Eukaryotic initiation factor 3
Eukaryotic initiation factor 3 (eIF3) is a multiprotein complex that functions during the initiation phase of eukaryotic translation. It is essential for most forms of cap-dependent and cap-independent translation initiation. In humans, eIF3 consists of 13 nonidentical subunits (eIF3a-m) with a combined molecular weight of ~800 kDa, making it the largest translation initiation factor. The eIF3 complex is broadly conserved across eukaryotes, but the conservation of individual subunits varies across organisms. For instance, while most mammalian eIF3 complexes are composed of 13 subunits, budding yeast's eIF3 has only six subunits (eIF3a, b, c, g, i, j).
eIF3 stimulates nearly all steps of translation initiation. eIF3 also appears to participate in other phases of translation, such as recycling, where it promotes the splitting of post-termination ribosomes. In specialized cases of reinitiation following uORFs, eIF3 may remain bounds to the ribosome through elongation and termination to promote subsequent initiation events. Research has also indicated that eIF3 plays a role in programmed stop codon readthrough in yeast, by interacting with pre-termination complexes and interfering with decoding.
eIF3 binds the small ribosomal subunit (40S) at and near its solvent side and serves as a scaffold for several other initiation factors and mRNA. eIF3 is a component of the multifactor complex (MFC) and 43S and 48S pre-initiation complexes (PICs). The interactions of eIF3 with other initiation factors can vary amongst species; for example, mammalian eIF3 directly interacts with the eIF4F complex (via eIF4G), while budding yeast lacks this connection. However, both mammalian and yeast eIF3 independently bind eIF1, eIF4B, and eIF5.
Several subunits of eIF3 contain RNA recognition motifs (RRMs) and other RNA binding domains to form a multisubunit RNA binding interface through which eIF3 interacts with cellular and viral IRES mRNA, including the HCV IRES. eIF3 has also been shown to specifically bind m6A modified RNA within 5'UTRs to promote cap-independent translation.
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