A Correlation between Mode of Growth and Regional Ultrastructure of the Plasma Membrane of Schizosaccharomyces pombe as Revealed by Freeze-fracturing before and after Filipin Treatment

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Abstract
The ultrastructure of the plasma membrane of S. pombe 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, ultrastructural 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. 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 existence of 2 states of the plasma membrane of S. pombe is suggested, a fact which correlates well with mode of growth of this organism.