In the intricate world of cellular biology, the phenomenon of translation termination is a pivotal event in the protein synthesis cycle, particularly in eukaryotes. This critical process is predominantly facilitated by eukaryotic release factor 1 (eRF1), which plays a significant role in the recognition of stop codons. Delivered to the ribosome by the translational GTPase eRF3, eRF1 serves as the key player in ending the translation of messenger RNA (mRNA) into a polypeptide chain. The precision with which eRF1 recognizes the stop signals ensures that protein synthesis proceeds without errors, highlighting the importance of this mechanism.
Upon reaching the A-site of the ribosome, eRF1 catalyzes the hydrolysis of peptidyl-tRNA through its conserved GGQ (Gly–Gly–Gln) motif. This enzymatic action leads to the release of the newly synthesized polypeptide from the ribosomal complex. The termination event is crucial as it signifies the completion of the protein synthesis process. Following this, the ribosome undergoes a recycling phase, wherein the ATPase ABCE1, also known as Rli1 in yeast, facilitates the dissociation of the ribosomal complex into its constituent subunits—60S and 40S. Such recycling is essential for the efficient use of ribosomal components in subsequent rounds of translation.
However, the termination process is not entirely straightforward. eRF1 exclusively recognizes stop codons and is ineffective in resolving stalled ribosomes, which can emerge during translation, particularly in complex or aberrant coding sequences. Instead of eRF1, a homologous protein known as Pelota, found in metazoans, or Dom34 in yeast, steps in to address stalled ribosomes. These proteins are delivered by the GTPase Hbs1, an eRF3 homolog, signaling a different route to resolve translation stalls. Their involvement is critical in ensuring that stalled ribosomes, which may result from truncated mRNAs, do not persist indefinitely, leading to potential cellular dysfunction.
Once Pelota or Dom34 is inserted into the empty A-site of the stalled ribosome, the ATPase ABCE1/Rli1 once again mediates the splitting of the ribosome into its subunits. Unlike eRF1, which actively promotes peptidyl-tRNA hydrolysis, Pelota and Dom34 lack the required GGQ motif for this reaction. Consequently, the peptidyl-tRNA remains bound to the large ribosomal subunit following their action, which necessitates subsequent degradation of the attached polypeptide. This mechanism represents an essential quality control step in the protein synthesis process.
In instances where ribosomes stall within a coding sequence rather than at the 3’ end, complications arise. The accessibility of the A-site might be obstructed, complicating the intervention of Pelota or Dom34. To tackle stalled ribosomes in such scenarios, ribosome collisions activate an alternative splitting mechanism. This occurs when two ribosomes collide, creating a unique 40S–40S interface that is recognized by an E3 ubiquitin ligase. This recognition initiates a quality control pathway that is crucial for maintaining cellular integrity.
In mammals, the E3 ligase ZNF598 plays a central role in tagging these collided ribosomes by ubiquitylating specific ribosomal proteins. The ubiquitylated residues, namely eS10 and uS10 at designated lysine positions, serve as signals for the recruitment of downstream quality control machinery. Similarly, in yeast, the E3 ligase Hel2 targets similar proteins, ensuring that ribosomes under stress are promptly recognized for disassembly. This mechanism is vital in preventing the accumulation of defective ribosomes, which could otherwise compromise cellular health.
As ribosomal proteins in disomes or trisomes undergo ubiquitylation, they attract the ribosome-splitting machinery, consisting of distinct complexes in mammals (ASC-1 complex) and yeast (RQC-trigger complex). The intricate composition of the ASCC or RQT complex enables them to effectively disassemble the collided ribosomes. Key players in these complexes include RNA helicases and ubiquitin-binding proteins, which work in concert to facilitate ribosome splitting, ensuring that the initiation of the quality control mechanisms is efficient and effective.
In the case of the ASCC complex found in mammals, the helitron component, ASCC3, exerts a pulling force on the mRNA that extends from the lead ribosome. This ATP hydrolysis-driven action pulls the stalled ribosome forward, effectively generating enough force to separate the colliding ribosomes. This mechanism not only serves to disassemble the ribosomal complex but also emphasizes the kinetic demands placed on these quality control processes during translation.
Notably, the fate of the peptidyl-tRNA attached to the large ribosomal subunit remains a critical aspect of post-termination processing. Following the splitting facilitated by either the Pelota-mediated mechanism or the ASCC/RQT complex, the peptidyl-tRNA is retained, necessitating its degradation. This ensures that incomplete proteins do not accumulate, thereby maintaining the quality of synthesized proteins within the cell. The subsequent degradation process is vital for cellular homeostasis and is tightly regulated.
Equally important is the deubiquitination process for the small subunit ribosomal proteins marked by ZNF598 or Hel2. In mammals, deubiquitinating enzymes such as USP10 recycle the smallest ribosomal subunits, whereas in yeast, enzymes like Ubp2 and Ubp3 serve a similar function. This recycling is crucial to restore ribosomal components for subsequent rounds of translation, thereby guaranteeing cellular efficiency in protein synthesis.
The orchestration of these various pathways highlights the sophistication of eukaryotic cells in managing translation and fostering quality control. The cellular mechanisms activated in response to ribosomal stalling and collisions illustrate a robust system designed to ensure fidelity in protein synthesis. Despite the challenges posed by premature and stalled ribosomes, the cellular quality control systems exemplify the evolutionary adaptations that sustain life at the molecular level.
Through ongoing research, scientists continue to uncover the complexities of these mechanisms, providing deeper insights into the molecular underpinnings of translation fidelity. As our understanding of ribosomal quality control expands, it becomes increasingly clear how vital these processes are in maintaining cellular health and preventing the potential diseases that may arise from translation errors.
The exploration of ribosome stalling pathways not only enhances our understanding of fundamental biology but may also open new avenues for therapeutic interventions. By unraveling the intricate connections between ribosome function and quality control, future studies hold the promise of advancing our ability to target translation-related diseases effectively.
As we broaden our knowledge of the molecular mechanisms governing translation, the focus on stalled ribosomes and their resolution underscores the importance of maintaining cellular integrity. Rigorous quality control processes are essential for the accurate expression of genes and the proper functioning of proteins, crucial elements in the sophisticated design of eukaryotic life forms.
Ultimately, the elegance of these systems highlights the dynamic nature of cellular biology, where complex interactions and finely-tuned responses converge to facilitate life’s fundamental processes. The continued investigation into translation termination and ribosome quality control promises to enrich our understanding of molecular biology and its implications on health and disease.
Subject of Research:
Translation termination and quality control pathways in eukaryotic ribosomes.
Article Title:
Quality control and signaling pathways at stalled ribosomes.
Article References:
Chang, W.D., Choe, YJ. Quality control and signaling pathways at stalled ribosomes. Exp Mol Med (2026). https://doi.org/10.1038/s12276-025-01623-w
Image Credits:
AI Generated
DOI:
15 January 2026
Keywords:
Ribosome, translation termination, eukaryotic release factor, quality control, stalled ribosomes, peptidyl-tRNA, ubiquitin ligase, ribosome recycling, cellular integrity, translation fidelity.
