Abstract
The initiation of protein synthesis requires the involvement of the eukaryotic translation initiation factor (eIF) 4G to promote assembly of the factors needed to recruit a 40S ribosomal subunit to an mRNA. Although many eukaryotes express two eIF4G isoforms that are highly similar, those in plants, referred to as eIF4G and eIFiso4G, are highly divergent in size, sequence, and domain organization. Species of the Brassicaceae and the Cleomaceae also express a divergent eIFiso4G isoform, referred to as eIFiso4G2, not found elsewhere in the plant kingdom. Despite their divergence, eIF4G and eIFiso4G interact with eIF4A, eIF4B, and eIF4E isoforms needed for binding an mRNA. eIF4G and eIFiso4G also interact with the poly(A)-binding protein (PABP) which promotes ribosome recruitment to an mRNA. Increasing the complexity of such an interaction, however, Arabidopsis also expresses three PABP isoforms (PAB2, PAB4, and PAB8) in vegetative and reproductive tissues. In this study, the functional interactions among the eIF4G and the widely-expressed PABP isoforms were examined. Loss of PAB2 or PAB8 in combination with loss of eIF4G or eIFiso4G had little to no effect on growth or fertility whereas pab2 pab8 eif4g or pab2 pab8 eifiso4g1/2 mutants exhibited smaller stature and reduced fertility. Although the pab4 eifiso4g1 mutant grows normally and is fertile, pab4 eif4g or pab4 eifiso4g2 mutants could not be isolated. Even pab4/PAB4 eif4g/eIF4G heterozygous plants exhibited growth defects and low fertility. Mutant co-inheritance analysis in reciprocal crosses with wild-type plants revealed that most ovaries and pollen from pab4/PAB4 eif4g/eIF4G plants were PAB4 eif4g. Similarly, co-inheritance studies with pab4/PAB4 eifiso4g2/eIFiso4G2 plants suggested most ovaries were PAB4 eifiso4g2. These results suggest that a functional interaction between PAB4 and eIF4G and between PAB4 and eIFiso4G2 is required for growth and normal fertility.
Highlights
Protein synthesis in eukaryotes requires multiple translation initiation factors that promote 40 S ribosomal subunit recruitment to an mRNA, initiation codon recognition, and 80 S ribosome assembly [1, 2, 3]
Specific functional interactions between PABP, eIF4G, and eIFiso4G isoforms unwinding secondary structure in a 50-leader that might impede 40 S subunit scanning; and eIF4G, which interacts with eIF4E, eIF4A, eIF4B, eIF3, and the poly(A)-binding protein (PABP) [2, 4, 5, 6]
To examine whether the loss of expression of any one class II PABP gene member in combination with eIF4G would affect growth not observed for the individual mutants, single and double pab mutant plants were crossed with eif4g plants to generate each combinatorial mutant
Summary
Protein synthesis in eukaryotes requires multiple translation initiation factors that promote 40 S ribosomal subunit recruitment to an mRNA, initiation codon recognition, and 80 S ribosome assembly [1, 2, 3]. Unlike other eukaryotes, the plant eIF4G isoforms, referred to as eIF4G and eIFiso4G, differ substantially in size and sequence and each interacts with distinct eIF4E isoforms [13] Both eIF4G isoforms from wheat interact with eIF4B, eIF4A, and PABP. Such divergence has suggested functional specialization in mRNA selection, a conclusion supported by the observation that alfalfa mosaic virus RNA 4 and Arabidopsis HSP21 use wheat eIF4G or eIFiso4G well whereas barley α-amylase, oat globulin mRNAs, and the tobacco etch virus 50-leader use eIF4G preferentially [16, 17]
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