Elongation Factor 1 from Krebs II Mouse Ascites Cells. Interaction with Guanosine Nucleotides and Aminoacyl-tRNA

The three major forms of elongation factor 1 (EF-1) occurring in Krebs II mouse ascites tumor cells were investigated with respect to their affinities for guanosine nucleotides and aminoacyl-tRNA. The binding of GTP, GDP and 5′-guanylyl-methylene-diphosphonate (Guo-5′-P2-CH2-P) to a pure preparation of the tetramer form of EF-1 and to partially purified dimer and hexamer preparations of the same factor was studied by equilibrium dialysis techniques. Another series of experiments was designed to describe the interaction of the tetramer and dimer form of EF-1 with aminoacyl-tRNAs. The following results were obtained. 1 When measured at 2 °C, the association constants for the complexes between pure EF-I and GTP or GDP were both 92 mM−1. 2 There is one guanosine nucleotide binding site per molecule of the EF-1 tetramer which can be occupied by GTP, GDP or Guo-5′-P2-CH2-P. 3 The affinities of the dimer and hexamer forms of EF-I for GTP and GDP were of the same order of magnitude as the corresponding values relating to the tetramer. 4 Large (tetramer) and small (dimer) aggregates of EF-1 provide equal protection of aminoacyl-tRNA against hydrolytic deacylation. This protective effect depends on the previous interaction of the factor with GTP. Although GDP and Guo-5′-P2-CH2-P bind to EF-1, they cannot replace GTP in this reaction. 5 The protective effect of EF-1 on aminoacyl-tRNAs is limited to tRNA species which can function in polypeptide chain elongation. Met-tRNAtMet and the chemically formylated Met-tRNAMet are only poorly protected by EF-1 indicating that these tRNAs are not well recognized by the factor. The significance of these results with respect to our understanding of the function of elongation factor 1 in mammalian cells is discussed.