GCN20, a novel ATP binding cassette protein, and GCN1 reside in a complex that mediates activation of the eIF‐2 alpha kinase GCN2 in amino acid‐starved cells.

GCN2 is a protein kinase that phosphorylates the alpha‐subunit of translation initiation factor 2 (eIF‐2) and thereby stimulates translation of GCN4 mRNA in amino acid‐starved cells. We isolated a null mutation in a previously unidentified gene, GCN20, that suppresses the growth‐inhibitory effect of eIF‐2 alpha hyperphosphorylation catalyzed by mutationally activated forms of GCN2. The deletion of GCN20 in otherwise wild‐type strains impairs derepression of GCN4 translation and reduces the level of eIF‐2 alpha phosphorylation in vivo, showing that GCN20 is a positive effector of GCN2 kinase function. In accordance with this conclusion, GCN20 was co‐immunoprecipitated from cell extracts with GCN1, another factor required to activate GCN2, and the two proteins interacted in the yeast two‐hybrid system. We conclude that GCN1 and GCN20 are components of a protein complex that couples the kinase activity of GCN2 to the availability of amino acids. GCN20 is a member of the ATP binding cassette (ABC) family of proteins and is closely related to ABC proteins identified in Caenorhabditis elegans, rice and humans, suggesting that the function of GCN20 may be conserved among diverse eukaryotic organisms.