Catalytic subunit of the alternative triglyceride biosynthesis pathway, which mediates formation of triacylglycerol from diacylglycerol and membrane phospholipids (PubMed:37648867). Synthesizes triacylglycerol at the expense of membrane phospholipids, such as phosphatidylcholine (PC) and its ether-linked form (ePC), thereby altering the composition of membranes (PubMed:37648867). The alternative triglyceride biosynthesis pathway is probably required to provide the energy required for rapid growth when fuel sources are limiting (PubMed:37648867). It maintains mitochondrial function during periods of extracellular lipid starvation (PubMed:37648867). Can also use acyl-CoA as donor: acts as a acyl-CoA:monoacylglycerol acyltransferase (MGAT), but also shows acyl-CoA:diacylglycerol acyltransferase (DGAT) activity (By similarity).