Salts induce structural changes in elongation factor 1 alpha from the hyperthermophilic archaeon Sulfolobus solfataricus: A fourier transform infrared spectroscopic study

Elongation factor I a from the hyperthermophilic archaeon Sulfolobus solfat aricus (SsEF-1 alpha) carries the aminoacyl tRNA to the ribosome; it binds GDP or GTP, and it is also endowed with an intrinsic GTPase activity that i s triggered in vitro by NaCl at molar concentrations [Masullo, M., De Vendi ttis, E., and Bocchini, V. (1994) J. Biol. Chem. 269, 20376-20379]. The str uctural propel-ties of SsEF-1 alpha were investigated by Fourier transform infrared spectroscopy. The estimation of the secondary structure of the SsE F-1 alpha . GDP complex, made by curve fitting of the an-tide I' band or by factor analysis of the an-tide I band, indicated a content of 34-36% alpha -helix, 35-40% beta -sheet, 14-19% turn, and 7% unordered structure. The s ubstitution of the GDP bound with the slowly hydrolyzable GTP analogue Gpp( NH)p induced a slight increase in the a-helix and fi-sheet content. On the other hand, the cc-helix content of the SsEF-1 alpha . GDP complex increase d upon addition of salts, and the highest effect was produced by 5 M NaCl. The thermal stability of the SsEF-1 alpha . GDP complex was significantly r educed when the GDP was replaced with Gpp(NH)p or in the presence of NaBr o r NH4Cl, whereas a lower destabilizing, effect was provoked by NaCl and KCl . Therefore, the extent of the destabilizing effect of salts depended on th e nature of both the cation and the anion. The data suggested that the sodi um ion was responsible for the induction of the GTPase activity, whereas th e anion modulated the enzymatic activity through destabilization of particu lar regions of SsEF-1 alpha. Finally, the infrared data suggested that, in particular region(s) of the polypeptide chain, the SsEF-1 alpha -Gpp(NH)p c omplex possesses structural conformations which are different from those pr esent in the SsEF-1 alpha -GDP complex.