Identification of a Translation Initiation Factor 3 (eIF3) Core Complex, Conserved in Yeast and Mammals, That Interacts with eIF5

Abstract

Only five of the nine subunits of human eukaryotic translation initiation factor 3 (eIF3) have recognizable homologs encoded in the Saccharomyces cerevisiae genome, and only two of these (Prt1p and Tif34p) were identified previously as subunits of yeast eIF3. We purified a polyhistidine-tagged form of Prt1p (His-Prt1p) by Ni 2؉ affinity and gel filtration chromatography and obtained a complex of Ϸ600 kDa composed of six polypeptides whose copurification was completely dependent on the polyhistidine tag on His-Prt1p. All five polypeptides associated with His-Prt1p were identified by mass spectrometry, and four were found to be the other putative homologs of human eIF3 subunits encoded in S. cerevisiae: YBR079c/Tif32p, Nip1p, Tif34p, and YDR429c/Tif35p. The fifth Prt1p-associated protein was eIF5, an initiation factor not previously known to interact with eIF3. The purified complex could rescue Met-tRNA i Met binding to 40S ribosomes in defective extracts from a prt1 mutant or extracts from which Nip1p had been depleted, indicating that it possesses a known biochemical activity of eIF3. These findings suggest that Tif32p, Nip1p, Prt1p, Tif34p, and Tif35p comprise an eIF3 core complex, conserved between yeast and mammals, that stably interacts with eIF5. Nip1p bound to eIF5 in yeast two-hybrid and in vitro protein binding assays. Interestingly, Sui1p also interacts with Nip1p, and both eIF5 and Sui1p have been implicated in accurate recognition of the AUG start codon. Thus, eIF5 and Sui1p may be recruited to the 40S ribosomes through physical interactions with the Nip1p subunit of eIF3.The initiation of protein synthesis in eukaryotic cells is dependent on multiple initiation factors (eIFs) that stimulate the binding of mRNA and methionyl-initiator tRNA (tRNA i Met ) to 40S ribosomes to form the 48S preinitiation complex (39). The Met-tRNA i Met is delivered to 40S ribosomes in a ternary complex with eIF2 and GTP, whereas the binding of mRNA to ribosomes is stimulated by eIF4F, eIF4A, eIF4B (39), and the poly(A)-binding protein Pab1p (54). Joining of the 60S subunit to form an 80S initiation complex requires hydrolysis of the GTP bound to eIF2, dissociation of the ternary complex, and release of the eIF2-GDP binary complex, and eIF5 promotes these events by stimulating GTP hydrolysis on ternary complexes bound to 40S ribosomes (39).Mammalian eIF3 is a multisubunit complex that has been implicated in several aspects of 48S complex formation. The purified factor promotes dissociation of 80S ribosomes into 40S and 60S subunits, forming a complex with the 40S subunits, and stabilizes binding of the eIF2-GTP-Met-tRNA i Met ternary complex to the 40S ribosome. It also stimulates binding of mRNA to 40S subunits (9, 56), presumably through its interactions with the cap-binding initiation factor eIF4F (36, 38) or eIF4B (41). A mammalian eIF3 complex, purified by its ability to promote methionylpuromycin (Met-puromycin) synthesis by an 80S initiation complex in an assay containing purified eIF1A, eIF2, eIF5, eIF5A, and ribos...

The essential Gcd10p–Gcd14p nuclear complex is required for 1-methyladenosine modification and maturation of initiator methionyl-tRNA

“…Recently, we purified a yeast eIF3 complex from a strain expressing a polyhistidine-tagged form of the PRT1-encoded subunit ) and found that it contains all five S. cerevisiae proteins homologous to subunits of mammalian eIF3, but lacks Gcd10p. In addition, Gcd10p was not coimmunoprecipitated with an epitope-tagged version of the TIF34 subunit of yeast eIF3 Phan et al 1998). Together, these findings suggested that Gcd10p is not an integral subunit of yeast eIF3, and may have a distinct function involved in the formation of ternary complexes or their recruitment to the ribosome.…”

Starting the protein synthesis machine: eukaryotic translation initiation

“…1), which consists of a core of five non-identical subunits (eIF3a, eIF3b, eIF3c, eIF3i and eIF3g in yeast) (20) and up to six additional subunits in mammals (see Ref. 21 for a current nomenclature of eIF3 subunits).…”

Eukaryote-specific extensions in ribosomal proteins of the small subunit: Structure and function

“…The five core subunits (a, b, c, i and g) are conserved across all eukaryotes and are essential in yeast. 25,40,41 As described above, eIF3 acts as a scaffold for other initiation factors, promotes their binding to the 40S ribosomal subunit and serves as a bridge between the mRNA and the 40S subunit. eIF3 also impedes 40S-60S subunit association.…”

“…22,23 Consistent with this idea, eIF3b mutation (prt1-1) has been shown to compromise mRNA recruitment to the 43S complex in yeast. 40 eIF3 architecture and its interaction with the 40S ribosomal subunit is of primary interest and has been the focus of many recent research studies. 14,25 eIF3 was originally assumed to cover and interact with a relative large surface of the 40S subunit.…”