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) (By similarity). ERG13 condenses acetyl-CoA with acetoacetyl-CoA to form hydroxymethylglutaryl-CoA (HMG-CoA) (By similarity). The first module starts with the action of the cytosolic acetyl-CoA acetyltransferase ERG10 that catalyzes the formation of acetoacetyl-CoA. The hydroxymethylglutaryl-CoA synthase ERG13 then condenses acetyl-CoA with acetoacetyl-CoA to form HMG-CoA. The 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase HMG1 finally reduces HMG-CoA to produce mevalonate (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. Expressed during exponential and stationary growth phase (PubMed:17588813). Expression is repressed by amphotericin B and caspofungin (PubMed:15917516). Expression is also repressed during biofilm formation (PubMed:22265407). Is a probable target for sumoylation. Belongs to the thiolase-like superfamily. HMG-CoA synthase family.