Eukaryotic initiation factor (eIF)-4F. Implications for a role in internal initiation of translation

Area of Science:

• Molecular Biology
• Gene Expression
• Protein Synthesis

Background:

• Eukaryotic translation initiation is a complex process involving cap-dependent and cap-independent mechanisms.
• Internal ribosome entry sites (IRES) mediate cap-independent translation initiation.
• Understanding internal initiation is crucial for comprehending gene expression regulation.

Purpose of the Study:

• To investigate the mechanisms of eukaryotic internal, re-initiation, and coupled internal initiation.
• To identify the roles of specific initiation factors in cap-independent translation.
• To construct and utilize model mRNAs encoding rabbit alpha globin for studying translation.

Main Methods:

• Construction of model mRNAs with two non-overlapping open reading frames (ORFs).
• Utilized bicistronic constructs TK/CAT and TK/P2CAT.
• Employed an in vitro rabbit reticulocyte lysate translation system, distinguishing cap-dependent and cap-independent translation using a cap analog (m7GTP).
• Performed initiation factor addition studies with eukaryotic initiation factor (eIF)-4F and eIF-4B.

Main Results:

• Successfully generated mRNAs that translate both ORFs via cap-dependent mechanisms.
• Obtained mRNAs translating the second ORF cap-independently while the first ORF was translated cap-dependently.
• Demonstrated that eIF-4F and, to a lesser extent, eIF-4B can stimulate internal ORF translation independently of upstream ORF translation and cap recognition.

Conclusions:

• Eukaryotic initiation factor 4F (eIF-4F) and eIF-4B play significant roles in facilitating cap-independent internal initiation.
• These factors enable the translation of an open reading frame without reliance on the mRNA's 5' cap structure.
• The findings provide insights into the molecular mechanisms governing internal translation initiation in eukaryotes.