A study of lipid secretion from the lichen symbionts, ascomycetous fungus Myelochroa leucotyliza and green alga Trebouxia sp.

Lichens are symbionts of fungi and algae. Although wild fungi secrete lipids to form crystals, those grown in culture either alone or with algae do not secrete enough lipids to be crystallized. To investigate the mode of lipid secretion, we stimulated fungi cultured alone to form crystals. (1) The fungi had serpentine invaginations on the P-faces. These were formed as a consequence of secretory granule exocytosis. (2) Fungi cultured alone in normally used medium had on their P-faces intramembrane particle-cleared parts that also showed a serpentine configuration. (3) After the medium was fortified by further addition of glucose, the fungi cultured alone produced multiple lipid bodies and secretory granules, though no crystals were formed. (4) After the addition of filtered algal culture medium into the fungal culture medium that had been fortified, the fungi grown under this condition showed extracellular crystals. As fungi also showed extensive exocytotic activity, protein secretion seemed essential prior to lipid secretion. The place with no intramembrane particles was postulated to be the site through which lipids penetrate to the outside. (5) As we had identified crystals as atranorin, we made atranorin-containing liposome. The liposome released atranorin to the aqueous phase by the addition of albumin or albumin-like proteins.