A PROTEIN COMPLEX OF TRANSLATIONAL REGULATORS OF GCN4 MESSENGER-RNA IS THE GUANINE NUCLEOTIDE-EXCHANGE FACTOR FOR TRANSLATION INITIATION FACTOR-II IN YEAST

In Saccharomyces cerevisiae, phosphorylation of the alpha subunit of translation initiation factor 2 (eIF-2) by protein kinase GCN2 stimulates translation of GCN4 mRNA. In mammalian cells, phosphorylation of eIF-2alpha inhibits the activity of eIF-2B, the GDP-GTP exchange factor for eIF-2. We present biochemical evidence that five translational regulators of GCN4 encoded by GCD1, GCD2, GCD6, GCD7, and GCN3 are components of a protein complex that stably interacts with eIF-2 and represents the yeast equivalent of eIF-2B. In vitro, this complex catalyzes guanine nucleotide exchange on eIF-2 and overcomes the inhibitory effect of GDP on formation of eIEF-2.GTP.Met-initiator tRNA(Met) ternary complexes. This finding suggests that mutations in GCD-encoded subunits of the complex derepress GCN4 translation because they mimic eIF-2alpha phosphorylation in decreasing eIF-2B activity. Our results indicate that translational control of GCN4 involves a reduction in eIF-2B function, a mechanism used in mammalian cells to regulate total protein synthesis in response to stress.