Effects of domain exchanges between Escherichia coli and mammalian mitochondrial EF-Tu on interactions with guanine nucleotides, aminoacyl-tRNA and ribosomes

Escherichia coli elongation factor (EF-Tu) and the corresponding mammalian mitochondrial factor, EF-Tu mt, show distinct differences in their affinities for guanine nucleotides and in their interactions with elongation factor Ts (EF-Ts) and mitochondrial tRNAs. To investigate the roles of the three domains of EF-Tu in these differences, six chimeric proteins were prepared in which the three domains were systematically switched. E. coli EF-Tu binds GDP much more tightly than EF-Tu mt. This difference does not reside in domain I alone but is regulated by interactions with domains II and III. All the chimeric proteins formed ternary complexes with GTP and aminoacyl-tRNA although some had an increased or decreased activity in this assay. The activity of E. coli EF-Tu but not of EF-Tu mt is stimulated by E. coli EF-Ts. The presence of any one of the domains of EF-Tu mt in the prokaryotic factor reduced its interaction with E. coli EF-Ts 2–3-fold. In contrast, the presence of any of the three domains of E. coli EF-Tu in EF-Tu mt allowed the mitochondrial factor to interact with bacterial EF-Ts. This observation indicates that even domain II which is not in contact with EF-Ts plays an important role in the nucleotide exchange reaction. EF-Ts mt interacts with all of the chimeras produced. However, with the exception of domain III exchanges, it inhibits the activities of the chimeras indicating that it could not be productively released to allow formation of the ternary complex. The unique ability of EF-Tu mt to promote binding of mitochondrial Phe-tRNA Phe to the A-site of the ribosome resides in domains I and II. These studies indicate that the interactions of EF-Tu with its ligands is a complex process involving cross-talk between all three domains.