The eIF4F and eIFiso4F Complexes of Plants: An Evolutionary Perspective

Abstract

Translation initiation in eukaryotes requires a number of initiation factors to recruit the assembled ribosome to mRNA. The eIF4F complex plays a key role in initiation and is a common target point for regulation of protein synthesis. Most work on the translation machinery of plants to date has focused on flowering plants, which have both the eIF4F complex (eIF4E and eIF4G) as well as the plant-specific eIFiso4F complex (eIFiso4E and eIFiso4G). The increasing availability of plant genome sequence data has made it possible to trace the evolutionary history of these two complexes in plants, leading to several interesting discoveries. eIFiso4G is conserved throughout plants, while eIFiso4E only appears with the evolution of flowering plants. The eIF4G N-terminus, which has been difficult to annotate, appears to be well conserved throughout the plant lineage and contains two motifs of unknown function. Comparison of eIFiso4G and eIF4G sequence data suggests conserved features unique to eIFiso4G and eIF4G proteins. These findings have answered some questions about the evolutionary history of the two eIF4F complexes of plants, while raising new ones.

Two Arabidopsis Loci Encode Novel Eukaryotic Initiation Factor 4E Isoforms That Are Functionally Distinct from the Conserved Plant Eukaryotic Initiation Factor 4E

“…Alignment and phylogenetic tree construction of eIF4E and eIFiso4E sequences (Fig. 1) show that the eIF4E1b-type protein sequences cluster together, separately from the conserved eIF4E of flowering plants and from eIFiso4E, which diverged from eIF4E early in the flowering plant lineage (Patrick and Browning, 2012). In addition to completed and draft genomes, there is EST evidence of EIF4E1B and/or EIF4E1C in Brassica oleracea, Brassica napus, Raphanus raphanistrum, and Raphanus sativus, as well as Genome Survey Sequence support for the presence of an EIF4E1B-type gene in Sisymbrium irio (Supplemental Fig.…”

“…The eIF4E proteins generally thought to be involved in translation initiation are Class I eIF4E proteins (Joshi et al, 2005), of which two exist in flowering plants: eIF4E, which pairs with eIF4G to form the eIF4F complex, and the plant-specific isoform eIFiso4E, which pairs with eIFiso4G to form eIFiso4F (Mayberry et al, 2011;Patrick and Browning, 2012). Class I eIF4E family members have conserved Trp residues at positions equivalent to Trp-43 and Trp-56 of Homo sapiens eIF4E (Joshi et al, 2005), and the canonical members of this class, such as plant eIF4E and eIFiso4E, have the ability to promote translation through binding of mRNA cap structure and eIF4G (or eIFiso4G).…”

Mechanism of Cytoplasmic mRNA Translation

“…This twoprotein complex, eIFiso4F, is comprised of eIFiso4G and eIFiso4E (Allen et al, 1992;van Heerden and Browning, 1994;Patrick and Browning, 2012). The cap binding proteins, eIF4E and eIFiso4E, have ~50% amino acid sequence similarity and form distinct and specific complexes with their respective binding partners, eIF4G and eIFiso4G (Mayberry et al, 2011).…”

“…The larger scaffold subunits (eIF4G and eIFiso4G) share similarity in the C-terminal half with the eIF4E binding site and two HEAT (Huntington, elongation factor 3, protein phosphatase 2A, and the yeast TOR1 kinase) domains; however, the N-terminal half of eIF4G is absent in eIFiso4G (Patrick and Browning, 2012). eIFiso4F appears to be necessary for proper growth and development of Arabidopsis, as deletion of both eIFiso4G genes results in slow and stunted growth, pale green rosettes and significant reproductive defects (Lellis et al, 2010).…”

Translational Regulation of Cytoplasmic mRNAs

“…For the remainder of this section, recent references are cited simply to serve as a primer into the plant literature, since the factual statements qualify as pan-eukaryotic textbook knowledge. The 5' cap of the mRNA is recognized by the eIF4F complex, which consists of the cap binding protein, eIF4E, and a large scaffold protein, eIF4G (Patrick and Browning, 2012). In preparation for initiation, the small (40S) subunit of the ribosome is loaded with a charged methionyl-tRNA, which is delivered by the trimeric GTP-binding protein, eIF2.…”