Translation termination in eukaryotes (Termination of protein synthesis)
- The elongation cycle is repeated until one of the three stop-codons (UAA, UAG and UGA) is positioned in the ribosomal A-site.
- The stop codons have no corresponding tRNAs.
- Once termination codon appear on the A site, Pre-assembled ternary complex comprising of eRF1, eRF3 and GTP binds to the ribosome.
- In the ternary complex, these stop codons are recognized by eRF-1 (eukaryotic release factor 1). A single release factor (eRF) recognizes all three termination codons.
(Eukaryotes have only one class I release factor (eRF – 1). It is unrelated in sequence to RF-1 and RF-2).
- The activity of eRF-1 is stimulated by eRF-3 (Class II release factor).
- eRF3 is a GTP-binding protein that forms a complex with eRF-1.
- eRF3 is a ribosome-dependent GTPase that helps eRF1 release the completed polypeptide.
- eRF3 resembles RF-3 in both sequence and function. eRF3 is essential for eukaryotic cell viability (RF3 is not essential for prokaryotic viability).
The ribosome-bound ternary complex stimulates the cleavage of the ester bond between the growing peptide chain and the tRNA (peptidyl-tRNA) in the P site thereby releasing the new protein from the ribosome.
The hydrolysis of RF3 – GTP facilitates peptidyl-tRNA hydrolysis at the peptidyl transferase centre (PTC).
Following GTP hydrolysis and peptide release, the RF3-GDP will dissociate from the protein, leaving behind eRF1, which remains bound to the ribosome (known as the post-termination complex).
Post termination ribosomal complex: Consists of 80S ribosome still bound to the mRNA, with deacylated tRNA in its P-site and at least eRF1 in its A-site.
The post – TCs needs to be disassembled and released from the mRNA to allow further rounds of translation.
- Recycling of post-termination complexes (post-TCs) is promoted by the concerted action of ABCE1 (ATP-binding cassette family of proteins) and eRF1 which remains associated with post-TCs after peptide release.
- ABCE1 and eRF1 dissociate post-TCs into free 60S and tRNA/mRNA-bound 40S subunits.
That means first 60S subunit dissociates from the post-TCs leaving a 40S/deacylated tRNA complex bound to the mRNA.
Subsequent tRNA/mRNA release from 40S subunits can be promoted by initiation factor3 (IF3)
eIF3 is the principal factor that promotes splitting of post-termination ribosomes into 60S subunits and tRNA- and mRNA-bound 40S subunits.
Recycling occurs at low Mg2+ concentration (Mg2+ ions promote ribosomal subunit association).
The recycled ribosome can now start a new translational cycle. In most cases the ribosome initiates a new round of translation on the same mRNA.
Translation termnation in Mitochondria
- Process is similar to prokaryotic termination process.
- There is only one release factor present and the name is mitochondrial release factor 1 (mRF1).
Figure 2: General representation of translation termination and ribosome recycling