Embraced by eIF3: structural and functional insights into the roles of eIF3 across the translation cycle

Abstract

Protein synthesis is mediated via numerous molecules including the ribosome, mRNA, tRNAs, as well as translation initiation, elongation and release factors. Some of these factors play several roles throughout the entire process to ensure proper assembly of the preinitiation complex on the right mRNA, accurate selection of the initiation codon, errorless production of the encoded polypeptide and its proper termination. Perhaps, the most intriguing of these multitasking factors is the eukaryotic initiation factor eIF3. Recent evidence strongly suggests that this factor, which coordinates the progress of most of the initiation steps, does not come off the initiation complex upon subunit joining, but instead it remains bound to 80S ribosomes and gradually falls off during the first few elongation cycles to: (1) promote resumption of scanning on the same mRNA molecule for reinitiation downstream—in case of translation of upstream ORFs short enough to preserve eIF3 bound; or (2) come back during termination on long ORFs to fine tune its fidelity or, if signaled, promote programmed stop codon readthrough. Here, we unite recent structural views of the eIF3–40S complex and discus all known eIF3 roles to provide a broad picture of the eIF3’s impact on translational control in eukaryotic cells.

Selective Translation Complex Profiling Reveals Staged Initiation and Co-translational Assembly of Initiation Factor Complexes

“…Furthermore, the observation that this enrichment is significantly more pronounced for eIF2::40Ses than eIF3::40Ses in yeast ( Figure 3L and Figure S10E) indicates that some start codon associated PICs still contain eIF3 but not eIF2. This is consistent with the generally accepted model that eIF2, together with eIF5, must leave the PIC upon AUG recognition, whereas a certain proportion of eIF3 complexes lingers on even during the early elongation cycles (for reviews see (Hinnebusch, 2017;Shirokikh and Preiss, 2018;Valasek et al, 2017)).…”

“…The eIF3 complex, composed of five subunits in yeast (a/Tif32, b/Prt1, c/Nip1, g/Tif35, i/Tif34) and twelve in mammals (a, b, c, d, e, f, g, h, i, k, l, m) (Figure S1A,B), is known to be critical for efficient progression of most of the initiation steps, yet its complete structure has not been determined from any organism (reviewed in (Cate, 2017;Valasek et al, 2017)). The assembly pathway for human and N. crassa (similar in composition to human) eIF3 was recently described (Smith et al, 2016;Wagner et al, 2014;Wagner et al, 2016) but it remains to be examined for the most extensively studied S. cerevisiae eIF3 complex (Zeman et al, 2019).…”

“…Both modules are flexibly connected by the poorly resolved extended Cterminal half of the eIF3a subunit, allowing the quaternary module (b-i-g-a-Cter.) to relocate to the interface surface of the 40S subunit, and back, at stages of initiation yet to be fully identified ( Figure S1D) Simonetti et al, 2016;Valasek et al, 2017). Importantly, besides initiation, eIF3 was also shown to i) control translation termination and ribosomal recycling (Beznosková et al, 2013;Beznoskova et al, 2015;Pisarev et al, 2007); and to ii) stimulate reinitiation (REI) on downstream cistrons after translation of short upstream uORFs -due to its ability to remain bound to elongating ribosomes immediately following termination Park et al, 2001;Szamecz et al, 2008).…”

“…Importantly, besides initiation, eIF3 was also shown to i) control translation termination and ribosomal recycling (Beznosková et al, 2013;Beznoskova et al, 2015;Pisarev et al, 2007); and to ii) stimulate reinitiation (REI) on downstream cistrons after translation of short upstream uORFs -due to its ability to remain bound to elongating ribosomes immediately following termination Park et al, 2001;Szamecz et al, 2008). Owing to the manifold functions of eIF3, deregulated eIF3 expression is associated with numerous pathological conditions (reviewed in (Gomes-Duarte et al, 2018;Robichaud and Sonenberg, 2017;Valasek et al, 2017)).…”

Translation arrest as a protein quality control system for aberrant translation of the 3′‐UTR in mammalian cells

“…Protein synthesis is a fundamental process for all living species. Termination of translation at a stop codon is an essential step ensuring accurate protein expression . However, ribosomes stochastically read through a stop codon in the presence of errors or genetic mutations.…”