Genetic and biochemical evidence for yeast GCN2 protein kinase polymerization.

The GCN2 (general control kinase 2) protein is an eIF2-alpha (eukaryotic initiation factor alpha) kinase which mediates translational derepression of the yeast general control transcriptional activator, GCN4, upon amino-acid starvation. We isolated and characterized GCN2 mutations differentially affecting GCN2 function. Mutations mapping in, or close to, the ATP-binding site of the kinase moiety result in constitutively activated GCN2 molecules. A C-terminal regulatory mutation dramatically affects translation initiation rates resulting in pleiotropic phenotypes. The effect of mutations in both regions were found to depend on eIF2-alpha phosphorylation. We have demonstrated that GCN2 mutants have altered autophosphorylation activities in vitro, depending on the presence or absence of a wild-type GCN2 gene and that GCN2 elutes in gel-filtration chromatography fractions with high apparent molecular mass. Both these genetic and biochemical findings suggest that GCN2 functioning might involve polymerization to form dimers or tetramers.