RESUMEN
Ribosome stalling at tandem CGA codons or poly(A) sequences activates quality controls for nascent polypeptides including ribosome-associated quality control (RQC) and no-go mRNA decay (NGD). In RQC pathway, Hel2-dependent uS10 ubiquitination and the RQC-trigger (RQT) complex are essential for subunit dissociation, and Ltn1-dependent ubiquitination of peptidyl-tRNA in the 60S subunit requires Rqc2. Here, we report that polytryptophan sequences induce Rqc2-independent RQC. More than 11 consecutive tryptophan residues induced RQC in a manner dependent on Hel2-mediated ribosome ubiquitination and the RQT complex. Polytryptophan sequence-mediated RQC was not coupled with CAT-tailing, and Rqc2 was not required for Ltn1-dependent degradation of the arrest products. Eight consecutive tryptophan residues located at the region proximal to the peptidyl transferase center in the ribosome tunnel inhibited CAT-tailing by tandem CGA codons. Polytryptophan sequences also induced Hel2-mediated canonical RQC-coupled NGD and RQC-uncoupled NGD outside the stalled ribosomes. We propose that poly-tryptophan sequences induce Rqc2-independent RQC, suggesting that CAT-tailing in the 60S subunit could be modulated by the polypeptide in the ribosome exit tunnel.
Asunto(s)
Péptidos/metabolismo , Biosíntesis de Proteínas , Proteínas de Unión al ARN/fisiología , Subunidades Ribosómicas Grandes de Eucariotas/metabolismo , Proteínas de Saccharomyces cerevisiae/fisiología , Codón , Péptidos/química , Estabilidad del ARN , ARN Mensajero/metabolismo , Aminoacil-ARN de Transferencia/metabolismo , Saccharomyces cerevisiae/genética , Saccharomyces cerevisiae/metabolismo , Triptófano/metabolismo , UbiquitinaciónRESUMEN
Comprehensive genome-wide analysis has revealed the presence of translational elements in the 3' untranslated regions (UTRs) of human transcripts. However, the mechanisms by which translation is initiated in 3' UTRs and the physiological function of their products remain unclear. This study showed that eIF4G drives the translation of various downstream open reading frames (dORFs) in 3' UTRs. The 3' UTR of GCH1, which encodes GTP cyclohydrolase 1, contains an internal ribosome entry site (IRES) that initiates the translation of dORFs. An in vitro reconstituted translation system showed that the IRES in the 3' UTR of GCH1 required eIF4G and conventional translation initiation factors, except eIF4E, for AUG-initiated translation of dORFs. The 3' UTR of GCH1-mediated translation was resistant to the mTOR inhibitor Torin 1, which inhibits cap-dependent initiation by increasing eIF4E-unbound eIF4G. eIF4G was also required for the activity of various elements, including polyU and poliovirus type 2, a short element thought to recruit ribosomes by base-pairing with 18S rRNA. These findings indicate that eIF4G mediates translation initiation of various ORFs in mammalian cells, suggesting that the 3' UTRs of mRNAs may encode various products.
Asunto(s)
Factor 4G Eucariótico de Iniciación/genética , GTP Ciclohidrolasa/genética , Sistemas de Lectura Abierta/genética , Serina-Treonina Quinasas TOR/genética , Regiones no Traducidas 3'/genética , Factor 4E Eucariótico de Iniciación/genética , Humanos , Naftiridinas/farmacología , Poliovirus/genética , Biosíntesis de Proteínas/genética , Caperuzas de ARN/genética , ARN Mensajero/genética , ARN Ribosómico 18S/genética , Ribosomas/genética , Serina-Treonina Quinasas TOR/antagonistas & inhibidoresRESUMEN
Ribosome-associated quality control (RQC) represents a rescue pathway in eukaryotic cells that is triggered upon translational stalling. Collided ribosomes are recognized for subsequent dissociation followed by degradation of nascent peptides. However, endogenous RQC-inducing sequences and the mechanism underlying the ubiquitin-dependent ribosome dissociation remain poorly understood. Here, we identified SDD1 messenger RNA from Saccharomyces cerevisiae as an endogenous RQC substrate and reveal the mechanism of its mRNA-dependent and nascent peptide-dependent translational stalling. In vitro translation of SDD1 mRNA enabled the reconstitution of Hel2-dependent polyubiquitination of collided disomes and, preferentially, trisomes. The distinct trisome architecture, visualized using cryo-EM, provides the structural basis for the more-efficient recognition by Hel2 compared with that of disomes. Subsequently, the Slh1 helicase subunit of the RQC trigger (RQT) complex preferentially dissociates the first stalled polyubiquitinated ribosome in an ATP-dependent manner. Together, these findings provide fundamental mechanistic insights into RQC and its physiological role in maintaining cellular protein homeostasis.
Asunto(s)
Proteínas de Ciclo Celular/ultraestructura , Biosíntesis de Proteínas , Ribosomas/genética , Proteínas de Saccharomyces cerevisiae/ultraestructura , Serina Endopeptidasas/ultraestructura , Ubiquitina-Proteína Ligasas/ultraestructura , Adenosina Trifosfato/química , Adenosina Trifosfato/genética , Proteínas de Ciclo Celular/química , Proteínas de Ciclo Celular/genética , Péptidos/química , Péptidos/genética , ARN Mensajero/genética , Ribosomas/química , Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/química , Proteínas de Saccharomyces cerevisiae/genética , Serina Endopeptidasas/química , Serina Endopeptidasas/genética , Ubiquitina/química , Ubiquitina/genética , Ubiquitina-Proteína Ligasas/química , Ubiquitina-Proteína Ligasas/genética , Ubiquitinación/genéticaRESUMEN
A dermoscopy of linear unilateral syringoma has never been described in the English literature. The authors report the case to elucidate the dermoscopic features of syringoma. Clinical, dermoscopic and pathological features of a 38-year-old Japanese female case of linear unilateral syringoma on the left upper arm were described and compared with those of lichen planus linearis. Although the clinical appearance of linearly arranged dusky erythemas in the present case was quite similar to that of lichen planus linearis, dermoscopy did not show Wickham's striae but rather homogeneous light brown pigmentation with multifocal whitish areas and a delicate pigment network at the periphery. Dermoscopy might be helpful to differentiate linear unilateral syringoma from lichen planus linearis.
