GCN1, a translational activator of GCN4 in Saccharomyces cerevisiae, is required for phosphorylation of eukaryotic translation initiation factor 2 by protein kinase GCN2.
Phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF-2 alpha) by the protein kinase GCN2 mediates increased translation of the transcriptional activator GCN4 in amino acid-starved yeast cells. We show that this key phosphorylation event and the attendant translational induction of GCN4 are dependent on the product of a previously uncharacterized gene, GCN1. Inactivation of GCN1 did not affect the level of eIF-2 alpha phosphorylation when mammalian eIF-2 alpha kinases were expressed in yeast cells in place of GCN2, arguing against an involvement of GCN1 in dephosphorylation of eIF-2 alpha. In addition, while GCN1 is required in vivo for phosphorylation of eIF-2 alpha by GCN2, cell extracts from gcn1 delta strains contained wild-type levels of GCN2 eIF-2 alpha-kinase activity. On the basis of these results, we propose that GCN1 is not needed for GCN2 kinase activity per se but is required for in vivo activation of GCN2 in response to the starvation signal, uncharged tRNA. GCN1 encodes a protein of 297 kDa with an 88-kDa region that is highly similar in sequence to translation elongation factor 3 identified in several fungal species. This sequence similarity raises the possibility that GCN1 interacts with ribosomes or tRNA molecules and functions in conjunction with GCN2 in monitoring uncharged tRNA levels during the process of translation elongation.
Section on Molecular Genetics of Lower Eukaryotes, National Institute of Child Health and Human Development, Bethesda, Maryland 20892.
M J
MJ
Marton
D
D
Crouch
A G
AG
Hinnebusch
D000595:Amino Acid Sequence; D001483:Base Sequence; D015825:Chromosomes, Fungal; D003001:Cloning, Molecular; D004271:DNA, Fungal / Q000235:genetics / Q000302:isolation & purification; D004268:DNA-Binding Proteins; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism*; D005656:Fungal Proteins / Q000235:genetics* / Q000378:metabolism*; D017353:Gene Deletion; D005800:Genes, Fungal; D005838:Genotype; D008969:Molecular Sequence Data; D010445:Peptide Elongation Factors; D010766:Phosphorylation; D010957:Plasmids; D011494:Protein Kinases / Q000235:genetics* / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D015183:Restriction Mapping; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins; D017386:Sequence Homology, Amino Acid; D015534:Trans-Activators / Q000235:genetics* / Q000378:metabolism*
D004271:DNA, Fungal; D004268:DNA-Binding Proteins; D015852:Eukaryotic Initiation Factor-2; D005656:Fungal Proteins; C501389:GCN1 protein, S cerevisiae; D010445:Peptide Elongation Factors; D012333:RNA, Messenger; D029701:Saccharomyces cerevisiae Proteins; D015534:Trans-Activators; D011494:Protein Kinases; C512912:GCN2 protein, S cerevisiae; D017346:Protein Serine-Threonine Kinases