This enzyme condenses acetyl-CoA with acetoacetyl-CoA to form HMG-CoA, which is the substrate for HMG-CoA reductase. acetoacetyl-CoA + acetyl-CoA + H2O = (3S)-hydroxy-3-methylglutaryl-CoA + CoA + H(+) Metabolic intermediate biosynthesis; (R)-mevalonate biosynthesis; (R)-mevalonate from acetyl-CoA: step 2/3. Ubiquitinated. RNAi-mediated knockdown results in sterility, slow development, reduced body size, severe paralysis and reduced pharyngeal pumping. Belongs to the thiolase-like superfamily. HMG-CoA synthase family.