New Insights into an Old Problem: Ternary Complex (Met-tRNA _f ·eIF-2 ·GTP) Formation in Animal Cells

Abstract

An early step in the initiation of protein synthesis in eukaryotic cells is the formation of a ternary complex between initiation Factor 2, Met-tRNA _f and GTP; Met-tRNA _f ·elF- 2 ·GTP. This complex is an obligate intermediate in the binding of Met-tRNA _f to 40S ribosomes and thus is important for translation of all mRNAs. Several reports have indicated that formation and/or stability of the ternary complex in vitro may require a number of additional protein factors (for a review, see Gupta et al, 1987). For example, it has been reported that the bulk of reticulocyte eIF-2 is purified as eIF-2 ·GDP and a protein factor eIF-2B is required to displace GDP from eIF-2 ·GDP and subsequently to form ternary complex in the presence of Mg²⁺. Recently, Roy et al (1988) reported that two additional protein factor preparations, Co-eIF-2A and Co-eIF-2C are required for the formation of a stable ternary complex in the presence of natural mRNAs. Co-eIF-2A and Co-eIF-2C are two highly purified protein preparations that possess eIF-2 stimulatory activities described in a number of papers from Gupta and co-workers (see Gupta et al, 1987, Roy et al, 1988). Co-eIF-2A possesses a mass of 94 kDa and appears to be unrelated to any of the other known initiation factors. On the other hand, Co-eIF-2C resembles the previously described initiation factor eIF-3. Because of the considerable controversy surrounding the roles of these auxiliary factors in ternary complex formation, our three laboratory groups collaborated to compare the structures of the relevant initiation factors prepared independently and to re-examine their roles in ternary complex formation.