Oxygen deprivation stimulates Ca2+-mediated phosphorylation of mRNA cap-binding protein elF4E in maize roots

Abstract

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 elF4E, we characterized the distinct mRNA cap-binding proteins elF4E and elFiso4E of maize. These proteins have 45% deduced amino acid sequence identity and are highly conserved at residues of elF4E that function in intermolecular interactions in animals. Maize elF4E is a phosphoprotein. O2 deprivation resulted in a decrease in the isoelectric point of elF4E, consistent with additional phosphorylation. Modification of elF4E 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 elF4E modification. In contrast, no isoelectric variants of elFiso4E were detected. The possible role of cytosolic Ca2+ and pH in regulation of mRNA cap-binding protein activity under O2 deprivation is discussed.