STUDIES ON THE POLYPEPTIDE ELONGATION-FACTOR-2 FROM SULFOLOBUS-SOLFATARICUS - INTERACTION WITH GUANOSINE NUCLEOTIDES AND GTPASE ACTIVITY STIMULATED BY RIBOSOMES

The elongation factor 2 from the thermoacidophilic archaeon Sulfolobus solfataricus (SsEF-2) binds [H-3]GDP at 1:1 molar ratio. The bound [H -3]GDP is displaced by GTP or its nonhydrolyzable analogue guanyl-5'-y l imidodiphosphate (Gpp(NH)p) but not by ATP, thus indicating that onl y the two guanosine nucleotides compete for the same binding site. The affinity of SsEF-2 for [H-3]GDP is higher than that for GTP and Gpp(N H)p. On the contrary, in the presence of ribosomes the affinity of SsE F-2 for GDP is lower than that for Gpp(NH)p. SsEF-2 is endowed with an intrinsic hardly detectable GTPase activity that is stimulated by rib osomes up to 2000-fold. The ribosome-stimulated SsEF-2 GTPase (GTPase( r)) reaches a maximum at pH 7.8 and is not affected by ATP but is comp etitively inhibited by either GDP or Gpp(NH)p. Both K-m for [gamma-P-3 2]GTP and k(cat) of GTPase(r) increase with increasing temperature, an d the highest catalytic efficiency is reached at 80 degrees C. The ADP -ribosylation of SsEF-2 does not significantly affect either the bindi ng of GDP and GTP or the kinetics of the GTPase(r). A hypothesis on th e stimulation by ribosome of SsEF-2 GTPase is proposed.