Oxygen deprivation stimulates Ca2+-mediated phosphorylation of mRNA cap-binding protein eIF4E in maize roots

Flooding of maize seedlings causes O2 deprivation that leads to a global reduction in protein synthesis and selective translation of cytoplasmic mRNAs. Since selective translation in animal cells can involve the cap-binding protein eIF4E, we characterized the distinct mRNA cap-binding proteins eIF4E and eIFiso4E of maize. These proteins have 45% deduced amino acid sequence identity and are highly conserved at residues of eIF4E that function in intermolecular interactions in animals. Maize eIF4E is a phosphoprotein. O2 deprivation resulted in a decrease in the isoelectric point of eIF4E, consistent with additional phosphorylation. Modification of eIF4E was mimicked by treatment with caffeine under aerobic conditions and blocked by treatment with ruthenium red under O2 deprivation, implicating Ca2+ as a second messenger in eIF4E modification. In contrast, no isoelectric variants of eIFiso4E were detected. The possible role of cytosolic Ca2+ and pH in regulation of mRNA cap-binding protein activity under O2 deprivation is discussed.