Question

Which factor recruits the first tRNA (tRNAMet) to the small ribosomal subunit during eukaryotic translation?

Answer 1

In eukaryotic translation, the first tRNA (tRNAMet) is recruited to the small ribosomal subunit by the formation of a ternary complex with eukaryotic initiation factor 2 (eIF2) and GTP.

Step-by-step explanation:

Initiation of Translation in Eukaryotic Cells

The process of translation initiation in eukaryotic cells starts with the binding of the small ribosomal subunit to the mRNA molecule. In eukaryotes, the first tRNA, known as tRNAMet, is recruited with the help of several eukaryotic initiation factors, particularly the eukaryotic initiation factor 2 (eIF2), which forms a ternary complex with GTP and Met-tRNA. The ternary complex then associates with the 40S ribosomal subunit which is attached to the 5' cap of the mRNA. The scanning of this pre-initiation complex leads to the recognition of the AUG start codon, where the anticodon of the initiator tRNA base pairs with the start codon. Once the AUG start codon has been identified and the initiator tRNA is correctly positioned, the large ribosomal subunit (60S) joins the assembly. This step completes the initiation process, leading to the release of initiation factors and the hydrolysis of GTP on eIF2.

It is crucial to note that the methionine amino acid attached to the initiator tRNA in eukaryotes is not formylated as it is in prokaryotes. Therefore, the eukaryotic initiator tRNA is known as Met-tRNA and is distinct from non-initiator methionine tRNAs, which do not associate with the initiation factors. The proper assembly of the initiation complex is a pivotal step in setting the stage for subsequent amino acid elongation and synthesis of the polypeptide chain.

SUMUP of the Key Points

• Eukaryotic initiation factor 2 (eIF2) and GTP bind with Met-tRNA to form a ternary complex.
• The small ribosomal subunit binds to the mRNA's 5' cap and, along with the ternary complex, scans for the AUG start codon.
• The large ribosomal subunit joins the initiation complex after the start codon is recognized, releasing the initiation factors and allowing translation to proceed.