Asp56 in actin is critical for the full activity of the amino acid starvation-responsive kinase Gcn2.

Eukaryotes harbour a conserved signalling pathway, called General Amino Acid Control (GAAC) in Saccharomyces cerevisiae, for overcoming amino acid starvation. Upon starvation, the protein kinase Gcn2, which phosphorylates the eukaryotic translation initiation factor eIF2α, becomes stimulated to trigger the GAAC response. Genetic studies suggest that Yih1, which is the yeast homolog of mammalian IMPACT and which binds monomeric actin, inhibits Gcn2 when released from actin. Here, we found that D56A substitution in actin (the act1-9 allele) leads to reduced eIF2α phosphorylation, suggesting that the Asp56 residue is required for full Gcn2 activation. In the act1-9 mutant, Yih1 overexpression further enhanced the sensitivity to amino acid starvation-inducing drugs and further impaired eIF2α phosphorylation, suggesting that Gcn2 inhibition was mediated via Yih1. The D56A substitution may impair the actin-Yih1 interaction, directly or indirectly, thereby increasing the amount of Yih1 available to inhibit Gcn2.

Novel characteristics of the biological properties of the yeast Saccharomyces cerevisiae eukaryotic initiation factor 2A.

Eukaryotic initiation factor 2A (eIF2A) has been shown to direct binding of the initiator methionyl-tRNA (Met-tRNA(i)) to 40 S ribosomal subunits in a codon-dependent manner, in contrast to eIF2, which requires GTP but not the AUG codon to bind initiator tRNA to 40 S subunits. We show here that yeast eIF2A genetically interacts with initiation factor eIF4E, suggesting that both proteins function in the same pathway. The double eIF2A/eIF4E-ts mutant strain displays a severe slow growth phenotype, which correlated with the accumulation of 85% of the double mutant cells arrested at the G(2)/M border. These cells also exhibited a disorganized actin cytoskeleton and elevated actin levels, suggesting that eIF2A might be involved in controlling the expression of genes involved in morphogenic processes. Further insights into eIF2A function were gained from the studies of eIF2A distribution in ribosomal fractions obtained from either an eIF5BDelta (fun12Delta) strain or a eIF3b-ts (prt1-1) strain. It was found that the binding of eIF2A to 40 and 80 S ribosomes was not impaired in either strain. We also found that eIF2A functions as a suppressor of Ure2p internal ribosome entry site-mediated translation in yeast cells. The regulation of expression from the URE2 internal ribosome entry site appears to be through the levels of eIF2A protein, which has been found to be inherently unstable with a half-life of approximately 17 min. It was hypothesized that this instability allows for translational control through the level of eIF2A protein in yeast cells.

New targets for antivirals: the ribosomal A-site and the factors that interact with it.

Many viruses use programmed -1 ribosomal frameshifting to ensure the correct ratio of viral structural to enzymatic proteins. Alteration of frameshift efficiencies changes these ratios, in turn inhibiting viral particle assembly and virus propagation. Previous studies determined that anisomycin, a peptidyl transferase inhibitor, specifically inhibited -1 frameshifting and the ability of yeast cells to propagate the L-A and M(1) dsRNA viruses (J. D. Dinman, M. J. Ruiz-Echevarria, K. Czaplinski, and S. W. Peltz, 1997, Proc. Natl. Acad. Sci. USA 94, 6606-6611). Here we show that preussin, a pyrollidine that is structurally similar to anisomycin (R. E. Schwartz, J. Liesch, O. Hensens, L. Zitano, S. Honeycutt, G. Garrity, R. A. Fromtling, J. Onishi, and R. Monaghan, 1988. J. Antibiot. (Tokyo) 41, 1774--1779), also inhibits -1 programmed ribosomal frameshifting and virus propagation by acting at the same site or through the same mechanism as anisomycin. Since anisomycin is known to assert its effect at the ribosomal A-site, we undertook a pharmacogenetic analysis of mutants of trans-acting eukaryotic elongation factors (eEFs) that function at this region of the ribosome. Among mutants of eEF1A, a correlation is observed between resistance/susceptibility profiles to preussin and anisomycin, and these in turn correlate with programmed -1 ribosomal frameshifting efficiencies and killer virus phenotypes. Among mutants of eEF2, the extent of resistance to preussin correlates with resistance to sordarin, an eEF2 inhibitor. These results suggest that structural features associated with the ribosomal A-site and with the trans-acting factors that interact with it may present a new set of molecular targets for the rational design of antiviral compounds.