Lipid bodies and lipid body formation in an oleaginous fungus, Mortierella ramanniana var. angulispora.

Mortierella ramanniana var. angulispora accumulates triacylglycerol (TG) in lipid bodies. Studies on lipid transport into lipid bodies are essential for elucidating mechanisms of lipid body formation. We used fluorescent dyes and fluorescent lipid analogs to visualize lipid body formation with a confocal laser scanning microscope. Different sizes of lipid bodies were stained by Nile red, a lipid body marker - one with a diameter of about 1 micrometer and the other with a diameter of about 2-3 micrometers. Lipid bodies matured into larger ones with culture. To metabolically monitor lipid bodies, we used 1-palmitoyl, 2-[5-(5,7-dimethyl boron dipyrromethene difluoride)-1-pentanoyl]-phosphatidic acid (C5-DMB-PA), and C5-DMB-phosphatidylcholine (C5-DMB-PC). These were taken up into fungal cells and incorporated into intracellular organelles at 30 degrees C. C5-DMB-PA was quickly incorporated into lipid bodies while C5-DMB-PC was initially incorporated into internal membranes, presumably endoplasmic reticulum membranes, and fluorescence was then gradually transported into lipid bodies. The transport of fluorescent lipids accompanied their metabolism into diacylglycerol (DG) and TG, which, taken together with the fluorescence distribution, suggested that conversion to TG was not necessary for transport into lipid bodies. It is likely that the synthesized DG was mainly located in lipid bodies and the conversion to TG took place in lipid bodies. C5-DMB-PA and C5-DMB-PC were converted to DG and TG in the membrane and lipid body fractions of this fungus, which agreed with in vivo metabolism of these fluorescent lipids and in vitro enzyme activity related to PA and PC metabolism. These results indicate that transport and metabolism of C5-DMB-PA and C5-DMB-PC represent two different routes for lipid body formation in this fungus.