Catalyzes the condensation of acetyl-CoA with acetoacetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). Functions in the mevalonate (MVA) pathway leading to isopentenyl diphosphate (IPP), a key precursor for the biosynthesis of isoprenoid compounds such as archaeal membrane lipids. acetoacetyl-CoA + acetyl-CoA + H2O = (3S)-hydroxy-3-methylglutaryl-CoA + CoA + H(+) In contrast to bacterial and eukaryotic HMG-CoA synthases, is insensitive to feedback substrate inhibition by acetoacetyl-CoA. Enzymatic activity is inhibited by hymeglusin, which also blocks the propagation of H.volcanii cells in vivo, indicating the critical role that the mevalonate pathway plays in isoprenoid biosynthesis by these archaea. kcat is 4.6 sec(-1) (at pH 8 and 30 degrees Celsius). Optimum pH is about 8.5. Optimum temperature is 45 degrees Celsius. Metabolic intermediate biosynthesis; (R)-mevalonate biosynthesis; (R)-mevalonate from acetyl-CoA: step 2/3. Belongs to the thiolase-like superfamily. HMG-CoA synthase family.