The roles of alternative cap-binding proteins of Arabidopsis thaliana

The mRNA cap-binding complexes eIF4F (made up of the cap-binding protein eIF4E and the large scaffold eIF4G) and eIFiso4F (made up of the plant-specific isoforms eIFiso4E and eIFiso4G) have established roles in translation initiation. However, other cap-binding proteins are known to be encoded in the Arabidopsis thaliana genome. We have chosen to investigate the biochemical properties and potential functions of these proteins. We have identified the eIF4E-like proteins, eIF4E1b and eIF4E1c, as Brassicaceae-specific eIF4E isoforms with the ability to form cap-binding complexes. These proteins are able to complement an eIF4E deletion in yeast. However, their limited expression in A. thaliana along with their relatively weak binding affinity for eIF4G and more limited ability to promote translation in vitro indicate a possible role outside of canonical translation initiation pathways. The alternative cap-binding protein 4EHP is conserved from animals to plants, but its role and binding partners in plants are not well defined. We demonstrate that a homologous complex to the 4EHP-GIGYF2 cap-binding complex observed in mammals is present in A. thaliana. The plant complex appears important to proper development, as double knockouts show a noticeable developmental phenotype and dysregulation of gene expression, but the viability of these knockouts in A. thaliana may offer an opportunity to research the complex’s function that cannot be performed in animal systems as knockouts are lethal. RNA immunoprecipitation studies find that 4EHP and GIGYF associate with non-coding RNA in A. thaliana, and nucleocytoplasmic fractionation supports a possible nuclear role for the proteins. These findings indicate that the 4EHP-GIGYF complex may have an unexpected role in bridging non-coding RNA to gene expression in plants.