ABSTRACT
Previous studies have demonstrated that the alpha subunit of eukaryotic initiation factor 2 (eIF-2 alpha), encoded by the SUI2 gene in the yeast Saccharomyces cerevisiae, is phosphorylated at Ser-51 by the GCN2 kinase in response to general amino acid control. Here we describe that yeast eIF-2 alpha is a constitutively phosphorylated protein species that is multiply phosphorylated by a GCN2-independent mechanism. 32Pi labeling and isoelectric focusing analysis of a SUI2+ delta gcn2 strain identifies eIF-2 alpha as radiolabeled and a single isoelectric protein species. Treatment of SUI2+ delta gcn2 strain extracts with phosphatase results in the identification of three additional isoelectric forms of eIF-2 alpha that correspond to the stepwise removal of three phosphates from the protein. Mutational analysis of SUI2 coupled with biochemical analysis of eIF-2 alpha maps the sites to the carboxyl region of SUI2 that correspond to Ser residues at amino acid positions 292, 294, and 301 that compose consensus casein kinase II sequences. 32Pi labeling or isoelectric focusing analysis of eIF-2 alpha from conditional casein kinase II mutants indicated that phosphorylation of eIF-2 alpha is abolished or dephosphorylated forms of eIF-2 alpha are detected when these strains are grown at the restrictive growth conditions. Furthermore, yeast casein kinase II phosphorylates recombinant wild-type eIF-2 alpha protein in vitro but does not phosphorylate recombinant eIF-2 alpha that contains Ser-to-Ala mutations at all three consensus casein kinase II sequences. These data strongly support the conclusion that casein kinase II directly phosphorylates eIF-2 alpha at one or all of these Ser amino acids in vivo. Although substitution of SUI2 genes mutated at these sites for the wild-type gene have no obvious effect on cell growth, one test that we have used appears to demonstrate that the inability to phosphorylate these sites has a physiological consequence on eIF-2 function in S. cerevisiae. Haploid strains constructed to contain Ser-to-Ala mutations at the consensus casein kinase II sequences in SUI2 in combination with a mutated allele of either the GCN2, GCN3, or GCD7 gene have synthetic growth defects. These genetic data appear to indicate that the modifications that we describe at the carboxyl end of the eIF-2 alpha protein are required for optimal eIF-2 function in S. cerevisiae.
