Regulation of Protein Synthesis in Mammalian Systems by the Guanine Nucleotide Exchange Factor

Abstract

In mammalian cells, the guanine nucleotide exchange factor (GEF) plays a major role in regulating protein synthesis. The first step in polypeptide chain initiation is the formation of a ternary (eIF-2 · GTP · Met-tRNA f complex where eIF-2 is eu- karyotic initiation factor (elF) 2. This is followed by the transfer of this complex to a 40 S ribosomal subunit (Wahba and Woodley, 1984; Pain, 1986). In the presence of mRNA, other initiation factors and the 60S ribosomal subunit, an 80S initiation complex is formed, thereby setting the stage for polypeptide chain elongation. Upon formation of the 80S initiation complex, GTP is hydrolyzed and eIF-2 is released as a binary complex of eIF-2·GDP. The binary complex is stable in the presence of Mg2+ and is functionally inactive. Regeneration of the ternary (eIF-2 · GTP · Met-tRNA f complex for a new cycle of initiation requires GEF which catalyzes the exchange of eIF-2-bound GDP for GTP (Dholakia and Wahba, 1989). It is at this point in the elF- 2 cycle that the initiation process is regulated (Fig. 1). Phosphorylation of the α-subu- nit of eIF-2 by either the heme-controlled repressor or the double-stranded RNA induced kinase is associated with the cessation of protein synthesis and is due to the inability of GEF to catalyze the GDP/GTP exchange from eIF-2(α-P) · GDP (Pain, 1986). Our studies demonstrate that GEF activity may also be regulated directly by phosphorylation of one of its subunits and the redox state of the cell (Dholakia et al., 1986; Dholakia and Wahba, 1988).