Translation is a biological process where cells convert genetic instructions from messenger RNA (mRNA) into functional proteins. This intricate mechanism is carried out by ribosomes, which assemble amino acids into a specific sequence. Protein synthesis requires a precise mechanism to stop, just as it begins with a start codon. This controlled termination ensures proteins are synthesized to their accurate lengths, important for their proper function.
The Stop Signals
The signals that halt protein synthesis are specific nucleotide sequences on the mRNA molecule known as stop codons. There are three primary stop codons: UAA, UAG, and UGA. These triplets do not code for any amino acid, unlike the other 61 codons that specify amino acids. Instead, they act as termination signals, indicating the end of the protein-coding region within an mRNA sequence.
These stop codons are often referred to as “nonsense codons” because they do not correspond to any transfer RNA (tRNA) molecule. Their presence at the end of a gene’s coding sequence ensures the ribosome concludes the polypeptide chain. Without these signals, protein synthesis would continue, resulting in an abnormally long and potentially non-functional protein.
Recognizing the Signals
When a ribosome encounters a stop codon in the mRNA’s A-site, specialized proteins called release factors recognize these signals. Unlike tRNAs, which deliver specific amino acids, release factors bind directly to the stop codons. In prokaryotes, two main class I release factors exist: RF1 recognizes UAA and UAG, while RF2 recognizes UAA and UGA. A class II release factor, RF3, enhances the activity of RF1 and RF2.
In eukaryotes, a single class I release factor, eRF1, recognizes all three stop codons. Eukaryotes also have a class II release factor, eRF3, which stimulates eRF1 activity. The binding of these release factors to the ribosome at the stop codon leads to the release of the newly synthesized protein. This recognition mechanism ensures accurate and specific translation termination.
The Termination Process
The binding of release factors to the stop codon in the ribosomal A-site triggers a series of events that culminate in the cessation of translation. The class I release factors, such as RF1/RF2 in prokaryotes or eRF1 in eukaryotes, possess a conserved GGQ motif. This motif is positioned within the ribosome’s peptidyl transferase center, where it facilitates the hydrolysis of the ester bond linking the growing polypeptide chain to the tRNA located in the P-site. This reaction cleaves the bond and releases the completed protein from the ribosome.
Following polypeptide release, the ribosome must disassemble so its components can be reused for new rounds of protein synthesis. In prokaryotes, ribosome recycling is facilitated by the ribosome recycling factor (RRF) and elongation factor G (EF-G), often with the help of initiation factor 3 (IF3). These factors work cooperatively to split the ribosome into its large and small subunits, and to release the mRNA and deacylated tRNA. In eukaryotes, the process also involves specific factors like ABCE1 for ribosome splitting. This disassembly ensures the ribosomal machinery is ready to begin translating another mRNA molecule.
Why Accurate Termination Matters
Accurate translation termination is important for producing functional proteins. Inaccurate termination can lead to proteins with altered structures and functions. For instance, premature termination, often caused by a “nonsense mutation” that introduces an early stop codon, results in a truncated protein. These shortened proteins are typically non-functional or misfolded, potentially leading to cellular dysfunction or various genetic disorders, such as cystic fibrosis or Duchenne muscular dystrophy.
Conversely, a failure to terminate translation at the intended stop codon, known as “read-through,” can lead to abnormally long proteins. This happens when the ribosome bypasses the stop codon and continues translating into normally untranslated regions of the mRNA. Such extended proteins may exhibit altered stability, incorrect localization within the cell, or impaired function, which can also contribute to disease. Therefore, the accurate operation of the termination mechanism influences cellular health and protein quality control.