ATPASE STRONGLY BOUND TO HIGHER EUKARYOTIC RIBOSOMES

80 S ribosomes from a number of higher eukaryotic organisms are able t o hydrolyse ATP and GTP without the addition of soluble protein factor s. ATPase seems to be an intrinsic activity of the ribosome, as indica ted by the findings that ATPase activity is not diminished upon dissoc iation of ribosomes and reassociation of subunits, by washing with 0.6 6 M (KCl + NH4Cl) or 0.6 M LiCl treatment and ethanol precipitation; 1 .5 M LiCl treatment removes only 40% ATPase activity. 80 S ribosomes a re able to bind a variety of NTPs, NDPs and NTP analogues, with a pref erence for ATP. Effective inhibitors of the ribosomal ATPase are ammon ium metavanadate and alcaloid emetine. The ATPase activity is present on both ribosomal subunits, which may reflect the existence of two cat alytical sites for ATP on the 80 S ribosome. Ribosomal ATPase is stimu lated by the occupancy of the A site, in particular with charged tRNA. The ATPase inhibitor adenylylimidodiphosphate almost completely preve nts elongation-factor(EF)-1-dependent binding of Phe-tRNA(Phe) to the A site. The hydrolysis of ATP, therefore, is likely to be involved in the mechanism of tRNA binding to the A site of the 80 S ribosome. As f ar as wide substrate specificity and possible participation in tRNA in teraction with the ribosome are concerned, the ribosomal ATPase seems to be similar to EF-3 found in fungi. A synergism in ATPase activities of yeast EF-3 and rabbit liver ribosomes at high ATP concentration an d certain ribosome/EF-3 ratios have been observed. Rabbit liver riboso mes seem to stimulate the ATPase activity of yeast EF-3 similar to the mechanism in yeast ribosomes, though less efficiently.