HELIX UNWINDING IN THE EFFECTOR REGION OF ELONGATION-FACTOR EF-TU-GDP

Background: Elongation factor Tu (EF-Tu) in its GTP conformation is a carrier of aminoacylated tRNAs (aa-tRNAs) to the ribosomal A site duri ng protein biosynthesis. The ribosome triggers GTP hydrolysis, resulti ng in the dissociation of EF-Tu-GDP from the ribosome. The affinity of EF-Tu for other molecules involved in this process, some of which are unknown, is regulated by two regions (Switch I and Switch II) that ha ve different conformations in the GTP and GDP forms, The structure of the GDP form of EF-Tu is known only as a trypsin-modified fragment, wh ich lacks the Switch I, or effector, domain. The aim of this work was to establish the overall structure of intact EF-Tu-GDP, in particular the structure of the effector domain. Results: The crystal structures of intact EF-Tu-GDP from Thermus aquaticus and Escherichia coli have b een determined at resolutions of 2.7 Angstrom and 3.8 Angstrom, respec tively. The structures confirm the domain orientation previously found in the structure of partially trypsin-digested EF-Tu-GDP. The structu res of the effector region in T. aquaticus and E. coli EF-Tu-GDP are v ery similar, The C-terminal part of the effector region of EF-Tu-GDP i s a beta hairpin; in EF-Tu-GTP, this region forms an oc helix, This co nformational change is not a consequence of crystal packing. Conclusio ns: EF-Tu undergoes major conformational changes upon GTP hydrolysis. Unlike other GTP-binding proteins, EF-Tu exhibits a dramatic conformat ional change in the effector region, involving an unwinding of a small helix and the formation of a beta hairpin structure. This change is p resumably involved in triggering the release of tRNA, and EF-Tu, from the ribosome.