Absence of filipin-sterol complexes from large coated pits on the surface of culture cells.

Monolayer cultures of normal or transformed fibroblasts and of liver cells fixed in a glutaraldehyde solution containing 300 microM filipin, a sterol-specific polyene antibiotic, were freeze-fractured to study the distribution of cholesterol within their plasma membranes. Filipin-sterol complexes, recognizable as 25- to 30-nm protuberances scattered in the fracture face of plasma membrane, were absent from invaginations corresponding to large, bristle-coated pits (and possibly also from small, flask-shaped invaginations). These results suggest that invaginating regions on the cell surface are specialized plasma membrane domains with a lower cholesterol content than the surrounding membrane. The localized change in membrane fluidity due to the low cholesterol concentration could play a role in endocytosis.