Preliminary X-ray Analysis of the Crystalline Complex between Poypeptide Chain Elongation Factor, Tu, and GDP

Abstract

DURING protein synthesis in bacteria, aminoacyl tRNA is transported to the ribosome complexed with a protein elongation factor Tu (EF-TU) and GTP^1–3. In the ribosome the aminoacyl tRNA is positioned onto its appropriate codon, GTP is hydrolysed, and EF-Tu is released as a stable complex with GDP. The EF-Tu·GDP complex is then reconverted to EF-Tu·GTP by means of another soluble protein, EF-Ts. Transfer factors analogous to EF-Tu but with different physical properties have also been isolated from eucaryotic systems. The EF-Tu·GDP complex has been purified to homogeneity from Escherichia coli^4,5. The protein factor has a molecular weight around 40,000 and is composed of a single polypeptide chain. It forms a stable 1 : 1 complex with GDP which has a dissociation constant of 3 × 10⁻⁹ M⁴. In the purification of EF-Tu from E. coli, needle shaped microcrystals have been reported^4,5 on addition of ammonium sulphate; these crystals were too small for X-ray diffraction analysis, however. Here we report the preparation of large crystals of an EF-Tu·GDP complex and describe their preliminary analysis by X-ray diffraction.