Helix unwinding in the effector region of elongation factor EF-Tu–GDP

Abstract

Background Elongation factor Tu (EF-Tu) in its GTP conformation is a carrier of aminoacylated tRNAs (aa-tRNAs) to the ribosomal A site during protein biosynthesis. The ribosome triggers GTP hydrolysis, resulting 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 have 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, which 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 been determined at resolutions of 2.7 å and 3.8 å, respectively. The structures confirm the domain orientation previously found in the structure of partially trypsin-digested EF-Tu–GDP. The structures of the effector region in T. aquaticus and E. coli EF-Tu–GDP are very similar. The C-terminal part of the effector region of EF-Tu–GDP is a β hairpin; in EF-Tu–GTP, this region forms an α helix. This conformational change is not a consequence of crystal packing. Conclusions EF-Tu undergoes major conformational changes upon GTP hydrolysis. Unlike other GTP-binding proteins, EF-Tu exhibits a dramatic conformational change in the effector region, involving an unwinding of a small helix and the formation of a β hairpin structure. This change is presumably involved in triggering the release of tRNA, and EF-Tu, from the ribosome.