Catalyzes the NADPH-dependent reductive deacylation of (S)-3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to (R)-mevalonate. Cannot use NADH instead of NADPH. 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. (R)-mevalonate + CoA + 2 NADP(+) = (3S)-hydroxy-3-methylglutaryl-CoA + 2 H(+) + 2 NADPH Is competitively inhibited by lovastatin (formerly called mevinolin). Lovastatin also blocks the growth of H.salinarum, and this effect is reversed by addition of mevalonate, indicating the critical role that the mevalonate pathway plays in isoprenoid biosynthesis by these archaea. KCl concentrations greater than 2 M are required for maximum reductase activity. Metabolic intermediate biosynthesis; (R)-mevalonate biosynthesis; (R)-mevalonate from acetyl-CoA: step 3/3. Homodimer. Belongs to the HMG-CoA reductase family.