Hydroxymethylglutaryl-CoA synthase; part of the first module of ergosterol biosynthesis pathway that includes the early steps of the pathway, conserved across all eukaryotes, and which results in the formation of mevalonate from acetyl-coenzyme A (acetyl-CoA) (PubMed:8750242). Hcs1 condenses acetyl-CoA with acetoacetyl-CoA to form hydroxymethylglutaryl-CoA (HMG-CoA) (PubMed:8750242). The first module starts with the action of the cytosolic acetyl-CoA acetyltransferase eg10 that catalyzes the formation of acetoacetyl-CoA. The hydroxymethylglutaryl-CoA synthases erg13 then condenses acetyl-CoA with acetoacetyl-CoA to form HMG-CoA. The rate-limiting step of the early module is the reduction to mevalonate by the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductases hcs1 (Probable). 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. Impairs growth in absence of mevalonate. Belongs to the thiolase-like superfamily. HMG-CoA synthase family.