Phosphorylation of the alpha-subunit of plant eukaryotic initiation factor 2 prevents its association with polysomes but does not considerably suppress protein synthesis

Phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2 alpha) and subsequent inhibition of protein synthesis is a major survival response to different stresses in animal and yeast cells. However, the role of this regulatory mechanism in plants is not unambiguously established to date. Here we describe a slight reduction of polysome abundance in Nicotiana benthamiana after the transient expression of a cDNA, AteIF2 alpha(S56D), encoding a phosphomimetic form of Arabidopsis thaliana eIF2 alpha. In contrast, the expression of a cDNA, AteIF2 alpha(S56A), that encodes a non-phosphorylatable form of AteIF2 alpha caused slightly elevated polysome formation compared to the control. Recombinant AteIF2 alpha(S56A) was detected in association with 40S ribosomal subunit-containing complexes and also in the polysomal fraction, while recombinant AteIF2 alpha(S56D) was detected mainly in complex with 40S subunits. Intentional phosphorylation of TaeIF2 alpha induced by L-histidinol in a wheat germ (Triticum aestivum) cell-free extract did not reduce the abundance of polysomes. Interestingly, the phosphorylated TaeIF2 (alpha P) was not detected in the polysomal fraction, similar to AteIF2 alpha(S56D) in the in vivo experiment. Using mRNAs with a 'Strepto-tag' in the 3' untranslated region, the 485 pre-initiation complexes isolated from histidinol-treated wheat germ extracts were shown to contain phosphorylated TaeIF2(alpha P). Thus, the phosphorylation of plant eIF2 does not greatly affect its ability to participate in the initiation of mRNA translation, in contrast to animals and yeast, in which eIF2 alpha phosphorylation results in profound suppression of protein synthesis.

Automated summary

The phosphorylation of plant eIF2 does not greatly affect its ability to participate in the initiation of mRNA translation, in contrast to animals and yeast, in which eIF2 alpha phosphorylation results in profound suppression of protein synthesis. This was shown through experiments in tobacco and wheat, where phosphorylated plant eIF2 was still able to participate in translation initiation without significant reduction in polysome abundance.