Papulacandin B: inhibitor of biogenesis of (1----3)-beta-D-glucan fibrillar component of the cell wall of Saccharomyces cerevisiae protoplasts.

The effect of papulacandin B on regenerating protoplasts of Saccharomyces cerevisiae was studied by light and electron microscopy. In liquid media it inhibited the biogenesis of (1----3)-beta-D-glucan fibrillar nets; as a result, the protoplasts did not grow polarly but only spherically. The effect was reversible. Instead of the nets the inhibited protoplasts synthesized only individual microfibrils soluble in hydroxide; these were not joined in the nets and were partially masked by amorphous material. The microfibrils disintegrated after lysis and did not maintain the shape of protoplasts. Protoplasts inhibited in solid media grew spherically up to 25 micron but they did not divide or revert, in spite of forming cell walls. These walls were amorphous and fragile and they disintegrated during preparation. Papulacandin B did not decrease the viability of protoplasts and did not interfere with their growth, biogenesis of alkali-soluble glucan microfibrils or amorphous wall matrix. It inhibited specifically the synthesis of alkali-insoluble branched (1----3)-beta-D-glucan, a necessary building unit required for the formation of the fibrillar component of the cell wall responsible for the cell wall shape, its rigidity and tensile strength.