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 tran slation 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 elF-2 alpha hyperphosphorylation catalyzed by mut ationally activated forms of GCN2. The deletion of GCN20 in otherwise wild-type strains impairs derepression of GCN4 translation and reduces the level of elF-2 alpha phosphorylation in vivo, showing that GCN20 is a positive effector of GCN2 kinase function. In accordance with thi s conclusion, GCN20 was co-immunoprecipitated from cell extracts with GCN1, another factor required to activate GCN2, and the two proteins i nteracted in the yeast two-hybrid system. We conclude that GCN1 and GC N20 are components of a protein complex that couples the kinase activi ty of GCN2 to the availability of amino acids. GCN20 is a member of th e 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.