Taking a re-look at cap-binding signatures of the mRNA cap-binding protein eIF4E orthologues in trypanosomatids

Protein translation leading to polypeptide synthesis involves three distinct events, namely, initiation, elongation, and termination. Translation initiation is a multi-step process that is carried out by ribosomes on the mRNA with the assistance of a large number of proteins called translation initiation factors. Trypanosomatids are kinetoplastidas (flagellated protozoans), some of which cause acute disease syndromes in humans. Vector-borne transmission of protozoan parasites like Leishmania and Trypanosoma causes diseases that affect a large section of the world population and lead to significant morbidity and mortality. The mechanisms of translation initiation in higher eukaryotes are relatively well understood. However, structural and functional conservation of initiation factors in trypanosomatids are only beginning to be understood. Studies carried out so far suggests that at least in Leishmania and Trypanosoma eIF4E function may not be restricted to canonical translation initiation and some of the homologues may have alternate/non-canonical functions. Nonetheless, all of them bind the cap analogs, albeit with different efficiencies, indicating that this property may play an important role in the functionality of eIF4Es. Here, I give a brief background of trypanosomatid eIF4Es and revisit the cap-binding signatures of eIF4E orthologues in trypanosomatids, whose genome sequences are available, in detail, in comparison to human eIF4E1 and Trypanosoma cruzi eIF4E5, with an expanded list of members of this group in light of newer findings. The group 1 and 2 eIF4Es may use either a variation of heIF4E1 or T. cruzi eIF4E5 cap-4-binding signatures, while eIF4E5 and eIF4E6 use distinct amino acid contacts.

An isoform of eukaryotic initiation factor 4E from Chrysanthemum morifolium interacts with Chrysanthemum virus B coat protein

Background

Eukaryotic translation initiation factor 4E (eIF4E) plays an important role in plant virus infection as well as the regulation of gene translation.

Methodology/Principal Findings

Here, we describe the isolation of a cDNA encoding CmeIF(iso)4E (GenBank accession no. JQ904592), an isoform of eIF4E from chrysanthemum, using RACE PCR. We used the CmeIF(iso)4E cDNA for expression profiling and to analyze the interaction between CmeIF(iso)4E and the Chrysanthemum virus B coat protein (CVBCP). Multiple sequence alignment and phylogenetic tree analysis showed that the sequence similarity of CmeIF(iso)4E with other reported plant eIF(iso)4E sequences varied between 69.12% and 89.18%, indicating that CmeIF(iso)4E belongs to the eIF(iso)4E subfamily of the eIF4E family. CmeIF(iso)4E was present in all chrysanthemum organs, but was particularly abundant in the roots and flowers. Confocal microscopy showed that a transiently transfected CmeIF(iso)4E-GFP fusion protein distributed throughout the whole cell in onion epidermis cells. A yeast two hybrid assay showed CVBCP interacted with CmeIF(iso)4E but not with CmeIF4E. BiFC assay further demonstrated the interaction between CmeIF(iso)4E and CVBCP. Luminescence assay showed that CVBCP increased the RLU of Luc-CVB, suggesting CVBCP might participate in the translation of viral proteins.

Conclusions/Significance

These results inferred that CmeIF(iso)4E as the cap-binding subunit eIF(iso)4F may be involved in Chrysanthemum Virus B infection in chrysanthemum through its interaction with CVBCP in spatial.

Selective involvement of members of the eukaryotic initiation factor 4E family in the infection of Arabidopsis thaliana by potyviruses

Arabidopsis thaliana plants with mutations in the genes encoding eukaryotic initiation factor (eIF4E) or isoform of eIF4E (eIF(iso)4E) were tested for susceptibility to Clover yellow vein virus (ClYVV), a member of the genus Potyvirus. ClYVV accumulated in both inoculated and upper uninoculated leaves of mutant plants lacking eIF(iso)4E, but not in mutant plants lacking eIF4E. In contrast, Turnip mosaic virus (TuMV), another member of the genus Potyvirus, multiplied in mutant plants lacking eIF4E but not in mutant plants lacking eIF(iso)4E. These results suggest the selective involvement of members of the eIF4E family in infection by potyviruses.

Characterization of mammalian eIF4E-family members

The translational factor eukaryotic initiation factor 4E (eIF4E) is a central component in the initiation and regulation of translation in eukaryotic cells. Through its interaction with the 5' cap structure of mRNA, eIF4E functions to recruit mRNAs to the ribosome. The accumulation of expressed sequence tag sequences has allowed the identification of three different eIF4E-family members in mammals termed eIF4E-1, eIF4E-2 (4EHP, 4E-LP) and eIF4E-3, which differ in their structural signatures, functional characteristics and expression patterns. Unlike eIF4E-1, which is found in all eukaryotes, orthologues for eIF4E-2 appear to be restricted to metazoans, while those for eIF4E-3 have been found only in chordates. Like prototypical eIF4E-1, eIF4E-2 was found to be ubiquitously expressed, with the highest levels in the testis. Expression of eIF4E-3 was detected only in heart, skeletal muscle, lung and spleen. Similarly to eIF4E-1, both eIF4E-2 and eIF4E-3 can bind to the mRNA cap-structure. However, in contrast to eIF4E-1 which interacts with both the scaffold protein, eIF4G and the translational repressor proteins, the eIF4E-binding proteins (4E-BPs), eIF4E-2 and eIF4E-3 each possesses a range of partial activities. eIF4E-2 does not interact with eIF4G, but does interact with 4E-BPs. Conversely, eIF4E-3 interacts with eIF4G, but not with 4E-BPs. Neither eIF4E-2 nor eIF4E-3 is able to rescue the lethality of eIF4E gene deletion in yeast. It is hypothesized that each eIF4E-family member fills a specialized niche in the recruitment of mRNAs by the ribosome through differences in their abilities to bind cap and/or to interact with eIF4G and the 4E-BPs.

Yeast "knockout-and-rescue" system for identification of eIF4E-family members possessing eIF4E-activity

Evidence from several laboratories and sequencing projects has revealed that many eukaryotes contain multiple proteins related in sequence to the human mRNA-cap binding translation initiation factor 4E (eIF4E-1). Although some have been shown to bind cap-analogues, whether all eIF4E-family members function as translation initiation factors is unclear Furthermore, the existence of proteins related to eIF4E complicates the identification of the translation factor by sequence-based approaches. Methods to assess the functionality of eIF4E are limited. The most informative, single assay to identify proteins with eIF4E-activity is that of rescue of the lethal disruption of the single Saccharomyces cerevisiae eIF4E gene. We have developed a simplified yeast eIF4E "knockout-and-rescue" system, the characteristics of which are (i) a haploid system that obviates the needfor a "plasmid shuffle", (ii) a simple G418-based selection for yeast lacking a chromosomal eIF4E gene, and (iii) a glucose-based selection to deplete the strain of a human eIF4E-1 substitute and to assess the eIF4E-activity of an untested elF4E-family member In this form, the yeast eIF4E knockout-and-rescue system becomes a tool available to any laboratory experienced in the selection of microbial strains with antibiotics and standard media for the identification and isolation of cDNAs encoding proteins with eIF4E-activity.