Studies on regulatory roles of the translation initiation factor 3 subunits E and H in Arabidopsis

The three protein complexes known as the PCI complexes (19S P&barbelow;roteasome lid, C&barbelow;OP9 signalosome (CSN), eukaryotic translation I&barbelow;nitiation factor 3) play important roles at the protein synthesis and degradation stages of gene expression. The PCI complexes are defined on the basis of sequence similarities between their paralogous subunits. Interestingly, biochemical interactions have been reported between all three PCI complexes but the physiological significance of those interactions is poorly understood. Two subunits of eIF3, eIF3e and eIF3h, were confirmed to interact with both eIF3 and the CSN. This study was conducted to define the roles of eIF3e and eIF3h in the regulation of translation during Arabidopsis development and to suggest further testable hypotheses regarding the interaction between eIF3 and the CSN. To this end, two T-DNA insertion mutants of the eIF3h gene were isolated and characterized morphologically and biochemically. The eif3h mutant produced pleiotropic developmental defects including delayed growth, reduced meristem activities, and malformed organs. The eif3h mutants share certain phenotypic characteristics with the can mutants, including hypersensitivity to exogenous sucrose, reduced elongation of hypocotyls and partial opening of cotyledons in the dark condition, suggesting their participation in common developmental pathways. A regulatory role of the eIF3h subunit of eIF3 was suggested given that the eif3h mutation did not affect the overall efficiency of translation but differentially regulates translation of a subset of mRNAs such as those of the transcription factors ATB2 and HY5. Concerning the eIF3e subunit, data presented suggest a tissue type specific nucleocytoplasmic distribution of eIF3e. In light of nuclear localization and nuclear export signals present in eIF3e, these results suggest that a dynamic nucleocytoplasmic transport mechanism may mediate a regulatory role of eIF3e during translation. Reverse genetic approaches targeted to eIF3e demonstrated a role of eIF3e during the transition between cell division and differentiation phases of Arabidopsis development. Therefore it is shown that translational control by regulatory subunits of eIF3 participates in the modulation of protein synthesis in order to control development in Arabidopsis. The data further suggest a new type of functional crosstalk between the machineries of translation (eIF3) and protein turnover (CSN) in Arabidopsis.