Catalyzes the terminal and only committed step in triacylglycerol (TAG) synthesis by using diacylglycerol (DAG) and fatty acyl-CoA as substrates. Required for storage lipid synthesis. Major DAG esterifying enzyme in stationary phase when TAG production is particularly active. Involved in lipid particle synthesis from the endoplasmic reticulum, promoting localized TAG production at discrete ER subdomains, and in ergosterol biosynthesis (PubMed:11741946, PubMed:11751830, PubMed:11751875, PubMed:14640980, PubMed:15155725, PubMed:21321129, PubMed:32349126). Also has monoacylglycerol acyltransferase (MGAT) activity, catalyzing the acyl-CoA-dependent esterification of monoacylglycerol to diacylglycerol (PubMed:20554061). Can also utilize ceramide instead of DAG, acylating the ceramides by attaching a fatty acid to the hydroxy group on the first carbon atom of the long-chain base to produce 1-O-acylceramides (PubMed:22738231). a 1,2-diacyl-sn-glycerol + an acyl-CoA = a triacyl-sn-glycerol + CoA a 2-acylglycerol + an acyl-CoA = a 1,2-diacyl-sn-glycerol + CoA (9Z)-octadecenoyl-CoA + 2-(9Z-octadecenoyl)-glycerol = 1,2-di-(9Z-octadecenoyl)-glycerol + CoA Glycerolipid metabolism; triacylglycerol biosynthesis. Localizes to sites of lipid droplet biogenesis in the endoplasmic reticulum. Present with 907 molecules/cell in log phase SD medium. Belongs to the diacylglycerol acyltransferase family.