MACROMOLECULAR MIMICRY IN PROTEIN-BIOSYNTHESIS

Elongation factor Tu (EF-Tu) is a G-protein which, in its active GTP c onformation, protects and carries aminoacylated tRNAs (aa-tRNAs) to th e ribosome during protein biosynthesis. EF-Tu consists of three struct ural domains of which the N-terminal domain consists of two special re gions (switch I and switch II) which are structurally dependent on the type of the bound nucleotide. Structural studies of the complete func tional cycle of EF-Tu reveal that it undergoes rather spectacular conf ormational changes when activated from the EF-Tu.GDP form to the EF-Tu .GTP form, In its active form, EF-Tu.GTP without much further structur al change interacts with aa-tRNAs in the so called ternary complex, Th e conformational changes of EF-Tu involve rearrangements of the second ary structures of both the switch I and switch II regions, As the swit ch II region forms part of the interface between domains 1 and 3, its structural rearrangement results in a very large change of the positio n of domain 1 relative to domains 2 and 3, The overall shape of the te rnary complex is surprisingly similar to the overall shape of elongati on factor G (EF-G), Thus, three domains of the protein EF-G seem to mi mic the tRNA part of the ternary complex, This macromolecular mimicry has profound implications for the function of the elongation factors o n the ribosome.