Sterol O-acyltransferase that catalyzes the formation of stery esters. an acyl-CoA + ergosterol = CoA + ergosteryl ester an acyl-CoA + zymosterol = CoA + zymosterol ester Present with 279 molecules/cell in log phase SD medium. Belongs to the membrane-bound acyltransferase family. Sterol o-acyltransferase subfamily.