Initiation of Protein Synthesis in Eukaryotes

The formation of a ternary complex between the initiator Met-tRNA, GTP and the translation initiation factor eIF2 is a crucial step in initiation of protein synthesis in eukaryotes. Our studies have identified the important role of a base pair conserved in eukaryotic initiator tRNAs in binding to eIF2. Other experiments have identified structural elements in mammalian initiator tRNA which block its participation in the elongation step in mammalian cells. We have also shown that protein synthesis can be initiated in mammalian cells with codons such as GUC and amino acids such as valine. Results of such studies provide opportunities for isolating suppressor mutations in yeast genes encoding components of the initiation machinery.

Other work along these lines involves a study of the very unusual phenomenon of “cap independent” initiation of protein synthesis without the involvement of the canonical initiator tRNA. This type of initiation is mediated by internal ribosome entry sequences present in some insect dicistronic viral RNAs. Using mutant reporters carrying UAG as the initiation codon and an amber suppressor tRNA, we have provided direct evidence that such initiation occurs from the ribosomal A site in mammalian cells in tissue culture.