Characterization of triacylglycerol biosynthesis in subcellular fractions of an oleaginous fungus, Mortierella ramanniana var. angulispora

Triacylglycerol (TG) biosynthetic enzymes were characterized in subcellular fractions of an oleaginous fungus, Mortierella ramanniana var. angulispora. When the membrane or lipid body fraction of this fungus was incubated with [¹⁴C]oleoyl-CoA without adding exogenous acyl acceptors, radioactivity was incorporated predominantly into TG, indicating that diacylglycerol acyltransferase (DGAT) used endogenous diacylglycerol to incorporate [¹⁴C]oleoyl-CoA into TG. Adding glycerol 3-phosphate or lysophosphatidic acid increased radiolabeled phosphatidic acid (PA) in the membrane fraction, which reflected the presence of glycerol-3-phosphate acyltransferase (GPAT) and lysophosphatidic acid acyltransferase (LPAAT). Label accumulation did not occur in lysophosphatidic acid when glycerol 3-phosphate was added, suggesting that GPAT was rate-limiting in sequential acylation. In the lipid body fraction, adding lysophosphatidic acid similarly increased radiolabeled PA, whereas adding glycerol 3-phosphate caused much lower increase in radiolabeled PA. Quantitative assays for GPAT, LPAAT, phosphatidic acid phosphatase (PAP), and DGAT essentially confirmed the results obtained from [1-¹⁴C]oleoyl-CoA incorporation; LPAAT had the highest activity in the membrane and lipid body fractions, GPAT was significantly lower in the lipid body fraction, and DGAT was much higher in the lipid body fraction. GPAT and LPAAT in the membrane fraction had a strong preference toward oleoyl-CoA as a substrate over palmitoyl-CoA. Results indicate that TG biosynthetic enzymes had different subcellular distribution with the sequence of enrichment in the lipid body fraction, i.e., GPAT<LPAAT≈PAP<DGAT. This may reflect a TG biosynthetic process from endoplasmic reticulum membranes to lipid bodies in the fungus.