ABSTRACT Low-temperature-adapted archaea are abundant in the environment, yet little is known about the thermal adaptation of their proteins. We have previously compared elongation factor 2 (EF-2) proteins from Antarctic ( Methanococcoides burtonii ) and thermophilic ( Methanosarcina thermophila ) methanogens and found that the M. burtonii EF-2 had greater intrinsic activity at low temperatures and lower thermal stability at high temperatures (T. Thomas and R. Cavicchioli, J. Bacteriol. 182:1328–1332, 2000). While the gross thermal properties correlated with growth temperature, the activity and stability profiles of the EF-2 proteins did not precisely match the optimal growth temperature of each organism. This indicated that intracellular components may affect the thermal characteristics of the EF-2 proteins, and in this study we examined the effects of ribosomes and intracellular solutes. At a high growth temperature the thermophile produced high levels of potassium glutamate, which, when assayed in vitro with EF-2, retarded thermal unfolding and increased catalytic efficiency. In contrast, for the Antarctic methanogen adaptation to growth at a low temperature did not involve the accumulation of stabilizing organic solutes but appeared to result from an increased affinity of EF-2 for GTP and high levels of EF-2 in the cell relative to its low growth rate. Furthermore, ribosomes greatly stimulated GTPase activity and moderately stabilized both EF-2 proteins. These findings illustrate the different physiological strategies that have evolved in two phylogenetically related but thermally distinct methanogens to enable EF-2 to function satisfactorily.