3. Polypeptide Chain Termination

Publisher Summary Peptide chain termination defines an event that results in the release of the completed peptide from its ultimate ribosomal bound tRNA. This chapter discusses the mechanism of peptide chain termination and its relationship with other protein synthesis. It presents model for the intermediate events of peptide chain termination based primarily upon data obtained from in vitro studies with both mammalian and bacterial cells and assumes a common mechanism for the two cells. All intermediate events and requirements have not been demonstrated for each cell type, and the sequence of intermediate events should be regarded as tentative. Both mammalian and bacterial cells utilize the same terminator codons. In bacterial cells, these codons are recognized by protein release factors, which are codon specific. In mammalian cells, the RF apparently recognizes all three terminator codons. A separate protein factor third soluble protein factor RF3, identified in bacterial extracts, has the capacity to facilitate the binding of RF to ribosomes and interacts with guanosine diphosphate (GDP) and guanosine triphosphate GTP. Because mammalian RF alone is stimulated by GTP, a functional RF3 equivalent may not be involved. The data from both cell types favor the involvement of GTP and its γ-phosphate hydrolysis in RF binding and dissociation rather than peptidyl-tRNA hydrolysis.