Regulation of protein synthesis by eukaryotic initiation factor-2 (eIF-2) phosphorylation is a highly conserved phenomenon in eukaryotes that occurs in response to various stress conditions. Protein kinases capable of phosphorylating eIF-2 have been characterized from mammals and yeast. However, the phenomenon of eIF2--mediated regulation of protein synthesis and the presence of an eIF-2 kinase has not been demonstrated in higher plants. We show that plant eIF-2 (peIF-2) and mammalian eIF-2 (meIF-2) are phosphorylated similarly by both the double-stranded RNA-binding kinase, pPKR, present in plant ribosome salt wash fractions and the meIF-2 kinase, PKR. By several criteria, phosphorylation of peIF-2 is directly correlated with pPKR protein and autophosphorylation levels. Significantly, pPKR is capable of specifically phosphorylating Ser in a synthetic eIF-2 peptide, a key characteristic of the eIF-2 kinase family. Taken together, these data support the concept that pPKR is a member of the eIF-2 kinase family. In addition, the inhibition of brome mosaic virus RNA in vitro translation in wheat germ lysates by the addition of double-stranded RNA, phosphorylated peIF-2, meIF-2, or activated human PKR suggests that plant protein synthesis may be regulated via phosphorylation of eIF-2.

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