Translational Regulation via 5′ mRNA Leader Sequences Revealed by Mutational Analysis of the Arabidopsis Translation Initiation Factor Subunit eIF3h

Abstract

Eukaryotic translation initiation factor 3 (eIF3) consists of core subunits that are conserved from yeast to man as well as less conserved, noncore, subunits with potential regulatory roles. Whereas core subunits tend to be indispensable for cell growth, the roles of the noncore subunits remain poorly understood. We addressed the hypothesis that eIF3 noncore subunits have accessory functions that help to regulate translation initiation, by focusing on the Arabidopsis thaliana eIF3h subunit. Indeed, eIF3h was not essential for general protein translation. However, results from transient expression assays and polysome fractionation indicated that the translation efficiency of specific 59 mRNA leader sequences was compromised in an eif3h mutant, including the mRNA for the basic domain leucine zipper (bZip) transcription factor ATB2/AtbZip11, translation of which is regulated by sucrose. Among other pleiotropic developmental defects, the eif3h mutant required exogenous sugar to transit from seedling to vegetative development, but it was hypersensitive to elevated levels of exogenous sugars. The ATB2 mRNA was rendered sensitive to the eIF3h level by a series of upstream open reading frames. Moreover, eIF3h could physically interact with subunits of the COP9 signalosome, a protein complex implicated primarily in the regulation of protein ubiquitination, supporting a direct biochemical connection between translation initiation and protein turnover. Together, these data implicate eIF3 in mRNA-associated translation initiation events, such as scanning, start codon recognition, or reinitiation and suggest that poor translation initiation of specific mRNAs contributes to the pleiotropic spectrum of phenotypic defects in the eif3h mutant.

An Oncogenic Role for the Phosphorylated h-Subunit of Human Translation Initiation Factor eIF3

“…eIF3h is absent in Saccharomyces cerevisiae and indeed is dispensable in Arabidopsis thaliana, although its level in plant cells affects the translation of specific mRNAs (30). In contrast, eIF3h was shown to be present in a minimal eIF3 complex capable of functioning in the in vitro binding of mRNA to ribosomes (31).…”

Arabidopsis eIF3e is regulated by the COP9 signalosome and has an impact on development and protein translation

“…During seedling development, wheat eIF3 subunits accumulate in an asynchronized fashion (Gallie et al, 1998), consistent with temporally dispensable and perhaps regulatory roles of some of its subunits. We recently showed that the h subunit of Arabidopsis eIF3 is dispensable for basal translation, but essential for translation of specific transcripts carrying multiple upstream open reading frames in their 5¢ leader (Kim et al, 2004).…”

Recent Discoveries on the Role of TOR (Target of Rapamycin) Signaling in Translation in Plants

“…Identification of noncanonical RPFs promoting efficient translation of uORF-mRNAs was pioneered in plants, implicating the 60S ribosomal protein L24B (Nishimura et al, 2005;Zhou et al, 2010), eIF3 subunit h (Kim et al, 2004;Zhou et al, 2010;Roy et al, 2010), and TOR (Schepetilnikov et al, 2013(Schepetilnikov et al, , 2017. Two critical Arabidopsis mutants, eif3h-1, which expresses C-terminal truncation alleles of eIF3h, and rpl24b (short valve 1), have contributed greatly to our understanding of reinitiation phenomena (Nishimura et al, 2005;Zhou et al, 2010).…”

“…eIF3 was implicated at every step of translation initiation, including 43S and 48S preinitiation complex formation assembly, AUG start codon recognition, and reinitiation of translation, when ribosomes terminating translation of the leader uORF resume scanning and reinitiate at a further downstream ORF on the same mRNA (Park et al, 2001;Kim et al, 2004;Pöyry et al, 2007;Schepetilnikov et al, 2013).…”