Abstract
Eukaryotic translation initiation factor 4G (eIF4G) functions to organize the assembly of initiation factors required for the recruitment of a 40S ribosomal subunit to an mRNA and for interacting with the poly(A) binding protein (PABP). Many eukaryotes express two highly similar eIF4G isoforms. eIFiso4G, one of two isoforms in plants, is highly divergent and unusually small in size. Unlike animal and yeast eIF4G, the domain organization of plant eIF4G proteins is largely unknown. Consequently, little is known about the conservation of plant eIF4G with those in other eukaryotes. In this study, we show that eIFiso4G is similar to other eIF4G proteins in that there are interaction domains for eIF4A and PABP and we identify, for the first time, the interaction domain for eIF4B. In contrast to previous reports, two eIF4A binding domains in eIFiso4G were identified, similar in number and organization to those of animal eIF4G. The eIFiso4G domain organization does differ, however, in that the N-terminal eIF4A binding domain overlaps with the eIF4B and PABP binding domains. Moreover, the eIF4B and PABP binding domains overlap. PABP and eIF4B compete with eIF4A for binding eIFiso4G in the absence of the C-terminal eIF4A binding domain but not when both eIF4A binding domains are present, suggesting that the C-terminal eIF4A interaction domain functions to stabilize the association of eIF4A with eIFiso4G in the presence of eIF4B or PABP. Competitive binding to eIFiso4G was also observed between eIF4B and PABP. These observations reveal an important function of the C-terminal eIF4A binding domain in maintaining the interaction of multiple partner proteins with eIFiso4G despite the substantial divergence in its size and domain organization.