The elongation factor 1 alpha from the archaebacterium Sulfolobus solfataricus (aEF-1 alpha) possesses an intrinsic GTPase activity that is triggered by NaCl up to 5.2 M and requires the presence of at least 1 mM MgCl2 or MnCl2. Chloride salts of other monovalent cations are inefficient, whereas other sodium salts are much less efficient or not efficient at all as compared with NaCl. This aEF-1 alpha GTPase (GTPaseNa) reaches a maximum in a broad pH range and is not affected by other nucleoside triphosphates but is competitively inhibited by GDP. The turnover of GTPaseNa is rate limited by the breakdown of GTP. The Km for GTP is in the range of 2.2-9.3 microM, depending on the NaCl concentration and temperature. The highest catalytic efficiency is reached at 87 degrees C, which is the optimum temperature for growth of S. solfataricus. The energetic parameters of GTPaseNa are similar to those reported in the literature for the GTPase of Escherichia coli elongation factor Tu (EF-Tu) triggered by 2 M KCl, thus suggesting that the GTPase activity supported by either EF-Tu or aEF-1 alpha undergoes a similar mechanism of activation by salt at high concentration. A molecular mechanism for this activation is proposed.