Developmental regulation of a plant encoded inhibitor of eukaryotic initiation factor 2α phosphorylation

• Published in: The Plant journal : for cell and molecular biology 1997-08, Vol.12 (2), p.393-400
• Main Authors: Langland, Jeffrey O., Langland, Lisa, Zeman, Cameron, Saha, Debabrata, Roth, Don A.
• Format: Article
• Language: English
• Subjects: Antibodies, Monoclonal; Biological and medical sciences; Cross Reactions; eIF-2 Kinase; Eukaryotic Initiation Factor-2 - antagonists & inhibitors; Fundamental and applied biological sciences. Psychology; Gene Expression Regulation, Developmental; Gene Expression Regulation, Plant; Glycosylation; Humans; Molecular and cellular biology; Molecular genetics; Phosphorylation; Plant Leaves; Plant Proteins - biosynthesis; Plant Proteins - isolation & purification; Plant Proteins - pharmacology; Protein-Serine-Threonine Kinases - antagonists & inhibitors; Reticulocytes - metabolism; Translation. Translation factors. Protein processing
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1046/j.1365-313X.1997.12020393.x

An inhibitor of elF‐2a phosphorylation was identified in various plant species. The plant protein (termed PKI) specifically cross‐reacts with monoclonal antiserum that recognizes the glycosylated, active form of a Mr 67 kD protein analog (p67) from reticulocytes. Northern blot analysis using a probe to the reticulocyte inhibitor cDNA further supports the presence of analogous transcripts in plant tissue. PKI specifically inhibits the phosphorylation of the plant encoded elF‐2α kinase (pPKR) as well as plant and human elF‐2α phosphorylation. The interaction between PKI and pPKR is indicated by their copurification on dsRNA agarose, despite evidence showing that PKI does not bind dsRNA. Further, wheat PKI inhibits human PKR phosphorylation but activity is recovered by immunodepletion of PKI from wheat germ fractions. PKI is temporally regulated during plant growth and development. It is maximally present in extracts from dormant seeds, however, it is not detectable soon after leaf emergence at approximately 48 h post‐imbibition. PKI levels are again detectable at the mid‐milk stage in seed development. Protein levels of pPKR in ribosomal salt wash and cytosolic extracts from healthy plant tissue remain essentially constant throughout the life cycle. In contrast, pPKR activity levels based upon autophosphorylation vary significantly and are inversely correlated with PKI protein levels. Phosphorylation of elF‐2α is a classical mechanism for the downregulation of protein synthesis suggesting that inhibition of pPKR activity by PKI may contribute to the dramatic and rapid increase in protein synthesis observed during seed germination.