Initiation of protein synthesis takes place when a ribosome (both large and small subunit) has assembled on the mRNA and the P site is occupied by a methionyl-tRNA (met-tRNA) molecule (Fig. 32.4). This complex is formed by the action of proteins known as initiation factors. In prokaryotic cells, the process (Fig. 32.4) involves three initiation factors. The initiation complex first forms just 5' to the coding region, as a result of the interaction of the 16S rRNA with the Shine-Dalgarno sequence on the mRNA. N-Formyl methionine (fmet) is the first amino acid in all bacterial proteins. In eukaryotic cells, there are at least 12 different initiation factors (eIFs), only a few of which have a known functon. The majority of eIFs help to promote the association of the small ribosomal subunit with the mRNA and a charged met-tRNA. For example, a complex containing the activated initiator, met-tRNA, and eIF-2 first binds to the small ribosomal subunit, which has eIF-3 bound to it. This complex then binds to the 7-methylguanine cap structure of eukaryotic mRNAs by the actions of several factors, including eIF-4F. The small ribosomal subunit moves down the mRNA until the first AUG codon is encountered, at which time the large ribosomal subunit joins the complex and protein synthesis is ready to begin. The assembly of the initiation complex is driven by the hydrolysis of guanosine triphosphate (GTP), and the movement of this complex down the mRNA is driven by the hydrolysis of ATP.
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| Figure 32.4 Initiation of protein synthesis in bacterial cells. The 30S ribosomal subunit, mRNA, and formylated met-tRNA (fmet-tRNA) are brought together by the action of initiation factors. Once these components are assembled, the 50S ribosomal subunit completes the initiation complex. Note that, at initiation, the P site is occupied by the initiator tRNA, met-tRNA. GDP, guanosine diphosphate; IF, initiation factor. |
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