Unique classes of mutations in the Saccharomyces cerevisiae G-protein translation elongation factor 1A suppress the requirement for guanine nucleotide exchange.

G-proteins play critical roles in many cellular processes and are regulated by accessory proteins that modulate the nucleotide-bound state. Such proteins, including eukaryotic translation elongation factor 1A (eEF1A), are frequently reactivated by guanine nucleotide exchange factors (GEFs). In the yeast Saccharomyces cerevisiae, only the catalytic subunit of the GEF complex, eEF1Balpha, is essential for viability. The requirement for the TEF5 gene encoding eEF1Balpha can be suppressed by the presence of excess substrate, eEF1A. These cells, however, have defects in growth and translation. Two independent unbiased screens performed to dissect the cause of these phenotypes yielded dominant suppressors that bypass the requirement for extra eEF1A. Surprisingly, all mutations are in the G-protein eEF1A and cluster in its GTP-binding domain. Five mutants were used to construct novel strains expressing only the eEF1A mutant at normal levels. These strains show no growth defects and little to no decreases in total translation, which raises questions as to the evolutionary expression of GEF complexity and other potential functions of this complex. The location of the mutations on the eEF1A-eEF1Balpha structure suggests that their mechanism of suppression may depend on effects on the conserved G-protein elements: the P-loop and NKXD nucleotide-binding element.

The translation elongation factor eEF1B plays a role in the oxidative stress response pathway.

The multi-subunit guanine nucleotide exchange factor eEF1B for Saccharomyces cerevisiae Translation Elongation Factor 1A (eEF1A) has catalytic (eEF1Balpha) and noncatalytic (eEF1Bgamma) subunits. Deletion of the two nonessential genes encoding eEF1Bgamma has no dramatic effects on total protein synthesis or translational fidelity. Instead, loss of each gene gives resistance to oxidative stress, and loss of both is additive. The level of stress resistance is similar to overexpression of the Yap1p stress transcription factor and is dependent on the presence of the YAP1gene. Cells lacking the catalytic eEF1Balpha subunit show even greater resistance to CdSO(4), with or without eEF1Bgamma present. Thus, the loss of guanine nucleotide exchange activity promotes the resistance. As nucleotide exchange is a critical regulator of most G-proteins, these results indicate a new mechanism in the growing list of examples of post-transcriptional responses to cellular stress.