First-line nab-paclitaxel plus carboplatin for patients with advanced non-small cell lung cancer: Final results of the NEPTUN study.

Real-world data on the first-line treatment of patients with advanced non-small cell lung cancer (NSCLC) are still limited. The NEPTUN study evaluated effectiveness and safety of first-line nab-paclitaxel (Abraxane) plus carboplatin (nab-P/C) in patients with advanced NSCLC in routine clinical practice in Germany. Patients included in our study were aged ≥18 years, diagnosed with locally advanced or metastatic NSCLC and with decision for first-line nab-P/C in routine clinical practice. Primary objective was 6-month progression-free survival rate (PFS6), secondary objectives included overall survival (OS), overall response rate (ORR) and safety. From 2016 to 2019, 408 patients from 75 sites were enrolled. PFS6 was 39.5% (95% CI: 34.2-44.8), median PFS was 5.1 months (95% CI: 4.6-5.6), ORR was 42.9% (95% CI: 37.7-48.2). Median OS was 10.5 months (95% CI: 9.2-11.6). In subgroup analyses, median OS for squamous vs non-squamous histology was 11.5 months (95% CI: 9.2-13.8) vs 9.8 months (95% CI: 8.1-11.3) and for patients aged ≥70 vs <70 years median OS was 12.4 months (95% CI: 9.8-15.1) vs 9.6 months (95% CI: 7.7-11.1). Adverse events (AEs) related to nab-paclitaxel were reported in 247 (66.4%) patients, while carboplatin-related AEs were documented in 224 (60.2%) patients. Most frequently related AEs were leukopenia (22.3%) for nab-paclitaxel and anemia (20.2%) for carboplatin. Nab-P/C-related deaths were reported in 2 (0.5%) patients (sepsis and neutropenic sepsis). No new or unexpected safety signals emerged. These results support the effectiveness and safety of first-line nab-P/C in patients with advanced NSCLC reported in the pivotal trial and highlight the clinical value of this regimen in the real-world setting.

FOLFIRINOX or gemcitabine/nab-paclitaxel in advanced pancreatic adenocarcinoma: A novel validated prognostic score to facilitate treatment decision-making in real-world.

There is no prospective, randomised head-to-head trial comparing first-line FOLFIRINOX and gemcitabine/nab-paclitaxel in advanced pancreatic cancer. We assess real-world effectiveness and quality of life (QoL) of both regimens using a new prognostic score. This analysis includes 1540 patients with advanced pancreatic cancer from the prospective, clinical cohort study Tumour Registry Pancreatic Cancer separated into learning (n = 1027) and validation sample (n = 513). The Pancreatic Cancer Score (PCS) was developed using multivariate Cox regression. We compared overall survival (OS) and time to deterioration (TTD) for longitudinal QoL between first-line FOLFIRINOX (n = 407) and gemcitabine/nab-paclitaxel (n = 655) according to patients' prognostic risk, after inverse probability of treatment weighting (IPTW) by propensity score analysis. The PCS includes nine independent prognostic factors for survival: female sex, BMI ≥24/unknown, ECOG performance status ≥1, Charlson comorbidity index ≥1, tumour staging IV/unknown at primary diagnosis, liver metastases, bilirubin >1.5× upper limit of normal (ULN), leukocytes >ULN and neutrophil-to-lymphocyte ratio ≥4. Median OS of the validation sample was 11.4 (95% confidence interval [CI]: 10.4-14.4), 8.5 (95% CI: 6.8-9.6) and 5.9 months (95% CI: 4.0-7.4) for favourable- (0-3 risk factors), intermediate- (4-5 factors) and poor-risk group (6-9 factors), respectively. After IPTW, only poor-risk patients had significantly longer median OS and TTD of overall QoL with FOLFIRINOX (OS: 6.9 months, 95% CI: 3.9-13.3; TTD: 10.6 months, 95% CI: 2.0-14.1) vs gemcitabine/nab-paclitaxel (OS: 4.0 months, 95% CI: 2.8-4.8; TTD: 4.1 months, 95% CI: 2.4-4.5). Our novel PCS may facilitate treatment decisions in clinical routine of advanced pancreatic cancer, since only poor-risk, but not favourable-risk patients, seem to benefit from intensified treatment with FOLFIRINOX.

A dual role of the ribosome-bound chaperones RAC/Ssb in maintaining the fidelity of translation termination.

The yeast ribosome-associated complex RAC and the Hsp70 homolog Ssb are anchored to the ribosome and together act as chaperones for the folding and co-translational assembly of nascent polypeptides. In addition, the RAC/Ssb system plays a crucial role in maintaining the fidelity of translation termination; however, the latter function is poorly understood. Here we show that the RAC/Ssb system promotes the fidelity of translation termination via two distinct mechanisms. First, via direct contacts with the ribosome and the nascent chain, RAC/Ssb facilitates the translation of stalling-prone poly-AAG/A sequences encoding for polylysine segments. Impairment of this function leads to enhanced ribosome stalling and to premature nascent polypeptide release at AAG/A codons. Second, RAC/Ssb is required for the assembly of fully functional ribosomes. When RAC/Ssb is absent, ribosome biogenesis is hampered such that core ribosomal particles are structurally altered at the decoding and peptidyl transferase centers. As a result, ribosomes assembled in the absence of RAC/Ssb bind to the aminoglycoside paromomycin with high affinity (KD = 76.6 nM) and display impaired discrimination between stop codons and sense codons. The combined data shed light on the multiple mechanisms by which the RAC/Ssb system promotes unimpeded biogenesis of newly synthesized polypeptides.

Final results from IMPROVE: a randomized, controlled, open-label, two-arm, cross-over phase IV study to determine patients' preference for everolimus in combination with exemestane or capecitabine in combination with bevacizumab in advanced HR-positive, HER2-negative breast cancer.

BACKGROUND: The objective of the IMPROVE study was patients' preference for either endocrine-based therapy or combined chemo- and anti-angiogenic therapy in advanced HR-positive/HER2-negative breast cancer. METHODS: In this randomized, cross-over phase IV study, 77 patients were recruited in 26 sites in Germany. Patients were randomized 1:1 to receive either capecitabine plus bevacizumab (Cap+Bev) as first-line therapy followed by cross-over to everolimus plus exemestane (Eve+Exe) as second-line therapy (Arm A) or the reverse sequence (Arm B). The primary endpoint was patients' preference for either regimen, assessed by the Patient Preference Questionnaire 12 weeks after cross-over. Key secondary endpoints included progression-free survival (PFS), overall survival (OS), safety, and quality of life (QoL). RESULTS: 61.5% of patients preferred Cap+Bev (p = 0.1653), whereas 15.4% preferred Eve+Exe and 23.1% were indecisive. Physicians showed a similar tendency towards Cap+Bev (58.1%) as the preferred regimen versus Eve+Exe (32.3%). Median first-line PFS was longer for Cap+Bev than for Eve+Exe (11.1 months versus 3.5 months). Median second-line PFS was similar between Cap+Bev and Eve+Exe (3.6 months versus 3.7 months). Median OS was comparable between Arm A (28.8 months) and Arm B (24.7 months). 73.0% and 52.6% (first-/second-line, Cap+Bev) and 54.1% and 52.9% (first-/second-line, Eve+Exe) of patients experienced grade 3/4 TEAEs. No treatment-related deaths occurred. QoL and treatment satisfaction were not significantly different between arms or treatment lines. CONCLUSIONS: Patients tended to favor Cap+Bev over Eve+Exe, which was in line with physicians' preference. Cap+Bev showed superior first-line PFS, while QoL was similar in both arms. No new safety signals were reported. TRIAL REGISTRATION: EudraCT No: 2013-005329-22. Registered on 19 August 20.

Comparison of complex modeling strategies for prediction of a binary outcome based on a few, highly correlated predictors.

Motivated by a clinical prediction problem, a simulation study was performed to compare different approaches for building risk prediction models. Robust prediction models for hospital survival in patients with acute heart failure were to be derived from three highly correlated blood parameters measured up to four times, with predictive ability having explicit priority over interpretability. Methods that relied only on the original predictors were compared with methods using an expanded predictor space including transformations and interactions. Predictors were simulated as transformations and combinations of multivariate normal variables which were fitted to the partly skewed and bimodally distributed original data in such a way that the simulated data mimicked the original covariate structure. Different penalized versions of logistic regression as well as random forests and generalized additive models were investigated using classical logistic regression as a benchmark. Their performance was assessed based on measures of predictive accuracy, model discrimination, and model calibration. Three different scenarios using different subsets of the original data with different numbers of observations and events per variable were investigated. In the investigated setting, where a risk prediction model should be based on a small set of highly correlated and interconnected predictors, Elastic Net and also Ridge logistic regression showed good performance compared to their competitors, while other methods did not lead to substantial improvements or even performed worse than standard logistic regression. Our work demonstrates how simulation studies that mimic relevant features of a specific data set can support the choice of a good modeling strategy.

The Hsp70 homolog Ssb and the 14-3-3 protein Bmh1 jointly regulate transcription of glucose repressed genes in Saccharomyces cerevisiae.

Chaperones of the Hsp70 family interact with a multitude of newly synthesized polypeptides and prevent their aggregation. Saccharomyces cerevisiae cells lacking the Hsp70 homolog Ssb suffer from pleiotropic defects, among others a defect in glucose-repression. The highly conserved heterotrimeric kinase SNF1/AMPK (AMP-activated protein kinase) is required for the release from glucose-repression in yeast and is a key regulator of energy balance also in mammalian cells. When glucose is available the phosphatase Glc7 keeps SNF1 in its inactive, dephosphorylated state. Dephosphorylation depends on Reg1, which mediates targeting of Glc7 to its substrate SNF1. Here we show that the defect in glucose-repression in the absence of Ssb is due to the ability of the chaperone to bridge between the SNF1 and Glc7 complexes. Ssb performs this post-translational function in concert with the 14-3-3 protein Bmh, to which Ssb binds via its very C-terminus. Raising the intracellular concentration of Ssb or Bmh enabled Glc7 to dephosphorylate SNF1 even in the absence of Reg1. By that Ssb and Bmh efficiently suppressed transcriptional deregulation of Δreg1 cells. The findings reveal that Ssb and Bmh comprise a new chaperone module, which is involved in the fine tuning of a phosphorylation-dependent switch between respiration and fermentation.

The Hsp70 homolog Ssb is essential for glucose sensing via the SNF1 kinase network.

Yeast senses the availability of external energy sources via multiple interconnected signaling networks. One of the central components is SNF1, the homolog of mammalian AMP-activated protein kinase, which in yeast is essential for the expression of glucose-repressed genes. When glucose is available hyperphosphorylated SNF1 is rendered inactive by the type 1 protein phosphatase Glc7. Dephosphorylation requires Reg1, which physically targets Glc7 to SNF1. Here we show that the chaperone Ssb is required to keep SNF1 in the nonphosphorylated state in the presence of glucose. Using a proteome approach we found that the Deltassb1Deltassb2 strain displays alterations in protein expression and suffers from phenotypic characteristics reminiscent of glucose repression mutants. Microarray analysis revealed a correlation between deregulation on the protein and on the transcript level. Supporting studies uncovered that SSB1 was an effective multicopy suppressor of severe growth defects caused by the Deltareg1 mutation. Suppression of Deltareg1 by high levels of Ssb was coupled to a reduction of Snf1 hyperphosphorylation back to the wild-type phosphorylation level. The data are consistent with a model in which Ssb is crucial for efficient regulation within the SNF1 signaling network, thereby allowing an appropriate response to changing glucose levels.

Stress-resistant Translation of Cathepsin L mRNA in Breast Cancer Progression.

The cysteine protease cathepsin L (CTSL) is often thought to act as a tumor promoter by enhancing tumor progression and metastasis. This goes along with increased CTSL activity in various tumor entities; however, the mechanisms leading to high CTSL levels are incompletely understood. With the help of the polyoma middle T oncogene driven breast cancer mouse model expressing a human CTSL genomic transgene, we show that CTSL indeed promotes breast cancer metastasis to the lung. During tumor formation and progression high expression levels of CTSL are maintained by enduring translation of CTSL mRNA. Interestingly, human breast cancer specimens expressed the same pattern of 5' untranslated region (UTR) splice variants as the transgenic mice and the human cancer cell line MDA-MB 321. By polyribosome profiling of tumor tissues and human breast cancer cells, we observe an intrinsic resistance of CTSL to stress-induced shutdown of translation. This ability can be attributed to all 5' UTR variants of CTSL and is not dependent on a previously described internal ribosomal entry site motif. In conclusion, we provide in vivo functional evidence for overexpressed CTSL as a promoter of lung metastasis, whereas high CTSL levels are maintained during tumor progression due to stress-resistant mRNA translation.

Final Results from RIBBIT: A Randomized Phase III Study to Evaluate Efficacy and Quality of Life in Patients with Metastatic Hormone Receptor-Positive, HER2-Negative Breast Cancer Receiving Ribociclib in Combination with Endocrine Therapy or Chemotherapy with or without Bevacizumab in the First-Line Setting.

Background: We investigated the efficacy and health-related quality of life (HRQoL) in patients receiving either ribociclib plus endocrine therapy (ET) or chemotherapy with/without bevacizumab as first-line treatment of metastatic hormone receptor (HR)-positive, HER2-negative breast cancer (BC). Patients and Methods: In this randomized, phase III study (RIBBIT), 38 patients diagnosed with metastatic HR-positive, HER2-negative BC with presence of visceral metastases recruited between May 2018 and December 2020 were randomly assigned in a 1:1 ratio to either arm A (ribociclib + ET) or arm B (chemotherapy with/without bevacizumab) at 12 sites in Germany. The primary endpoint was progression-free survival (PFS). Secondary endpoints included overall response rate (ORR), overall survival (OS), patient-reported HRQoL, and frequency and type of adverse events. During study conduction, the recruitment rate was persistently and considerably lower than originally expected. Therefore, the recruitment was ended prematurely. The study was initially designed to enroll and randomize 158 patients. Results: Median [95% CI] PFS was 27.3 months [19.1 - NA, parameter not estimable] in arm A and 15.8 months [8.2 - NA] in arm B. Complete responses were achieved only in arm A (n = 2, 10.5%). The ORR [95% CI] between arm A (57.9% [33.5-79.7]) and arm B (52.6% [28.9-75.6]) was comparable. Median OS [95% CI] was not reached in arm A, while in arm B median OS was 28.4 months [25.0 - NA]. Patients in arm A reported less burden by side-effects. No new safety signals emerged. Conclusion: Treatment of patients with visceral metastatic HR-positive, HER2-negative BC with ribociclib in combination with ET showed a tendency toward a more favorable clinical outcome. Despite small numbers of patients and sites, this head-to-head comparison with chemotherapy supports the use of ribociclib with ET in patients with visceral metastasis at risk of fast disease progression.

The ribosome-bound Hsp70 homolog Ssb of Saccharomyces cerevisiae.

The Hsp70 homolog Ssb directly binds to the ribosome and contacts a variety of newly synthesized polypeptide chains as soon as they emerge from the ribosomal exit tunnel. For this reason a general role of Ssb in the de novo folding of newly synthesized proteins is highly suggestive. However, for more than a decade client proteins which require Ssb for proper folding have remained elusive. It was therefore speculated that Ssb, despite its ability to interact with a large variety of nascent polypeptides, may assist the folding of only a small and specific subset. Alternatively, it has been suggested that Ssb's function may be limited to the protection of nascent polypeptides from aggregation until downstream chaperones take over and actively fold their substrates. There is also evidence that Ssb, in parallel to a classical chaperone function, is involved in the regulation of cellular signaling processes. Here we aim to summarize what is currently known about Ssb's multiple functions and what remains to be ascertained by future research.

Sleep Characteristics in Italian Children During Home Confinement Due to Covid-19 Outbreak.

OBJECTIVE: Italy faced one of the first large clusters of COVID-19 infections worldwide. Home confinement and social distancing could have negatively impacted sleep habits and prevalence of sleep disorders in children, which may be also linked with altered emotional processes. The present study focused on clinical aspects related to sleep, insomnia and emotions in Italian children aged 0-to-12 years during home confinement due to COVID-19 outbreak. METHOD: An online survey was systematically distributed in all Italian territories by contacting regional offices of the Italian Ministry of Instruction, University and Research (MIUR) and schools with available contact. All respondents had to be parents of at least one child aged 0 to 12 years old. Information on sociodemographic variables, sleep habits, sleep health behaviors, sleep disorders and mood were collected. RESULTS: Parents of 2361 children (mean age: 8.1 ± 2.62 years; 1148 females; 1213 males) answered the survey. 1.2% of children was between 0 and 2 years old; 15.3% within 3 to 5 years and 83.3% within 6 and 12 years. In all group ages, late bedtime was observed (most of them after 9 p.m.). 59.4% of all children presented at least one clinical diagnostic criterion for childhood insomnia. Logistic regression model showed that presence of at least one criterion for childhood insomnia was associated to younger age, negative mood, current parental insomnia, being the only child, presence of any other sleep disorder, and sleep hygiene behaviors. CONCLUSIONS: Data indicate an alarming increase of prevalence of insomnia related problems in Italian children during home confinement with respect to previous data. This was found to be associated with poor sleep hygiene and negative mood. Clinical programs targeting insomnia, sleep health behaviors and emotional processes should be implemented in pediatric primary care in order to prevent the development of sleep problems in a post-pandemic situation.

First-line nab-paclitaxel plus carboplatin for patients with advanced non-small cell lung cancer: Results of the NEPTUN study.

BACKGROUND: Platinum-based chemotherapy remains a first-line standard of care for approximately 30% of patients with non-small cell lung cancer (NSCLC) not harboring a druggable alteration. Favorable efficacy and safety of the nab-paclitaxel/carboplatin (nab-P/C) combination was shown in the pivotal phase 3 trial. However, information on effectiveness of nab-P/C in a real-world setting in Germany is missing. The NEPTUN study prospectively investigated the effectiveness and safety of nab-P/C in patients with advanced NSCLC in a real-world setting. METHODS: Patients with advanced or metastatic NSCLC received first-line nab-P/C according to clinical routine. The primary endpoint was 6-month progression-free survival rate (PFS6). Other endpoints included further effectiveness parameters, safety and quality of life. Data were analyzed descriptively. RESULTS: 408 patients were enrolled. PFS6 was 40.8% (95% confidence interval [CI], 35.3-46.2); median PFS was 5.2 months (95% CI, 4.5-5.7). overall response rate was 41.5% (95% CI, 36.3-46.8). Median overall survival (OS) was 10.5 months (95% CI, 9.2-11.6). Subgroup analyses revealed median OS for squamous versus non-squamous histology (11.8 months [95% CI, 9.2-13.8] vs. 9.6 months [95% CI, 7.7-11.2]) and age ≥70 versus <70 years (11.7 months [95% CI, 9.4-14.3] vs. 9.6 months [95% CI, 7.5-11.2]). Most common treatment-emergent adverse events (TEAEs) were anemia (26.5%), leukopenia (25.7%), and thrombocytopenia (16.6%). Mostly reported grade 3/4 TEAEs were leukopenia (10.2%), anemia (8.6%), and pneumonia (5.1%). nab-paclitaxel-related deaths as reported by the investigator occurred in 0.8% of patients. CONCLUSION: These real-world data support the effectiveness and safety of nab-P/C as first-line treatment for patients with advanced NSCLC independent of tumor histology. The results are comparable with the pivotal phase 3 trial. No new safety signals emerged.

Insomnia in the Italian Population During Covid-19 Outbreak: A Snapshot on One Major Risk Factor for Depression and Anxiety.

Objectives: One of the largest clusters of Covid-19 infections was observed in Italy. The population was forced to home confinement, exposing individuals to increased risk for insomnia, which is, in turn, associated with depression and anxiety. Through a cross-sectional online survey targeting all Italian adult population (≥18 yrs), insomnia prevalence and its interactions with relevant factors were investigated. Methods: The survey was distributed from 1st April to 4th May 2020. We collected information on insomnia severity, depression, anxiety, sleep hygiene behaviors, dysfunctional beliefs about sleep, circadian preference, emotion regulation, cognitive flexibility, perceived stress, health habits, self-report of mental disorders, and variables related to individual difference in life changes due to the pandemic's outbreak. Results: The final sample comprised 1,989 persons (38.4 ± 12.8 yrs). Prevalence of clinical insomnia was 18.6%. Results from multivariable linear regression showed that insomnia severity was associated with poor sleep hygiene behaviors [ß = 0.11, 95% CI (0.07-0.14)]; dysfunctional beliefs about sleep [ß = 0.09, 95% CI (0.08-0.11)]; self-reported mental disorder [ß = 2.51, 95% CI (1.8-3.1)]; anxiety [ß = 0.33, 95% CI (0.25-0.42)]; and depression [ß = 0.24, 95% CI (0.16-0.32)] symptoms. Conclusion: An alarming high prevalence of clinical insomnia was observed. Results suggest that clinical attention should be devoted to problems of insomnia in the Italian population with respect to both prevention and treatment.

Buparlisib in combination with tamoxifen in pretreated patients with hormone receptor-positive, HER2-negative advanced breast cancer molecularly stratified for PIK3CA mutations and loss of PTEN expression.

The PIKTAM study evaluated the efficacy and safety of the PI3K inhibitor buparlisib in combination with tamoxifen in hormone receptor-positive (HR+ ), HER2-negative advanced breast cancer patients after failure of prior endocrine therapy. In this open-label, single-arm phase II trial, 25 patients were enrolled in 11 sites in Germany. Patients were stratified according to PIK3CA mutation status (tissue and cfDNA from serum samples) and/or loss of PTEN expression. Patients received buparlisib (100 mg) and tamoxifen (20 mg) once daily on a continuous schedule (28-day cycle) until progression or unacceptable toxicity. Primary endpoint was overall 6-month progression-free survival (PFS) rate. Key secondary endpoints included the 6-month PFS rate in subpopulations, PFS, overall survival, overall response rate (ORR), disease control rate (DCR), and safety. Overall, the 6-month PFS rate was 33.3% (n/N = 7/21, one-sided 95% CI 16.8-100) and median PFS was 6.1 (CI 2.6-10.6) months. The ORR and DCR were 12.5% and 44%. The PIK3CA-mutated subgroup consistently showed the highest 6-month PFS rate (62.5%, n/N = 5/8), median PFS (8.7 months), ORR (40%), and DCR (80%). No new safety signals emerged. Most common adverse events were gastrointestinal disorders (56%), psychiatric/mood disorders (48%), skin rash/hypersensitivity (44%), cardiovascular (40%), and hepatic (32%) events. The trial was prematurely terminated due to the substantially altered risk-benefit profile of buparlisib. Nevertheless, PIK3CA mutations emerged as a clinically feasible and useful biomarker for combined PI3K inhibition and endocrine therapy in patients with HR+ breast cancer. Further biomarker-stratified studies with isoform-specific PI3K inhibitors are warranted. EudraCT No: 2014-000599-24.

Development and validation of a prognostic model for survival in patients treated with venoarterial extracorporeal membrane oxygenation: the PREDICT VA-ECMO score.

AIMS: Several scoring systems have been introduced for prognostication after initiating venoarterial extracorporeal membrane oxygenation (VA-ECMO) therapy. However, static scores offer limited guidance once VA-ECMO is implanted, although continued allocation of healthcare resources is critical. Patients requiring continued VA-ECMO support are extremely unstable, with minimal heart function and multi-organ failure in most cases. The aim of the present study was to develop and validate a dynamic prognostic model for patients treated with VA-ECMO. METHODS AND RESULTS: A derivation cohort included 205 all-comers undergoing VA-ECMO implantation at a tertiary referral hospital (51% received VA-ECMO during resuscitation and 43% had severe shock). Two prediction models based on point-of-care biomarkers were developed using penalised logistic regression in an elastic net approach. A validation cohort was recruited from an independent tertiary referral hospital. Comparators for the prediction of hospital survival were the SAVE score (area under the receiver operation characteristic curve (AUC) of 0.686), the SAPS score (AUC 0.679), the APACHE score (AUC 0.662) and the SOFA score (AUC 0.732) in 6-hour survivors. The 6-hour PREDICT VA-ECMO score (based on lactate, pH and standard bicarbonate concentration) outperformed the comparator scores with an AUC of 0.823. The 12-hour PREDICT VA-ECMO integrated lactate, pH and standard bicarbonate concentration at 1 hour, 6 hours and 12 hours after ECMO insertion allowed even better prognostication (AUC 0.839). Performance of the scores in the external validation cohort was good (AUCs 0.718 for the 6-hour score and 0.735 for the 12-hour score, respectively). CONCLUSION: In patients requiring VA-ECMO therapy, a dynamic score using three point-of-care biomarkers predicts hospital mortality with high reliability. Furthermore, the PREDICT scores are the first scores for extracorporeal cardiopulmonary resuscitation patients.

Release factor eRF3 mediates premature translation termination on polylysine-stalled ribosomes in Saccharomyces cerevisiae.

Ribosome stalling is an important incident enabling the cellular quality control machinery to detect aberrant mRNA. Saccharomyces cerevisiae Hbs1-Dom34 and Ski7 are homologs of the canonical release factor eRF3-eRF1, which recognize stalled ribosomes, promote ribosome release, and induce the decay of aberrant mRNA. Polyadenylated nonstop mRNA encodes aberrant proteins containing C-terminal polylysine segments which cause ribosome stalling due to electrostatic interaction with the ribosomal exit tunnel. Here we describe a novel mechanism, termed premature translation termination, which releases C-terminally truncated translation products from ribosomes stalled on polylysine segments. Premature termination during polylysine synthesis was abolished when ribosome stalling was prevented due to the absence of the ribosomal protein Asc1. In contrast, premature termination was enhanced, when the general rate of translation elongation was lowered. The unconventional termination event was independent of Hbs1-Dom34 and Ski7, but it was dependent on eRF3. Moreover, premature termination during polylysine synthesis was strongly increased in the absence of the ribosome-bound chaperones ribosome-associated complex (RAC) and Ssb (Ssb1 and Ssb2). On the basis of the data, we suggest a model in which eRF3-eRF1 can catalyze the release of nascent polypeptides even though the ribosomal A-site contains a sense codon when the rate of translation is abnormally low.

Ribosome-associated complex and Ssb are required for translational repression induced by polylysine segments within nascent chains.

When a polyadenylated nonstop transcript is fully translated, a complex consisting of the ribosome, the nonstop mRNA, and the C-terminally polylysine-tagged protein is generated. In Saccharomyces cerevisiae, a 3-step quality control system prevents formation of such dead-end complexes. Nonstop mRNA is rapidly degraded, translation of nonstop mRNA is repressed, and finally, nonstop proteins are cotranslationally degraded. Nonstop mRNA degradation depends on Ski7 and the exosome; nonstop protein degradation depends on the ribosome-bound E3 ligase Ltn1 and the proteasome. However, components which mediate translational repression of nonstop mRNA have previously not been identified. Here we show that the ribosome-bound chaperone system consisting of the ribosome-associated complex (RAC) and the Hsp70 homolog Ssb is required to stabilize translationally repressed ribosome-polylysine protein complexes, without affecting the folding or the degradation of polylysine proteins. As a consequence, in the absence of RAC/Ssb, polylysine proteins escaped translational repression and subsequently folded into their native conformation. This active role of RAC/Ssb in the quality control of polylysine proteins significantly contributed to the low level of expression of nonstop transcripts in vivo.