Catalyzes the NADPH-dependent reductive deacylation of (S)-3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to (R)-mevalonate. 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. Is also able to catalyze the reduction of mevaldehyde to mevalonate and the oxidative acylation of mevaldehyde to HMG-CoA. (R)-mevalonate + CoA + 2 NADP(+) = (3S)-hydroxy-3-methylglutaryl-CoA + 2 H(+) + 2 NADPH Is competitively inhibited by (R)-HMG-CoA and lovastatin (formerly called mevinolin). Optimum pH is 7.3 for the reductive deacylation of HMG-CoA (in the presence of 3 M KCl). Metabolic intermediate biosynthesis; (R)-mevalonate biosynthesis; (R)-mevalonate from acetyl-CoA: step 3/3. Up-regulated during growth at high salinity. Belongs to the HMG-CoA reductase family.