Polar confinement of a macromolecular machine by an SRP-type GTPase.

The basal structure of the bacterial flagellum includes a membrane embedded MS-ring (formed by multiple copies of FliF) and a cytoplasmic C-ring (composed of proteins FliG, FliM and FliN). The SRP-type GTPase FlhF is required for directing the initial flagellar protein FliF to the cell pole, but the mechanisms are unclear. Here, we show that FlhF anchors developing flagellar structures to the polar landmark protein HubP/FimV, thereby restricting their formation to the cell pole. Specifically, the GTPase domain of FlhF interacts with HubP, while a structured domain at the N-terminus of FlhF binds to FliG. FlhF-bound FliG subsequently engages with the MS-ring protein FliF. Thus, the interaction of FlhF with HubP and FliG recruits a FliF-FliG complex to the cell pole. In addition, the modulation of FlhF activity by the MinD-type ATPase FlhG controls the interaction of FliG with FliM-FliN, thereby regulating the progression of flagellar assembly at the pole.

The RNA Helicase Ded1 from Yeast Is Associated with the Signal Recognition Particle and Is Regulated by SRP21.

The DEAD-box RNA helicase Ded1 is an essential yeast protein involved in translation initiation that belongs to the DDX3 subfamily. The purified Ded1 protein is an ATP-dependent RNA-binding protein and an RNA-dependent ATPase, but it was previously found to lack substrate specificity and enzymatic regulation. Here we demonstrate through yeast genetics, yeast extract pull-down experiments, in situ localization, and in vitro biochemical approaches that Ded1 is associated with, and regulated by, the signal recognition particle (SRP), which is a universally conserved ribonucleoprotein complex required for the co-translational translocation of polypeptides into the endoplasmic reticulum lumen and membrane. Ded1 is physically associated with SRP components in vivo and in vitro. Ded1 is genetically linked with SRP proteins. Finally, the enzymatic activity of Ded1 is inhibited by SRP21 in the presence of SCR1 RNA. We propose a model where Ded1 actively participates in the translocation of proteins during translation. Our results provide a new understanding of the role of Ded1 during translation.

Significance of signal recognition particle 9 nuclear translocation: Implications for pancreatic cancer prognosis and functionality.

Signal recognition particles (SRPs) are essential for regulating intracellular protein transport and secretion. Patients with tumors with high SRP9 expression tend to have a poorer overall survival. However, to the best of our knowledge, no reports have described the relationship between SRP9 localization and prognosis in pancreatic cancer. Thus, the present study aimed to investigate this relationship. Immunohistochemical staining for SRP9 using excised specimens from pancreatic cancer surgery cases without preoperative chemotherapy or radiotherapy showed that SRP9 was preferentially expressed in the nucleus of the cancerous regions in some cases, which was hardly detected in other cases, indicating that SRP9 was transported to the nucleus in the former cases. To compare the prognosis of patients with SRP9 nuclear translocation, patients were divided into two groups: Those with a nuclear translocation rate of >50% and those with a nuclear translocation rate of ≤50%. The nuclear translocation rate of >50% group had a significantly better recurrence­free survival than the nuclear translocation rate of ≤50% group (P=0.037). Subsequent in vitro experiments were conducted; notably, the nuclear translocation rate of SRP9 was reduced under amino acid­deficient conditions, suggesting that multiple factors are involved in this phenomenon. To further study the function of SRP9 nuclear translocation, in vitro experiments were performed by introducing SRP9 splicing variants (v1 and v2) and their deletion mutants lacking C­terminal regions into MiaPaCa pancreatic cancer cells. The results demonstrated that both splicing variants showed nuclear translocation regardless of the C­terminal deletions, suggesting the role of the N­terminal regions. Given that SRP9 is an RNA­binding protein, the study of RNA immunoprecipitation revealed that signaling pathways involved in cancer progression and protein translation were downregulated in nuclear­translocated v1 and v2. Undoubtedly, further studies of the nuclear translocation of SRP9 will open an avenue to optimize the precise evaluation and therapeutic control of pancreatic cancer.

The nucleolar phase of signal recognition particle assembly.

The signal recognition particle is essential for targeting transmembrane and secreted proteins to the endoplasmic reticulum. Remarkably, because they work together in the cytoplasm, the SRP and ribosomes are assembled in the same biomolecular condensate: the nucleolus. How important is the nucleolus for SRP assembly is not known. Using quantitative proteomics, we have investigated the interactomes of SRP components. We reveal that SRP proteins are associated with scores of nucleolar proteins important for ribosome biogenesis and nucleolar structure. Having monitored the subcellular distribution of SRP proteins upon controlled nucleolar disruption, we conclude that an intact organelle is required for their proper localization. Lastly, we have detected two SRP proteins in Cajal bodies, which indicates that previously undocumented steps of SRP assembly may occur in these bodies. This work highlights the importance of a structurally and functionally intact nucleolus for efficient SRP production and suggests that the biogenesis of SRP and ribosomes may be coordinated in the nucleolus by common assembly factors.

[Immune-Mediated Necrotizing Myopathy].

Immune-mediated necrotizing myopathy (IMNM) is a form of autoimmune myositis characterized by the presence of necrotic and regenerating process as a major finding in the muscle. Anti-SRP and anti-HMGCR have been identified as IMNM-specific autoantibodies. Patients with this disease often present with severe muscle weakness and markedly elevated serum creatine kinase (CK) levels. Differentiation from muscular dystrophy is challenging in certain cases. When patients meet the condition "subacute onset", "hyperCKemia over 1000 IU/L", and "clinical diagnosis of muscular dystrophy lacking molecular diagnosis", the possibility of IMNM should be considered. Autoantibody measurement, including of anti-SRP and HMGCR antibodies, is recommended. Treatment with corticosteroid in combination with immunosuppressants, intravenous immunoglobulin, and rituximab can be performed.

A Guide to Computational Cotranscriptional Folding Featuring the SRP RNA.

Although RNA molecules are synthesized via transcription, little is known about the general impact of cotranscriptional folding in vivo. We present different computational approaches for the simulation of changing structure ensembles during transcription, including interpretations with respect to experimental data from literature. Specifically, we analyze different mutations of the E. coli SRP RNA, which has been studied comparatively well in previous literature, yet the details of which specific metastable structures form as well as when they form are still under debate. Here, we combine thermodynamic and kinetic, deterministic, and stochastic models with automated and visual inspection of those systems to derive the most likely scenario of which substructures form at which point during transcription. The simulations do not only provide explanations for present experimental observations but also suggest previously unnoticed conformations that may be verified through future experimental studies.

Effects of PNA Sequence and Target Site Selection on Function of a 4.5S Non-Coding RNA.

Peptide nucleic acid (PNA) based antisense strategy is a promising therapeutic approach to specifically inhibit target gene expression. However, unlike protein coding genes, identification of an ideal PNA binding site for non-coding RNA is not straightforward. Here, we compare the inhibitory activities of PNA molecules that bind a non-coding 4.5S RNA called SRP RNA, a key component of the bacterial signal recognition particle (SRP). A 9-mer PNA (PNA9) complementary to the tetraloop region of the RNA was more potent in inhibiting its interaction with the SRP protein, compared to an 8-mer PNA (PNA8) targeting a stem-loop. PNA9, which contained a homo-pyrimidine sequence could form a triplex with the complementary stretch of RNA in vitro as confirmed using a fluorescent derivative of PNA9 (F-PNA13). The RNA-PNA complex formation resulted in inhibition of SRP function with PNA9 and F-PNA13, but not PNA8 highlighting the importance of target site selection. Surprisingly, F-PNA13 which was more potent in inhibiting SRP function in vitro, showed weaker antibacterial activity compared to PNA9 likely due to poor cell penetration of the longer PNA. Our results underscore the importance of suitable target site selection and optimum PNA length to develop better antisense molecules against non-coding RNA.

Pathologic Features of Anti-Ku Myositis.

OBJECTIVE: Characteristics of myositis with anti-Ku antibodies are poorly understood. The purpose of this study was to elucidate the pathologic features of myositis associated with anti-Ku antibodies, compared with immune-mediated necrotizing myopathy (IMNM) with anti-signal recognition particle (SRP) and anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) antibodies, in muscle biopsy-oriented registration cohorts in Japan and Germany. METHODS: We performed a retrospective pathology review of patients with anti-Ku myositis samples diagnosed in the Japanese and German cohorts. We evaluated histologic features and performed HLA phenotyping. RESULTS: Fifty biopsied muscle samples in the Japanese cohort and 10 in the German cohort were obtained. After exclusion of myositis-specific autoantibodies or other autoimmune connective tissue diseases, 26 samples (43%) of anti-Ku antibody-positive myositis were analyzed. All the samples shared some common features with IMNM, whereas they showed expression of MHC class II and clusters of perivascular inflammatory cells more frequently than the anti-SRP/HMGCR IMNM samples (71% vs 7%/16%; p < 0.005/<0.005; 64% vs 0%/0%; p < 0.005/<0.005). Anti-Ku myositis biopsies could be divided into 2 subgroups based on the extent of necrosis and regeneration. The group with more abundant necrosis and regeneration showed a higher frequency of MHC class II expression and perivascular inflammatory cell clusters. HLA phenotyping in the 44 available patients showed possible associations of HLA-DRB1*03:01, HLA-DRB1*11:01, and HLA-DQB1*03:01 (p = 0.0045, 0.019, and 0.027; odds ratio [OR] 50.2, 4.6, and 2.8; 95% CI 2.6-2942.1, 1.1-14.5, and 1.0-7.0) in the group with less conspicuous necrosis and regeneration. On the contrary, in the group of more abundant necrosis and regeneration, the allele frequencies of HLA-A*24:02, HLA-B*52:01, HLA-C*12:02, and HLA-DRB1*15:02 were lower than those of healthy controls (p = 0.0036, 0.027, 0.016, and 0.026; OR = 0.27, 0, 0, and 0; 95% CI 0.1-0.7, 0-0.8, 0-0.8, and 0-0.8). However, these HLA associations did not remain significant after statistical correction for multiple testing. DISCUSSION: While anti-Ku myositis shows necrotizing myopathy features, they can be distinguished from anti-SRP/HMGCR IMNM by their MHC class II expression and clusters of perivascular inflammatory cells. The HLA analyses suggest that anti-Ku myositis may have different subsets associated with myopathologic subgroups.

Whole-Exome Sequencing (WES) Reveals Novel Sex-Specific Gene Variants in Non-Alcoholic Steatohepatitis (MASH).

Non-alcoholic steatohepatitis (NASH, also known as MASH) is a severe form of non-alcoholic fatty liver disease (NAFLD, also known as MASLD). Emerging data indicate that the progression of the disease to MASH is higher in postmenopausal women and that genetic susceptibility increases the risk of MASH-related cirrhosis. This study aimed to investigate the association between genetic polymorphisms in MASH and sexual dimorphism. We applied whole-exome sequencing (WES) to identify gene variants in 8 age-adjusted matched pairs of livers from both male and female patients. Sequencing alignment, variant calling, and annotation were performed using standard methods. Polymerase chain reaction (PCR) coupled with Sanger sequencing and immunoblot analysis were used to validate specific gene variants. cBioPortal and Gene Set Enrichment Analysis (GSEA) were used for actionable target analysis. We identified 148,881 gene variants, representing 57,121 and 50,150 variants in the female and male cohorts, respectively, of which 251 were highly significant and MASH sex-specific (p < 0.0286). Polymorphisms in CAPN14, SLC37A3, BAZ1A, SRP54, MYH11, ABCC1, and RNFT1 were highly expressed in male liver samples. In female samples, Polymorphisms in RGSL1, SLC17A2, HFE, NLRC5, ACTN4, SBF1, and ALPK2 were identified. A heterozygous variant 1151G>T located on 18q21.32 for ALPK2 (rs3809983) was validated by Sanger sequencing and expressed only in female samples. Immunoblot analysis confirmed that the protein level of ß-catenin in female samples was 2-fold higher than normal, whereas ALPK2 expression was 0.5-fold lower than normal. No changes in the protein levels of either ALPK2 or ß-catenin were observed in male samples. Our study suggests that the perturbation of canonical Wnt/ß-catenin signaling observed in postmenopausal women with MASH could be the result of polymorphisms in ALPK2.

RN7SL1 may be translated under oncogenic conditions.

RN7SL1 (RNA component of signal recognition particle 7SL1), a component of the signal recognition particle, is a non-coding RNA possessing a small ORF (smORF). However, whether it is translated into peptides is unknown. Here, we generated the RN7SL1-Green Fluorescent Protein (GFP) gene, in which the smORF of RN7SL1 was replaced by GFP, introduced it into 293T cells, and observed cells emitting GFP fluorescence. Furthermore, RNA-seq of GFP-positive cells revealed that they were in an oncogenic state, suggesting that RN7SL1 smORF may be translated under special conditions.

Nonsense-mediated mRNA decay of mRNAs encoding a signal peptide occurs primarily after mRNA targeting to the endoplasmic reticulum.

Translation of messenger ribonucleic acids (mRNAs) encoding integral membrane proteins or secreted proteins occurs on the surface of the endoplasmic reticulum (ER). When a nascent signal peptide is synthesized from the mRNAs, the ribosome-nascent chain complex (RNC) is recognized by the signal recognition particle (SRP) and then transported to the surface of the ER. The appropriate targeting of the RNC-SRP complex to the ER is monitored by a quality control pathway, a nuclear cap-binding complex (CBC)-ensured translational repression of RNC-SRP (CENTRE). In this study, using ribosome profiling of CBC-associated and eukaryotic translation initiation factor 4E-associated mRNAs, we reveal that, at the transcriptomic level, CENTRE is in charge of the translational repression of the CBC-RNC-SRP until the complex is specifically transported to the ER. We also find that CENTRE inhibits the nonsense-mediated mRNA decay (NMD) of mRNAs within the CBC-RNC-SRP. The NMD occurs only after the CBC-RNC-SRP is targeted to the ER and after eukaryotic translation initiation factor 4E replaces CBC. Our data indicate dual surveillance for properly targeting mRNAs encoding integral membrane or secretory proteins to the ER. CENTRE blocks gene expression at the translation level before the CBC-RNC-SRP delivery to the ER, and NMD monitors mRNA quality after its delivery to the ER.

Loss of Preproinsulin Interaction with Signal Recognition Particle Activates Protein Quality Control, Decreasing mRNA Stability.

Many insulin gene variants alter the protein sequence and result in monogenic diabetes due to insulin insufficiency. However, the molecular mechanisms of various disease-causing mutations are unknown. Insulin is synthesized as preproinsulin containing a signal peptide (SP). SPs of secreted proteins are recognized by the signal recognition particle (SRP) or by another factor in a SRP-independent pathway. If preproinsulin uses SRP-dependent or independent pathways is still debatable. We demonstrate by the use of site-specific photocrosslinking that the SRP subunit, SRP54, interacts with the preproinsulin SP. Moreover, SRP54 depletion leads to the decrease of insulin mRNA and protein expression, supporting the involvement of the RAPP protein quality control in insulin biogenesis. RAPP regulates the quality of secretory proteins through degradation of their mRNA. We tested five disease-causing mutations in the preproinsulin SP on recognition by SRP and on their effects on mRNA and protein levels. We demonstrate that the effects of mutations are associated with their position in the SP and their severity. The data support diverse molecular mechanisms involved in the pathogenesis of these mutations. We show for the first time the involvement of the RAPP protein quality control pathway in insulin biogenesis that is implicated in the development of neonatal diabetes caused by the Leu13Arg mutation.

Cryo-EM structure of SRP68/72 reveals an extended dimerization domain with RNA-binding activity.

The signal recognition particle (SRP) is a critical component in protein sorting pathways in all domains of life. Human SRP contains six proteins bound to the 7S RNA and their structures and functions have been mostly elucidated. The SRP68/72 dimer is the largest SRP component and is essential for SRP function. Although the structures of the SRP68/72 RNA binding and dimerization domains have been previously reported, the structure and function of large portions of the SRP68/72 dimer remain unknown. Here, we analyse full-length SRP68/72 using cryo-EM and report that SRP68/72 depend on each other for stability and form an extended dimerization domain. This newly observed dimerization domain is both a protein- and RNA-binding domain. Comparative analysis with current structural models suggests that this dimerization domain undergoes dramatic translocation upon SRP docking onto SRP receptor and eventually comes close to the Alu domain. We propose that the SRP68/72 dimerization domain functions by binding and detaching the Alu domain and SRP9/14 from the ribosomal surface, thus releasing elongation arrest upon docking onto the ER membrane.

Rheumatoid Arthritis Associated With Anti-Signal Recognition Particle Immune-Mediated Necrotizing Myopathy: A Case Report.

Immune-mediated necrotizing myopathy (IMNM) is a rare subtype of idiopathic inflammatory myopathy that is characterized by severe subacute proximal weakness, myofiber necrosis, and significantly elevated serum creatine kinase. Anti-signal recognition particle (SRP) and anti-3-hydroxy-3-methylglutaryl-coenzyme-A reductase autoantibodies have been found in about two-thirds of patients with IMNM. This myopathy is usually idiopathic and there is a scarce literature concerning its association with connective tissue diseases. Herein, we report an unusual case of a young woman who presented with both rheumatoid arthritis and severe anti-SRP IMNM. Thankfully to a therapeutic protocol combining rituximab and cyclophosphamide, an important improvement was achieved, and notably no serious side effect was observed.

Structure of the GDP-bound state of the SRP GTPase FlhF.

The GTPase FlhF, a signal recognition particle (SRP)-type enzyme, is pivotal for spatial-numerical control and bacterial flagella assembly across diverse species, including pathogens. This study presents the X-ray structure of FlhF in its GDP-bound state at a resolution of 2.28 Å. The structure exhibits the classical N- and G-domain fold, consistent with related SRP GTPases such as Ffh and FtsY. Comparative analysis with GTP-loaded FlhF elucidates the conformational changes associated with GTP hydrolysis. These topological reconfigurations are similarly evident in Ffh and FtsY, and play a pivotal role in regulating the functions of these hydrolases.

Circulating mitochondrial long non-coding 7S RNA in primary health care patients with depression/anxiety.

BACKGROUND: The significant role of long non-coding 7S RNA in controlling mitochondrial transcription highlights its importance in mitochondrial function. Considering the suggested connection between mitochondrial dysfunction and the onset of mental disorders, this study aimed to explore the potential involvement of 7S RNA in the context of depression/anxiety. RESULTS: A total of 181 patients in primary health care (age 20-64 years) with depression/anxiety and 59 healthy controls were included in the study. 7S RNA was measured using quantitative real-time PCR in plasma samples collected before (baseline) and after 8 weeks of treatment (mindfulness or cognitive-based behavioral therapy). Upon adjustment for age and sex, the baseline plasma levels of 7S RNA were significantly higher in patients than in healthy controls (p < 0.001). Notably, post-treatment, there was a significant reduction in 7S RNA levels (p = 0.03). These changes in 7S RNA were related to the treatment response, as indicated by HADS-D (Hospital Anxiety and Depression Scale) scores (ß = -0.04, p = 0.04), even after accounting for baseline scores and other cofounders. CONCLUSION: The findings of this study indicate an association between plasma 7S RNA levels and depression/anxiety, as well as treatment response. While further confirmatory analyses are necessary, plasma 7S RNA holds promise as a potential predictive biomarker for both depression/anxiety and the treatment response within these disorders.

The P124A mutation of SRP14 alters its migration on SDS-PAGE without impacting its function.

SRP14 is a crucial protein subunit of the signal recognition particle (SRP), a ribonucleoprotein complex essential for co-translational translocation to the endoplasmic reticulum. During our investigation of SRP14 expression across diverse cell lines, we observe variations in its migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), with some cells exhibiting slower migration and others migrating faster. However, the cause of this phenomenon remains elusive. Our research rules out alternative splicing as the cause and, instead, identifies the presence of a P124A mutation in SRP14 (SRP14 P124A) among the faster-migrating variants, while the slower-migrating variants lack this mutation. Subsequent ectopic expression of wild-type SRP14 P124 or SRP14 WT and SRP14 P124A in various cell lines confirms that the P124A mutation indeed leads to faster migration of SRP14. Further mutagenesis analysis shows that the P117A and A121P mutations within the alanine-rich domain at the C-terminus of SRP14 are responsible for migration alterations on SDS-PAGE, whereas mutations outside this domain, such as P39A, Y27F, and T45A, have no such effect. Furthermore, the ectopic expression of SRP14 WT and SRP14 P124A yields similar outcomes in terms of SRP RNA stability, cell morphology, and cell growth, indicating that SRP14 P124A represents a natural variant of SRP14 and retains comparable functionality. In conclusion, the substitution of proline for alanine in the alanine-rich tail of SRP14 results in faster migration on SDS-PAGE, but has little effect on its function.

Differences in muscle magnetic resonance imaging findings between anti-signal recognition particle antibody-positive myopathy and anti-aminoacyl-tRNA synthetase antibody-positive myositis.

OBJECTIVES: To compare the findings of muscle magnetic resonance imaging (MRI) between anti-signal recognition particle antibody-positive myopathy (anti-SRP myopathy) and anti-aminoacyl-tRNA synthetase antibody-positive myositis (anti-ARS myositis). METHODS: Of the patients newly diagnosed with polymyositis (PM)/dermatomyositis (DM) and immune-mediated necrotising myopathy (IMNM) admitted to our Department between April 2012 and December 2021, those who met the eligibility criteria of positive for anti-SRP or anti-ARS antibodies and thigh MRI at the time of diagnosis were included. We compared the lesion sites and MRI findings of the thigh muscles that were classified into oedema, fascial oedema, fatty replacement, and muscle atrophy between the three groups of anti-SRP myopathy, anti-Jo-1 antibody-positive myositis, and non-Jo-1 antibody-positive myositis. RESULTS: Of the 98 PM/DM and IMNM patients, five anti-SRP myopathy patients and 11 anti-Jo-1-positive and 22 non-Jo-1 antibody-positive patients with myositis were included. The SRP group showed significantly higher blood levels of myogenic enzymes such as serum creatinine kinase (CK) than the other groups (p=0.01). In thigh MRI findings, despite oedema in most cases in anti-SRP and anti-ARS groups, fascial oedema was identified only in the ARS group, frequently in Jo-1 positive patients in particular. Moreover, gluteus maximus muscle lesions occurred more frequently in the SRP group than in the ARS group (p=0.008). CONCLUSIONS: A comparison of thigh MRI between anti-SRP myopathy and anti-ARS myositis showed different findings and lesion sites reflecting the different pathophysiology that may contribute to their diagnosis.

Membrane protein chaperone and sodium chloride modulate the kinetics and morphology of amyloid beta aggregation.

Protein aggregation is a biological phenomenon caused by the accumulation of misfolded proteins. Amyloid beta (Aß) peptides are derived from the cleavage of a larger membrane protein molecule and accumulate to form plaques extracellularly. According to the amyloid hypothesis, accumulation of Aß aggregates in the brain is primarily responsible for the pathogenesis of Alzheimer's disease (AD). Therefore, the disassembly of Aß aggregates may provide opportunities for alleviating or treating AD. Here, we show that the novel protein targeting machinery from chloroplast, chloroplast signal recognition particle 43 (cpSRP43), is an ATP-independent membrane protein chaperone that can both prevent and reverse Aß aggregation effectively. Using of thioflavin T dye, we obtained the aggregation kinetics of Aß aggregation and determined that the chaperone prevents Aß aggregation in a concentration-dependent manner. Size exclusion chromatography and sedimentation assays showed that 10-fold excess of cpSRP43 can keep Aß in the soluble monomeric form. Electron microscopy showed that the fibril structure was disrupted in the presence of this chaperone. Importantly, cpSRP43 utilizes the binding energy to actively remodel the preformed Aß aggregates without assistance by a co-chaperone and ATP, emphasizing its unique function among protein chaperones. Moreover, when sodium chloride concentration is higher than 25 mm, the Aß aggregation rate increases drastically to form tightly associated aggregates and generate more oligomers. Our results demonstrate that the presence of cpSRP43 and low NaCl levels inhibit or retard Aß peptide aggregation, potentially opening new avenues to strategically develop an effective treatment for AD.