The final stages of translation, like initiation and elongation, require highly specific molecular signals that decide the fate of the mRNA–ribosome–peptidyl-tRNA complex. Two types of specific protein release factors (RFs) have been discovered. Eukaryotic eRF1, whose shape is similar to that of tRNAs, acts by binding to the ribosomal A site and recognizing stop codons directly. Like some of the initiation and elongation factors discussed previously, the second eukaryotic release factor, eRF3, is a GTP-binding protein. The eRF3·GTP complex acts in concert with eRF1 to promote cleavage of the peptidyl-tRNA bond, thus releasing the completed protein chain and terminating translation (Figure 5-26). Bacteria have two release factors (RF1 and RF2) that are functionally analogous to eRF1 and a GTP-binding factor (RF3) that is analogous to eRF3. Once again, the eRF3 GTPase monitors the correct recognition of a stop codon by eRF1. The peptidyl-tRNA bond of the tRNA in the P site is not cleaved until one of the three stop codons is correctly recognized by eRF1, another example of a proofreading step in protein synthesis.

Release of the completed protein leaves a free tRNA in the P site and the mRNA still associated with the 80S ribosome, to which eRF1 and eRF3·GDP are still bound in the A site. In eukaryotes, ribosome recycling occurs when this post-termination complex is bound by a protein called ABCE1, which uses energy from ATP hydrolysis to separate the subunits and release the mRNA and tRNA in the P site. Initiation factors eIF1, eIF1A, and eIF3, which are also required for separation of the subunits, load onto the 40S subunit, making it ready for another round of initiation (see Figure 5-24, top). In reality, a free mRNA is never released as diagrammed in Figure 5-26 for simplicity. Rather, the mRNA has other ribosomes associated with it in various stages of elongation, PABPC bound to the poly(A) tail, and the eIF4 complex associated with the 5′ cap, ready to associate with another 43S preinitiation complex (see Figure 5-24).

In addition to these functions in protein synthesis, ribosomes also associate transiently with protein chaperones that assist in folding the polypeptide chain as it emerges from the ribosome surface (see Figure 3-17). As we will see in Chapter 13, ribosomes that synthesize proteins destined to be inserted into the endoplasmic reticulum (ER), transported into the ER lumen and later secreted from the cell, or introduced into other organelles such as lysosomes, also associate with a ribonucleoprotein complex called SRP (signal recognition particle) that arrests protein synthesis until the nascent polypeptide encounters specialized channels for insertion into the ER. SRP also assists with the insertion and threading of these proteins through these ER channels when protein synthesis is permitted to resume.