compared to those in NCs (33.9 ± 9.3 AU), RA (35 ± 8.7 AU), BD (37.5 ± 11.6 AU), and SSc (43 ± 11.9 AU). These data suggest that the anti-TCP1 antibody is a potential diagnostic biomarker for SLE.

Mood Disorder in Systemic Lupus Erythematosus Induced by Antiribosomal P Protein Antibodies Associated with Decreased Serum and Brain Tryptophan

Antiribosomal P protein (anti-P) autoantibodies commonly develop in patients with systemic lupus erythematosus. We have previously established hybridoma clones producing anti-P mAbs. In this study, we explored the pathogenesis of behavioral disorders induced by anti-P Abs using these mAbs. New Zealand Black × New Zealand White F1, New Zealand White, C57BL/6, and BALB/c mice were treated with 1 mg of anti-P Abs once every 2 wk. The behavioral disorder was evaluated by the tail suspension test, forced swim test, and open field test. Following administration of anti-P Abs, New Zealand Black × New Zealand White F1 and C57BL/6 mice developed depressive behavior and showed increased anxiety with elevated serum TNF-α and IL-6 levels. Anti-P Abs were not deposited in the affected brain tissue; instead, this mood disorder was associated with lower serum and brain tryptophan concentrations. Tryptophan supplementation recovered serum tryptophan levels and prevented the behavioral disorder. TNF-α and IL-6 were essential for the decreased serum tryptophan and disease development, which were ameliorated by treatment with anti-TNF-α neutralizing Abs or dexamethasone. Peritoneal macrophages from C57BL/6 mice produced TNF-α, IL-6, and IDO-1 via interaction with anti-P Abs through activating FcγRs, which were required for disease development. IVIg, which has an immunosuppressive effect partly through the regulation of FcγR expression, also prevented the decrease in serum tryptophan and disease development. Furthermore, serum tryptophan concentrations were decreased in the sera of systemic lupus erythematosus patients with anti-P Abs, and lower tryptophan levels correlated with disease activity. Our study revealed some of the molecular mechanisms of mood disorder induced by anti-P Abs.

Selective regulation in ribosome biogenesis and protein production for efficient viral translation

As intracellular parasites, viruses depend heavily on host cell structures and their functions to complete their life cycle and produce new viral particles. Viruses utilize or modulate cellular translational machinery to achieve efficient replication; the role of ribosome biogenesis and protein synthesis in viral replication particularly highlights the importance of the ribosome quantity and/or quality in controlling viral protein synthesis. Recently reported studies have demonstrated that ribosome biogenesis factors (RBFs) and ribosomal proteins (RPs) act as multifaceted regulators in selective translation of viral transcripts. Here we summarize the recent literature on RBFs and RPs and their association with subcellular redistribution, post-translational modification, enzyme catalysis, and direct interaction with viral proteins. The advances described in this literature establish a rationale for targeting ribosome production and function in the design of the next generation of antiviral agents.

Protein expression profiles that underpin the preventive and therapeutic potential of Moringa oleifera Lam against azoxymethane and dextran sodium sulfate-induced mouse colon carcinogenesis

Previous studies in a mouse model have indicated the anticancer potential of boiled Moringa oleifera pod (bMO)-supplemented diets; however, its molecular mechanisms are still unclear. Therefore, the present study aimed to explore the protein expression profiles responsible for the suppressive effect of bMO supplementation on azoxymethane (AOM)/dextran sodium sulfate (DSS)-induced mouse colon carcinogenesis. Analysis by gel electrophoresis and liquid chromatography-tandem mass spectrophotometry demonstrated that there were 125 proteins that were differentially expressed in mouse colon tissues between 14 experimental groups of mice. The differentially expressed proteins are involved in various biological processes, such as signal transduction, metabolism, transcription and translation. Venn diagram analysis of the differentially expressed proteins was performed in six selected mouse groups, including negative control, positive control mice induced by AOM/DSS, the AOM/DSS groups receiving preventive or therapeutic bMO diets and their bMO-supplemented control groups. This analysis identified 7 proteins; 60S acidic ribosomal protein P1 (Rplp1), fragile X mental retardation, cystatin 9, round spermatids protein, zinc finger protein 638, protein phosphatase 2C (Ppm1g) and unnamed protein product as being potentially associated with the preventive and therapeutic effects of bMO in AOM/DSS-induced mouse colon cancer. Analysis based on the search tool for interactions of chemicals (STITCH) database predicted that Rplp1 interacted with the apoptotic and inflammatory pathways, whereas Ppm1g was associated only with inflammatory networks. This proteomic analysis revealed candidate proteins that are responsible for the effects of bMO supplementation, potentially by regulating apoptotic and inflammatory signaling networks in colorectal cancer prevention and therapy.

Investigation of Structure of the Ribosomal L12/P Stalk

This review contains recent data on the structure of the functionally important ribosomal domain, L12/P stalk, of the large ribosomal subunit. It is the most mobile site of the ribosome; it has been found in ribosomes of all living cells, and it is involved in the interaction between ribosomes and translation factors. The difference between the structures of the ribosomal proteins forming this protuberance (despite their general resemblance) determines the specificity of interaction between eukaryotic and prokaryotic ribosomes and the respective protein factors of translation. In this review, works on the structures of ribosomal proteins forming the L12/P-stalk in bacteria, archaea, and eukaryotes and data on structural aspects of interactions between these proteins and rRNA are described in detail.

Disruption of the ribosomal P complex leads to stress-induced autophagy

The human ribosomal P complex, which consists of the acidic ribosomal P proteins RPLP0, RPLP1, and RPLP2 (RPLP proteins), recruits translational factors, facilitating protein synthesis. Recently, we showed that overexpression of RPLP1 immortalizes primary cells and contributes to transformation. Moreover, RPLP proteins are overexpressed in human cancer, with the highest incidence in breast carcinomas. It is thought that disruption of the P complex would directly affect protein synthesis, causing cell growth arrest and eventually apoptosis. Here, we report a distinct mechanism by which cancer cells undergo cell cycle arrest and induced autophagy when RPLP proteins are downregulated. We found that absence of RPLP0, RPLP1, or RPLP2 resulted in reactive oxygen species (ROS) accumulation and MAPK1/ERK2 signaling pathway activation. Moreover, ROS generation led to endoplasmic reticulum (ER) stress that involved the EIF2AK3/PERK-EIF2S1/eIF2α-EIF2S2-EIF2S3-ATF4/ATF-4- and ATF6/ATF-6-dependent arms of the unfolded protein response (UPR). RPLP protein-deficient cells treated with autophagy inhibitors experienced apoptotic cell death as an alternative to autophagy. Strikingly, antioxidant treatment prevented UPR activation and autophagy while restoring the proliferative capacity of these cells. Our results indicate that ROS are a critical signal generated by disruption of the P complex that causes a cellular response that follows a sequential order: first ROS, then ER stress/UPR activation, and finally autophagy. Importantly, inhibition of the first step alone is able to restore the proliferative capacity of the cells, preventing UPR activation and autophagy. Overall, our results support a role for autophagy as a survival mechanism in response to stress due to RPLP protein deficiency.

Natural humoral immune response to ribosomal P0 protein in colorectal cancer patients

Background

Tumor associated antigens are useful in colorectal cancer (CRC) management. The ribosomal P proteins (P0, P1, P2) play an important role in protein synthesis and tumor formation. The immunogenicity of the ribosomal P0 protein in head and neck, in breast and prostate cancer patients and the overexpression of the carboxyl-terminal P0 epitope (C-22 P0) in some tumors were reported.

Methods

Sera from 72 colorectal tumor patients (67 malignant and 5 benign tumors) were compared with 73 healthy donor sera for the presence of antibodies to CEA, EGFR, ErbB2 and ribosomal P proteins by western blotting or ELISA. Expression of the C-22 P0 epitope on tissues and colon cancer cells was determined by immunoperoxidase staining and indirect immunofluorescence/western blotting, respectively, employing MAb 2B2. Biological effects of MAb 2B2 on colon cancer cells were assessed by the Sulforhodamine B cell proliferation assay, trypan blue exclusion test and cleaved caspase-3 detection. Fisher’s exact test was used to compare the number of auto-antibodies positive patients with healthy donors. Variation in the C-22 P0 expression, and in the number of apoptotic cells was evaluated by Student’s t-test. Variation in cell survival and cell death was evaluated by Newman-Keuls test.

Results

No significant humoral response was observed to CEA, EGFR and ErbB2 in CRC patients. Conversely, 7 out of 67 CRC patient sera reacted to ribosomal P proteins. The prevalence of P proteins auto-antibodies in CRC patients was significant. Five patients showed restricted P0 immunoreactivity, while two patients reacted simultaneously to all P proteins. The C-22 P0 epitope was homogenously expressed both in malignant tumors and the adjacent mucosa, but the intensity of expression was higher in the tumor. Starved colon cancer cells showed a higher C-22 P0 epitope plasma membrane expression compared to control cells. MAb 2B2 inhibited colon cancer cell growth and induced cell death in a dose dependent manner.

Conclusions

Our study shows a spontaneous humoral immune response to ribosomal P0 protein in CRC patients and the inhibition of in vitro cancer cell growth after C-22 P0 epitope targeting. The ribosomal P0 protein might be a useful immunological target in CRC patients.

Characterization of anti-P monoclonal antibodies directed against the ribosomal protein-RNA complex antigen and produced using Murphy Roths large autoimmune-prone mice

Autoantibodies, including anti-ribosomal P proteins (anti-P), are thought to be produced by an antigen-driven immune response in systemic lupus erythematosus (SLE). To test this hypothesis, we reconstituted the ribosomal antigenic complex in vitro using human P0, phosphorylated P1 and P2 and a 28S rRNA fragment covering the P0 binding site, and immunized Murphy Roths large (MRL)/lrp lupus mice with this complex without any added adjuvant to generate anti-P antibodies. Using hybridoma technology, we subsequently obtained 34 clones, each producing an anti-P monoclonal antibody (mAb) that recognized the conserved C-terminal tail sequence common to all three P proteins. We also obtained two P0-specific monoclonal antibodies, but no antibody specific to P1, P2 or rRNA fragment. Two types of mAbs were found among these anti-P antibodies: one type (e.g. 9D5) reacted more strongly with the phosphorylated P1 and P2 than that with their non-phosphorylated forms, whereas the other type (e.g. 4H11) reacted equally with both phosphorylated and non-phosphorylated forms of P1/P2. Both 9D5 and 4H11 inhibited the ribosome/eukaryotic elongation factor-2 (eEF-2)-coupled guanosine triphosphate (GTP)ase activity. However, preincubation with a synthetic peptide corresponding to the C-terminal sequence common to all three P proteins, but not the peptide that lacked the last three C-terminal amino acids, mostly prevented the mAb-induced inhibition of GTPase activity. Thus, at least two types of anti-P were produced preferentially following the immunization of MRL mice with the reconstituted antigenic complex. Presence of multiple copies of the C-termini, particularly that of the last three C-terminal amino acid residues, in the antigenic complex appears to contribute to the immunogenic stimulus.

Molecular insights into the interaction of the ribosomal stalk protein with elongation factor 1α

In all organisms, the large ribosomal subunit contains multiple copies of a flexible protein, the so-called ‘stalk’. The C-terminal domain (CTD) of the stalk interacts directly with the translational GTPase factors, and this interaction is required for factor-dependent activity on the ribosome. Here we have determined the structure of a complex of the CTD of the archaeal stalk protein aP1 and the GDP-bound archaeal elongation factor aEF1α at 2.3 Å resolution. The structure showed that the CTD of aP1 formed a long extended α-helix, which bound to a cleft between domains 1 and 3 of aEF1α, and bridged these domains. This binding between the CTD of aP1 and the aEF1α•GDP complex was formed mainly by hydrophobic interactions. The docking analysis showed that the CTD of aP1 can bind to aEF1α•GDP located on the ribosome. An additional biochemical assay demonstrated that the CTD of aP1 also bound to the aEF1α•GTP•aminoacyl-tRNA complex. These results suggest that the CTD of aP1 interacts with aEF1α at various stages in translation. Furthermore, phylogenetic perspectives and functional analyses suggested that the eukaryotic stalk protein also interacts directly with domains 1 and 3 of eEF1α, in a manner similar to the interaction of archaeal aP1 with aEF1α.

In vivo formation of Plasmodium falciparum ribosomal stalk - a unique mode of assembly without stable heterodimeric intermediates

BACKGROUND

The ribosomal stalk composed of P-proteins constitutes a structure on the large ribosomal particle responsible for recruitment of translation factors and stimulation of factor-dependent GTP hydrolysis during translation. The main components of the stalk are P-proteins, which form a pentamer. Despite the conserved basic function of the stalk, the P-proteins do not form a uniform entity, displaying heterogeneity in the primary structure across the eukaryotic lineage. The P-proteins from protozoan parasites are among the most evolutionarily divergent stalk proteins.

METHODS

We have assembled P-stalk complex of Plasmodium falciparum in vivo in bacterial system using tricistronic expression cassette and provided its characteristics by biochemical and biophysical methods.

RESULTS

All three individual P-proteins, namely uL10/P0, P1 and P2, are indispensable for acquisition of a stable structure of the P stalk complex and the pentameric uL10/P0-(P1-P2)₂form represents the most favorable architecture for parasite P-proteins.

CONCLUSION

The formation of P. falciparum P-stalk is driven by trilateral interaction between individual elements which represents unique mode of assembling, without stable P1-P2 heterodimeric intermediate.

GENERAL SIGNIFICANCE

On the basis of our mass-spectrometry analysis supported by the bacterial two-hybrid assay and biophysical analyses, a unique pathway of the parasite stalk assembling has been proposed. We suggest that the absence of P1/P2 heterodimer, and the formation of a stable pentamer in the presence of all three proteins, indicate a one-step formation to be the main pathway for the vital ribosomal stalk assembly, whereas the P2 homo-oligomer may represent an off-pathway product with physiologically important nonribosomal role.

The acidic ribosomal protein P2 from Euplotes octocarinatus is phosphorylated at its N-terminal domain

The eukaryotic acid ribosomal P0, P1, and P2 proteins share a conserved flexible C-terminal tail that is rich in acidic residues, which are involved in the interaction with elongation factor 2 during protein synthesis. Our previous work suggested that the acidic ribosomal P proteins from Euplotes octocarinatus have a special C-terminal domain. To further understand this characteristic feature, both P2 and elongation factor 2 from E. octocarinatus were overexpressed, for the first time, in Escherichia coli in this study. GST pull-down assay indicated that P2 protein from E. octocarinatus (EoP2) interacted specifically with the N-terminal domain of elongation factor 2 from E. octocarinatus (EoEF-2) in vitro. The interacting part of EoP2 is in the C-terminal domains, consistent with the observation in other organisms. Phosphorylation of the recombinant EoP2 was performed in vitro using multiple methods such as (31)P-NMR spectroscopy, native PAGE, and Phos-tag(TM) SDS-PAGE. Results showed that ribosomal protein EoP2 was phosphorylated by casein kinase II at serine 21 located at the N terminus. This phosphorylation site identified in EoP2 is quite different from that of P2 from other organisms, in which the phosphorylation site is located in the conserved C-terminal region.

Solution structure of human P1•P2 heterodimer provides insights into the role of eukaryotic stalk in recruiting the ribosome-inactivating protein trichosanthin to the ribosome

Lateral ribosomal stalk is responsible for binding and recruiting translation factors during protein synthesis. The eukaryotic stalk consists of one P0 protein with two copies of P1•P2 heterodimers to form a P0(P1•P2)₂ pentameric P-complex. Here, we have solved the structure of full-length P1•P2 by nuclear magnetic resonance spectroscopy. P1 and P2 dimerize via their helical N-terminal domains, whereas the C-terminal tails of P1•P2 are unstructured and can extend up to ∼125 Å away from the dimerization domains. ¹⁵N relaxation study reveals that the C-terminal tails are flexible, having a much faster internal mobility than the N-terminal domains. Replacement of prokaryotic L10(L7/L12)₄/L11 by eukaryotic P0(P1•P2)₂/eL12 rendered Escherichia coli ribosome, which is insensitive to trichosanthin (TCS), susceptible to depurination by TCS and the C-terminal tail was found to be responsible for this depurination. Truncation and insertion studies showed that depurination of hybrid ribosome is dependent on the length of the proline-alanine rich hinge region within the C-terminal tail. All together, we propose a model that recruitment of TCS to the sarcin-ricin loop required the flexible C-terminal tail, and the proline-alanine rich hinge region lengthens this C-terminal tail, allowing the tail to sweep around the ribosome to recruit TCS.

Molecular dissection of the silkworm ribosomal stalk complex: the role of multiple copies of the stalk proteins

In animal ribosomes, two stalk proteins P1 and P2 form a heterodimer, and the two dimers, with the anchor protein P0, constitute a pentameric complex crucial for recruitment of translational GTPase factors to the ribosome. To investigate the functional contribution of each copy of the stalk proteins, we constructed P0 mutants, in which one of the two C-terminal helices, namely helix I (N-terminal side) or helix II (C-terminal side) were unable to bind the P1–P2 dimer. We also constructed ‘one-C-terminal domain (CTD) stalk dimers’, P1–P2_ΔC and P1_ΔC–P2, composed of intact P1/P2 monomer and a CTD-truncated partner. Through combinations of P0 and P1–P2 variants, various complexes were reconstituted and their function tested in eEF-2-dependent GTPase and eEF-1α/eEF-2-dependent polyphenylalanine synthesis assays in vitro. Double/single-CTD dimers bound to helix I showed higher activity than that bound to helix II. Despite low polypeptide synthetic activity by a single one-CTD dimer, its binding to both helices considerably increased activity, suggesting that two stalk dimers cooperate, particularly in polypeptide synthesis. This promotion of activity by two stalk dimers was lost upon mutation of the conserved YPT sequence connecting the two helices of P0, suggesting a role for this sequence in cooperativity of two stalk dimers.

Arrest of nuclear division in Plasmodium through blockage of erythrocyte surface exposed ribosomal protein P2

Malaria parasites reside inside erythrocytes and the disease manifestations are linked to the growth inside infected erythrocytes (IE). The growth of the parasite is mostly confined to the trophozoite stage during which nuclear division occurs followed by the formation of cell bodies (schizogony). The mechanism and regulation of schizogony are poorly understood. Here we show a novel role for a Plasmodium falciparum 60S stalk ribosomal acidic protein P2 (PfP2) (PFC0400w), which gets exported to the IE surface for 6-8 hrs during early schizogony, starting around 26-28 hrs post-merozoite invasion. The surface exposure is demonstrated using multiple PfP2-specific monoclonal antibodies, and is confirmed through transfection using PfP2-GFP. The IE surface-exposed PfP2-protein occurs mainly as SDS-resistant P2-homo-tetramers. Treatment with anti-PfP2 monoclonals causes arrest of IEs at the first nuclear division. Upon removal of the antibodies, about 80-85% of synchronized parasites can be released even after 24 hrs of antibody treatment. It has been reported that a tubovesicular network (TVN) is set up in early trophozoites which is used for nutrient import. Anti-P2 monoclonal antibodies cause a complete fragmentation of TVN by 36 hrs, and impairs lipid import in IEs. These may be downstream causes for the cell-cycle arrest. Upon antibody removal, the TVN is reconstituted, and the cell division progresses. Each of the above properties is observed in the rodent malaria parasite species P. yoelii and P. berghei. The translocation of the P2 protein to the IE surface is therefore likely to be of fundamental importance in Plasmodium cell division.

NMR insights into folding and self-association of Plasmodium falciparum P2

The eukaryotic 60S-ribosomal stalk is composed of acidic ribosomal proteins (P1 and P2) and neutral protein P0, which are thought to be associated as a pentameric structure, [2P1, 2P2, P0]. Plasmodium falciparum P2 (PfP2) appears to play additional non-ribosomal functions associated with its tendency for homo-oligomerization. Recombinant bacterially expressed PfP2 protein also undergoes self-association, as shown by SDS-PAGE analysis and light scattering studies. Secondary structure prediction algorithms predict the native PfP2 protein to be largely helical and this is corroborated by circular dichroism investigation. The ¹H-¹⁵N HSQC spectrum of native P2 showed only 43 cross peaks compared to the expected 138. The observed peaks were found to belong to the C-terminal region, suggesting that this segment is flexible and solvent exposed. In 9 M urea denaturing conditions the chain exhibited mostly non-native β structural propensity. ¹⁵N Relaxation data for the denatured state indicated substantial variation in ms-µs time scale motion along the chain. Average area buried upon folding (AABUF) calculations on the monomer enabled identification of hydrophobic patches along the sequence. Interestingly, the segments of slower motion in the denatured state coincided with these hydrophobic patches, suggesting that in the denatured state the monomeric chain undergoes transient hydrophobic collapse. The implications of these results for the folding mechanism and self-association of PfP2 are discussed.

P1 and P2 protein heterodimer binding to the P0 protein of Saccharomyces cerevisiae is relatively non-specific and a source of ribosomal heterogeneity

The ribosomal stalk is formed by four acidic phosphoproteins in Saccharomyces cerevisiae, P1α, P1β, P2α and P2β, which form two heterodimers, P1α/P2β and P1β/P2α, that preferentially bind to sites A and B of the P0 protein, respectively. Using mutant strains carrying only one of the four possible P1/P2 combinations, we found a specific phenotype associated to each P1/P2 pair, indicating that not all acidic P proteins play the same role. The absence of one P1/P2 heterodimer reduced the rate of cell growth by varying degrees, depending on the proteins missing. Synthesis of the 60S ribosomal subunit also decreased, particularly in strains carrying the unusual P1α-P2α or P1β-P2β heterodimers, although the distinct P1/P2 dimers are bound with similar affinity to the mutant ribosome. While in wild-type strains the B site bound P1β/P2α in a highly specific manner and the A site bound the four P proteins similarly, both the A and B binding sites efficiently bound practically any P1/P2 pair in mutant strains expressing truncated P0 proteins. The reported results support that while most ribosomes contain a P1α/P2β-P0-P1β/P2α structure in normal conditions, the stalk assembly mechanism can generate alternative compositions, which have been previously detected in the cell.

Archaeal ribosomal stalk protein interacts with translation factors in a nucleotide-independent manner via its conserved C terminus

Protein synthesis on the ribosome requires translational GTPase factors to bind to the ribosome in the GTP-bound form, take individual actions that are coupled with GTP hydrolysis, and dissociate, usually in the GDP-bound form. The multiple copies of the flexible ribosomal stalk protein play an important role in these processes. Using biochemical approaches and the stalk protein from a hyperthermophilic archaeon, Pyrococcus horikoshii, we here provide evidence that the conserved C terminus of the stalk protein aP1 binds directly to domain I of the elongation factor aEF-2, irrespective of whether aEF-2 is bound to GTP or GDP. Site-directed mutagenesis revealed that four hydrophobic amino acids at the C terminus of aP1, Leu-100, 103, 106, and Phe-107, are crucial for the direct binding. P1 was also found to bind to the initiation factor aIF5B, as well as aEF-1α, but not aIF2γ, via its C terminus. Moreover, analytical ultracentrifugation and gel mobility shift analyses showed that a heptameric complex of aP1 and aP0, aP0(aP1)(2)(aP1)(2)(aP1)(2), can bind multiple aEF-2 molecules simultaneously, which suggests that individual copies of the stalk protein are accessible to the factor. The functional significance of the C terminus of the stalk protein was also shown using the eukaryotic proteins P1/P2 and P0. It is likely that the conserved C terminus of the stalk proteins of archaea and eukaryotes can bind to translation factors both before and after GTP hydrolysis. This consistent binding ability of the stalk protein may contribute to maintaining high concentrations of translation factors around the ribosome, thus promoting translational efficiency.

Charged and hydrophobic surfaces on the a chain of shiga-like toxin 1 recognize the C-terminal domain of ribosomal stalk proteins

Shiga-like toxins are ribosome-inactivating proteins (RIP) produced by pathogenic E. coli strains that are responsible for hemorrhagic colitis and hemolytic uremic syndrome. The catalytic A(1) chain of Shiga-like toxin 1 (SLT-1), a representative RIP, first docks onto a conserved peptide SD[D/E]DMGFGLFD located at the C-terminus of all three eukaryotic ribosomal stalk proteins and halts protein synthesis through the depurination of an adenine base in the sarcin-ricin loop of 28S rRNA. Here, we report that the A(1) chain of SLT-1 rapidly binds to and dissociates from the C-terminal peptide with a monomeric dissociation constant of 13 µM. An alanine scan performed on the conserved peptide revealed that the SLT-1 A(1) chain interacts with the anionic tripeptide DDD and the hydrophobic tetrapeptide motif FGLF within its sequence. Based on these 2 peptide motifs, SLT-1 A(1) variants were generated that displayed decreased affinities for the stalk protein C-terminus and also correlated with reduced ribosome-inactivating activities in relation to the wild-type A(1) chain. The toxin-peptide interaction and subsequent toxicity were shown to be mediated by cationic and hydrophobic docking surfaces on the SLT-1 catalytic domain. These docking surfaces are located on the opposite face of the catalytic cleft and suggest that the docking of the A(1) chain to SDDDMGFGLFD may reorient its catalytic domain to face its RNA substrate. More importantly, both the delineated A(1) chain ribosomal docking surfaces and the ribosomal peptide itself represent a target and a scaffold, respectively, for the design of generic inhibitors to block the action of RIPs.

Solution structure of the dimerization domain of the eukaryotic stalk P1/P2 complex reveals the structural organization of eukaryotic stalk complex

The lateral ribosomal stalk is responsible for the kingdom-specific binding of translation factors and activation of GTP hydrolysis during protein synthesis. The eukaryotic stalk is composed of three acidic ribosomal proteins P0, P1 and P2. P0 binds two copies of P1/P2 hetero-dimers to form a pentameric P-complex. The structure of the eukaryotic stalk is currently not known. To provide a better understanding on the structural organization of eukaryotic stalk, we have determined the solution structure of the N-terminal dimerization domain (NTD) of P1/P2 hetero-dimer. Helix-1, -2 and -4 from each of the NTD-P1 and NTD-P2 form the dimeric interface that buries 2200 A² of solvent accessible surface area. In contrast to the symmetric P2 homo-dimer, P1/P2 hetero-dimer is asymmetric. Three conserved hydrophobic residues on the surface of NTD-P1 are replaced by charged residues in NTD-P2. Moreover, NTD-P1 has an extra turn in helix-1, which forms extensive intermolecular interactions with helix-1 and -4 of NTD-P2. Truncation of this extra turn of P1 abolished the formation of P1/P2 hetero-dimer. Systematic truncation studies suggest that P0 contains two spine-helices that each binds one copy of P1/P2 hetero-dimer. Modeling studies suggest that a large hydrophobic cavity, which can accommodate the loop between the spine-helices of P0, can be found on NTD-P1 but not on NTD-P2 when the helix-4 adopts an ‘open’ conformation. Based on the asymmetric properties of NTD-P1/NTD-P2, a structural model of the eukaryotic P-complex with P2/P1:P1/P2 topology is proposed.

Solution structure of the dimerization domain of ribosomal protein P2 provides insights for the structural organization of eukaryotic stalk

The lateral stalk of ribosome is responsible for kingdom-specific binding of translation factors and activation of GTP hydrolysis that drives protein synthesis. In eukaryotes, the stalk is composed of acidic ribosomal proteins P0, P1 and P2 that constitute a pentameric P-complex in 1: 2: 2 ratio. We have determined the solution structure of the N-terminal dimerization domain of human P2 (NTD-P2), which provides insights into the structural organization of the eukaryotic stalk. Our structure revealed that eukaryotic stalk protein P2 forms a symmetric homodimer in solution, and is structurally distinct from the bacterial counterpart L12 homodimer. The two subunits of NTD-P2 form extensive hydrophobic interactions in the dimeric interface that buries 2400 Ų of solvent accessible surface area. We have showed that P1 can dissociate P2 homodimer spontaneously to form a more stable P1/P2 1 : 1 heterodimer. By homology modelling, we identified three exposed polar residues on helix-3 of P2 are substituted by conserved hydrophobic residues in P1. Confirmed by mutagenesis, we showed that these residues on helix-3 of P1 are not involved in the dimerization of P1/P2, but instead play a vital role in anchoring P1/P2 heterodimer to P0. Based on our results, models of the eukaryotic stalk complex were proposed.

In vivo formation of a stable pentameric (P2alpha/P1beta)-P0-(P1alpha/P2beta) ribosomal stalk complex in Saccharomyces cerevisiae

Heterodimers of acidic proteins P1alpha/P2beta and P1beta/P2alpha bind to P0 and are fundamental for the assembly of the ribosomal stalk. However, different inconsistencies are found in the literature regarding additional P protein heterodimer formations and their individual interactions with P0. Using the two-hybrid approach, we have found results that help to clarify these interactions. Thus, we have found that neither P1 nor P2 directly interact with P0 unless the endogenous heterodimer partner is being expressed in the cell. In addition, a P2-free amino end is a requisite in these heterodimers for binding to P0. With regard to the two-hybrid interactions between P1 and P2, the known canonical P1alpha-P2beta and P1beta-P2alpha interactions do not depend on either a free amino end or the presence of endogenous P0, P1 or P2 proteins. Furthermore, the non-canonical P1beta-P2beta pair also behaves similarly, although this interaction is weaker. Interestingly, P1alpha-P2alpha, P1alpha-P1beta and P2alpha-P2beta two-hybrid interactions were also detected, although in these cases the endogenous P proteins were involved. Thus, these positive interactions are the consequence of the interaction between two canonical heterodimers. As the ribosome anchorage protein P0 is also necessary, the results suggest that, in vivo, all five P proteins form a complex, independent of the ribosome, containing the two canonical heterodimers and P0. This complex has been isolated in cells expressing a P0 protein unable to bind to the ribosome.

Structural basis for translation factor recruitment to the eukaryotic/archaeal ribosomes

The archaeal ribosomal stalk complex has been shown to have an apparently conserved functional structure with eukaryotic pentameric stalk complex; it provides access to eukaryotic elongation factors at levels comparable to that of the eukaryotic stalk. The crystal structure of the archaeal heptameric (P0(P1)(2)(P1)(2)(P1)(2)) stalk complex shows that the rRNA anchor protein P0 consists of an N-terminal rRNA-anchoring domain followed by three separated spine helices on which three P1 dimers bind. Based on the structure, we have generated P0 mutants depleted of any binding site(s) for P1 dimer(s). Factor-dependent GTPase assay of such mutants suggested that the first P1 dimer has higher activity than the others. Furthermore, we constructed a model of the archaeal 50 S with stalk complex by superposing the rRNA-anchoring domain of P0 on the archaeal 50 S. This model indicates that the C termini of P1 dimers where translation factors bind are all localized to the region between the stalk base of the 50 S and P0 spine helices. Together with the mutational experiments we infer that the functional significance of multiple copies of P1 is in creating a factor pool within a limited space near the stalk base of the ribosome.

Structural and functional characterization of the amino terminal domain of the yeast ribosomal stalk P1 and P2 proteins

The essential ribosomal stalk is formed in eukaryotes by a pentamer of two P1-P2 protein heterodimers and the P0 rRNA binding protein. In contrast to the highly stable prokaryotic complex, the P1 and P2 proteins in the eukaryotic stalk undergo a cyclic process of assembly and disassembly during translation that seems to modulate the ribosome activity. To better understand this process, the regions of the Saccharomyces cerevisiae P1alpha and P2beta proteins that are directly involved in heterodimer formation and ribosome binding have been characterized using a series of P1alpha/P2beta chimeras. The region required for a stable interaction with the ribosome is formed by the first three predicted alpha-helices in the N-terminal domain of both proteins. The same region is required for heterodimer formation in P2beta but the third helix is dispensable for this association in P1alpha. It seems, therefore, that stable ribosome binding is more structurally demanding than heterodimerization. A fourth predicted alpha-helix in the N-terminal domain of P1alpha and P2beta appears not to be involved in the assembly process but rather, it contributes to the conformation of the proteins by apparently restricting the mobility of their C-terminal domain and paradoxically, by reducing their activity. In addition, the study of P1/P2 chimeras showed that the C-terminal domains of these two types of protein are functionally identical and that their protein specificity is exclusively determined by their N-terminal domains.