Summary of Eukaryotic Initiation Factors

Initiation of protein synthesis in mammalian cells proceeds by a complex process whereby the assembly of mRNA, the ribosome, and initiator met-tRNAf into an initiation complex is catalyzed by a group of proteins called initiation factors. By definition, these proteins are not required for polypeptide chain elongation. A detailed discussion of this process, and the role of individual initiation factors can be found in this volume (Kaempfer, 1984), or in other recent reviews (Benne and Hershey, 1978 Jagus et al., 1981) which also contain pertinent references. Nine initiation factors have been highly purified from rabbit reticulocytes, and still other factors have been described which may serve auxiliary functions, or indeed may qualify as initiation factors in their own right. A very brief (and oversimplifed) description of the role of the nine, characterized initiation factors is listed below, along with the step in initiation complex formation in which each participates  

Step 1. eIF-2 forms a ternary complex with met-tRNAf and GTP. 

Step 2. eIF-3 and eIF-4C react with the 40 S ribosomal subunit and allow binding of the ternary complex. 

Step 3. eIF-4A, eI-F4B, and eIF-4F function to promote binding of mRNA to the ribosomal subunit carrying initiator met-tRNA. eIF-3 is essential for this step, and eIF-1 and elF-4C appear to stimulate binding. 

Step 4. eIF-5 catalyzes the junction of the 60 S ribosomal subunit to form an 80 S initiation complex. 

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