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 Proteasome lid, COP9 signalosome (CSN), eukaryotic translation Initiation factor 3) play important roles at the protein synthesis and protein degradation stages of gene expression. The PCI complexes are defined on the basis of sequence similarities between their paralogous subunits. Interestingly, physical biochemical interactions have been reported between all three PCI complexes but the physiological significance of those interactions is poorly understood. Two subunits of elF3, elF3e and elF3h, were confirmed to interact with both elF3 and with the subunits of the CSN. This study was conducted to define the roles of el F3e and el F3h in the regulation of translation during Arabidopsis development and to suggest further testable hypotheses regarding the interaction between elF3 and the CSN. To this end, two T-DNA insertion mutants of the e/F3h gene were isolated and characterized morphologically and biochemically. The eif3h mutant produced pleiotropic developmental defects including delayed growth, reduced meristem activities, and malformed organs such as leaves, roots, fruits, and stamens. The eif3h mutants share certain phenotypic characteristics with the csn 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 elF3h subunit of elF3 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 el F3e subunit, data presented suggest a tissue type specific nucleocytoplasmic distribution of elF3e. In light of nuclear localization and nuclear export signals present in el F3e, these results suggest that a dynamic nucleocytoplasmic transport mechanism may mediate a regulatory role of elF3e during translation. Reverse genetic approaches targeted to el F3e demonstrated a role of el F3e during the transition between cell division and differentiation phases of Arabidopsis development. Therefore it is shown that translational control of specific target mRNAs by regulatory subunits of elF3 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 (elF3) and protein turnover (CSN) in Arabidopsis.