Summary: The ultrastructure of the plasma membrane of was studied by freeze-fracture using invaginations of the plasma membrane as natural markers and filipin-induced deformations as artificial markers. In accord with the mode of growth of this organism, ultra-structural aspects of the plasma membrane were related to the following ring zones: the growing pole, adjacent regions, proximal regions, the new cell pole, and the middle in dividing cells. The growing pole and adjacent regions had no or only a few invaginations. Filipin induced numerous deformations in these regions. By contrast, the proximal regions of the plasma membrane had several invaginations and resisted filipin-induced deformation. Concomitantly with commitment to cytokinesis, both the invaginations and the resistance to filipin-induced deformation disappeared in the middle. The results presented here strongly suggest the existence of two states of the plasma membrane of , a fact which correlates well with the mode of growth of this organism.


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