Plant Initiation Factor 3 Subunit Composition Resembles Mammalian Initiation Factor 3 and Has a Novel Subunit

Abstract

Eukaryotic initiation factor 3 (eIF3) is a multisubunit complex that is required for binding of mRNA to 40 S ribosomal subunits, stabilization of ternary complex binding to 40 S subunits, and dissociation of 40 and 60 S subunits. These functions and the complex nature of eIF3 suggest multiple interactions with many components of the translational machinery. Recently, the subunits of mammalian and Saccharomyces cerevisiae eIF3 were identified, and substantial differences in the subunit composition of mammalian and S. cerevisiae were observed. Mammalian eIF3 consists of 11 nonidentical subunits, whereas S. cerevisiae eIF3 consists of up to eight nonidentical subunits. Only five of the subunits of mammalian and S. cerevisiae are shared in common, and these five subunits comprise a "core" complex in S. cerevisiae. eIF3 from wheat consists of at least 10 subunits, but their relationship to either the mammalian or S. cerevisiae eIF3 subunits is unknown. Peptide sequences derived from purified wheat eIF3 subunits were used to correlate each subunit with mammalian and/or S. cerevisiae subunits. The peptide sequences were also used to identify Arabidopsis thaliana cDNAs for each of the eIF3 subunits. We report seven new cDNAs for A. thaliana eIF3 subunits. A. thaliana eIF3 was purified and characterized to confirm that the subunit composition and activity of wheat and A. thaliana eIF3 were similar. We report that plant eIF3 closely resembles the subunit composition of mammalian eIF3, having 10 out of 11 subunits in common. Further, we find a novel subunit in the plant eIF3 complex not present in either mammalian or S. cerevisiae eIF3. These results suggest that plant and mammalian eIF3 evolved similarly, whereas S. cerevisiae has diverged. Eukaryotic initiation factor 3 (eIF3)1 is the most complex and least understood of the protein synthesis initiation factors. eIF3 has been purified from wheat germ (1-3), HeLa cells (4, 5), rabbit reticulocytes (6 -12), and Saccharomyces cerevisiae (13)(14)(15)(16). Depending upon the organism and method of purification, there are 6 -11 nonidentical subunits present in functional eIF3 complexes that have been isolated and characterized. Recently, the subunits from the human and S. cerevisiae eIF3 complex have all been identified and sequenced. Surprisingly, there are substantial differences in the subunit composition between mammalian and S. cerevisiae eIF3 complexes. The mammalian eIF3 complex composition varies depending upon the method of purification (6, 7, 9 -12); however, the mammalian eIF3 complex appears to contain 11 subunits, p170 (17), p116 (18), p110 (19), p66 (20), p44 (21, 22), p48 (23), p47 (20), p40 (20), p36 (19), p35 (21) and p28.2 The S. cerevisiae eIF3 complex was determined to consist of a core complex of five subunits, Tif32p, Prt1p, Nip1p, Tif35p,24) that correspond to mammalian subunits p170, p116, p110, p44, and p36, respectively (see Table II). There are other proteins associated with the S. cerevisiae eIF3 complex, TIF31p (Clu1p; Ref. 25), GCD10p (15,26), eI...

The Human Protein HSPC021 Interacts with Int-6 and Is Associated with Eukaryotic Translation Initiation Factor 3

“…A strong support to the notion of HSPC021 being tightly associated with eIF3 comes also from results published during completion of this work on the A. thaliana eIF3. Indeed, it has been reported that eIF3 purified from this organism includes 12 subunits, one of them corresponding to the A. thaliana homolog of HSPC021 (38). In agreement with the data published by Burks et al, the results presented in this paper show that HSPC021 strongly binds the eIF3 translation initiation factor in mammals.…”

Fission Yeast Homolog of Murine Int-6 Protein, Encoded by Mouse Mammary Tumor Virus Integration Site, Is Associated with the Conserved Core Subunits of Eukaryotic Translation Initiation Factor 3

“…Thus, it is possible that the activities of eIF3 from higher eukaryotes are modulated by association of the five core subunits with Int-6 and the other eIF3 subunits not found in S. cerevisiae. Interestingly, the recent progress in sequencing the entire genome of fission yeast Schizosaccharomyces pombe has revealed that it encodes the four orthologs of noncore eIF3 subunits, in addition to orthologs of the five core subunits (14). Thus, S. pombe eIF3 may resemble mammalian eIF3 in containing several polypeptides in addition to the five core subunits present in budding yeast.…”

“…To identify the biological role of Int-6 and decipher the consequences of its truncation, it was important to establish the role of Int-6 in translation initiation as a part of eIF3. Thus far, eIF3 has been purified from mammalian cells (10,11), wheat germ (12,13), A. thaliana (14), and budding yeast Saccharomyces cerevisiae (15)(16)(17). Mammalian eIF3 associates with the 40 S ribosome and stimulates the binding of both mRNA and Met-tRNA i Met (10,11).…”

Arabidopsis eIF3e is regulated by the COP9 signalosome and has an impact on development and protein translation

“…With 12 subunits in plants, eIF3 is by far the largest of these factors (Burks et al, 2001). eIF3 stimulates binding of the ternary complex (eIF2, Met-tRNA Met , GTP) to the 40S ribosome, inhibits premature association of 40S and 60S subunits, and may serve as a bridge between eIF4G and the 40S ribosome during the early stage of initiation (Hinnebusch, 2006;Siridechadilok et al, 2005).…”