Question

For each of the following initiation factors, how would eukaryotic initiation of translation be affected if it were missing?

A. eIF2
B. eIF4
C. eIF5

Answer 1

The absence of eIF2, eIF4, and eIF5 would significantly disrupt the initiation phase of eukaryotic translation. eIF2 is critical for forming the ternary complex and start codon recognition; without it, the small subunit cannot initiate translation properly. eIF4 and eIF5 are similarly crucial for binding to mRNA and ribosome assembly, respectively.

Step-by-step explanation:

The initiation of eukaryotic translation is a critical process that involves various initiation factors. If certain elements, such as eIF2, eIF4, and eIF5, are missing, the process would be affected as follows:

• eIF2 — Essential for the early stage of initiation, eIF2 in its GTP-bound form associates with the initiator methionyl-tRNA (Met-tRNAi) to form a ternary complex. This complex allows the small (40S) ribosomal subunit to bind to the mRNA and scan for the start codon. Without eIF2, the small subunit would not be able to form the ternary complex, inhibiting the recognition of the start codon and thus impeding the initiation of translation.
• eIF4 — This factor is involved in the recognition of the cap structure of mRNA. It is part of a complex that circularizes the mRNA to enhance the recruitment of the ribosomal subunits. Absence of eIF4 would hinder the binding of the small ribosomal subunit to the mRNA, as well as interfering with the scanning process.
• eIF5 — Works in concert with other factors to promote the joining of the large (60S) ribosomal subunit to the small subunit initiation complex. Without eIF5, the assembly of the complete 80S ribosome would be compromised, preventing the transition from initiation to elongation phase in protein synthesis.

Consequently, the absence of any of these factors would result in significant disruption of the protein synthesis process. Specifically, alterations in eIF2 could lead to a decrease in the global rate of protein synthesis or even cause selective inhibition of the translation of certain proteins, which might have links to neurological disorders like Alzheimer's and Parkinson's disease.