Which Of The Events Occur During Eukaryotic Translation Elongation

Which Events Occur During Eukaryotic Translation Elongation?

Eukaryotic translation elongation is a complex and highly regulated process responsible for the synthesis of polypeptide chains. Understanding the intricacies of this stage is crucial for comprehending gene expression, protein synthesis, and various cellular processes. This article delves deep into the events that occur during eukaryotic translation elongation, exploring the key players, mechanisms, and regulatory aspects involved.

The Players: Key Components of Elongation

Before diving into the sequential events, let's introduce the main actors in eukaryotic translation elongation:

• Ribosome: The protein synthesis machinery, consisting of the 40S (small) and 60S (large) ribosomal subunits. The 60S subunit houses the peptidyl transferase center (PTC), crucial for peptide bond formation.
• mRNA: The messenger RNA molecule carrying the genetic code (codons) specifying the amino acid sequence of the polypeptide.
• tRNA: Transfer RNA molecules, each carrying a specific amino acid and possessing an anticodon that base-pairs with the corresponding codon on the mRNA.
• EF-Tu (eukaryotic Elongation Factor 1α): A GTPase that delivers aminoacyl-tRNAs to the ribosome's A (aminoacyl) site.
• EF-G (eukaryotic Elongation Factor 2): A GTPase that translocates the ribosome along the mRNA.
• Peptidyl transferase: An enzymatic activity residing within the large ribosomal subunit (specifically the 28S rRNA) that catalyzes peptide bond formation.
• Release Factors (eRFs): Although not directly involved in elongation, these factors are essential for the termination process, which signals the end of elongation.

The Stages of Eukaryotic Translation Elongation

Eukaryotic translation elongation is a cyclical process comprising three main steps:

1. Aminoacyl-tRNA Binding (Aminoacyl-tRNA Selection & Accommodation)

This crucial step involves the delivery of the correct aminoacyl-tRNA to the A site of the ribosome. The process is highly selective, ensuring the fidelity of protein synthesis.

• Codon Recognition: The anticodon of the incoming aminoacyl-tRNA must precisely base-pair with the codon residing in the A site of the ribosome. This interaction is facilitated by EF-Tu, which forms a ternary complex with the aminoacyl-tRNA and GTP.
• EF-Tu Binding and GTP Hydrolysis: The EF-Tu-aminoacyl-tRNA-GTP ternary complex binds to the A site. Accurate codon-anticodon base pairing triggers GTP hydrolysis by EF-Tu, leading to a conformational change that releases EF-Tu-GDP from the ribosome. Inaccurate pairing prevents GTP hydrolysis, and the incorrect aminoacyl-tRNA is rejected.
• Accommodation: Once the correct aminoacyl-tRNA is bound, it undergoes a conformational adjustment, settling into the A site. This process is crucial for proper positioning of the amino acid for peptide bond formation.
• Quality Control: The accuracy of codon-anticodon recognition is crucial. If an incorrect tRNA binds, the elongation process is stalled, allowing for proofreading mechanisms to correct the error or trigger mRNA degradation.

2. Peptide Bond Formation

This step involves the formation of a peptide bond between the amino acid in the A site and the growing polypeptide chain in the P (peptidyl) site.

• Peptidyl Transferase Activity: The peptidyl transferase center (PTC) within the large ribosomal subunit catalyzes the formation of the peptide bond. This is a remarkable reaction, considering it occurs without the need for additional enzymatic cofactors. The energy for this reaction is derived from the high-energy ester bond linking the amino acid to its tRNA.
• Transpeptidation: The reaction involves the transfer of the polypeptide chain from the tRNA in the P site to the amino acid in the A site, forming a new peptide bond.
• Uncharged tRNA in P site: After transpeptidation, the tRNA in the P site is now uncharged (deacylated), having released its amino acid.

3. Translocation

This final step of the elongation cycle involves the movement of the ribosome along the mRNA, shifting the mRNA by three nucleotides (one codon).

• EF-G-GTP Binding: EF-G, another GTPase, binds to the ribosome in its GTP-bound form.
• GTP Hydrolysis and Ribosome Movement: GTP hydrolysis by EF-G induces a conformational change in the ribosome, causing it to move three nucleotides along the mRNA. This movement shifts the peptidyl-tRNA from the A site to the P site, and the uncharged tRNA from the P site to the E (exit) site, where it is released.
• Positioning for the next cycle: The A site is now vacant, ready to receive the next aminoacyl-tRNA corresponding to the next codon on the mRNA, preparing the ribosome for another round of elongation.

Regulation of Eukaryotic Translation Elongation

Eukaryotic translation elongation is a tightly regulated process influenced by various factors:

• Phosphorylation: Phosphorylation of ribosomal proteins or elongation factors can modulate their activity and influence the rate of elongation.
• GTPases: The GTPase activity of EF-Tu and EF-G is critical for precise control of aminoacyl-tRNA binding and translocation, respectively. GTP hydrolysis serves as a molecular switch, regulating these steps.
• mRNA Secondary Structure: The secondary structure of the mRNA can affect ribosome movement and elongation efficiency. Certain mRNA structures can cause pausing or stalling of ribosomes.
• Environmental Factors: Cellular stress, nutrient availability, and other environmental factors can significantly impact the rate and fidelity of elongation. Cells can regulate protein synthesis in response to these changes, modulating elongation as a consequence.
• Translational Inhibitors: Several antibiotics and other molecules can specifically inhibit different steps of elongation, providing valuable tools for studying this process and potentially serving as therapeutic agents.

Errors and Quality Control During Elongation

Despite the high fidelity of the elongation process, errors can occur. The cell employs several mechanisms to minimize these errors and maintain the accuracy of protein synthesis.

• Proofreading by EF-Tu: As mentioned earlier, EF-Tu plays a crucial role in proofreading the codon-anticodon interaction. Incorrect base pairing prevents GTP hydrolysis, resulting in the rejection of the incorrect tRNA.
• Kinetic Proofreading: The rate of peptide bond formation is faster for correct aminoacyl-tRNAs compared to incorrect ones. This kinetic difference contributes to the overall accuracy of the process.
• Ribosome Stalling: If an error occurs, the ribosome can stall, providing an opportunity for rescue mechanisms or mRNA degradation. Stalled ribosomes can also trigger the activation of stress response pathways.
• Non-stop Decay: If translation terminates prematurely due to a lack of a stop codon (perhaps due to a mutation), the mRNA undergoes degradation through a pathway called "non-stop decay," preventing the synthesis of truncated and potentially harmful proteins.

Clinical Significance of Elongation Defects

Disruptions in eukaryotic translation elongation can have significant consequences, contributing to various diseases and disorders. Mutations affecting ribosomal proteins, elongation factors, or tRNA synthetases can lead to:

• Inherited Ribosomopathies: These are a group of disorders caused by mutations in genes encoding ribosomal proteins or other components of the translational machinery. These disorders can manifest in a wide range of symptoms, affecting various organ systems.
• Cancer: Dysregulation of translation elongation has been implicated in cancer development and progression. Changes in the expression levels or activity of elongation factors can contribute to uncontrolled cell growth and proliferation.
• Neurological Disorders: Defects in protein synthesis can affect neuronal development and function, contributing to various neurological disorders.

Conclusion

Eukaryotic translation elongation is a dynamic and precisely controlled process crucial for cellular life. The intricate interplay of ribosomes, tRNAs, elongation factors, and regulatory mechanisms ensures the accurate and efficient synthesis of proteins. Understanding the events involved in this process is fundamental to comprehending gene expression, cellular function, and the pathogenesis of various diseases. Future research will undoubtedly continue to reveal further intricacies and regulatory aspects of this fascinating and essential biological process. Further investigation into the specific interactions and regulatory networks is needed for a complete understanding. The field continues to evolve with new discoveries about the precise mechanisms and complexities involved, continually refining our knowledge of this vital aspect of cellular biology.