Membrane-associated proteins affect the formation of filipin-cholesterol complexes in viral membranes

The presence and distribution of cholesterol in biological membranes can be visualized by complex formation with the polyene antibiotic filipin after or during fixation with glutaraldehyde. In the envelopes of budding and immature retroviruses no filipin-cholesterol complexes are formed, but in the plasma membrane of host cells and in the envelopes of mature viruses filipin-cholesterol complexes are easily detected. However, after treatment of glutaraldehyde-fixed cells with pepsin, the presence of cholesterol in the envelopes of budding and immature viral particles could also be demonstrated. This indicates that in these structures the reaction of cholesterol with filipin is inhibited by proteins associated with the cholesterol-containing membrane. Treatment of fixed cells with trypsin, and of unfixed cells with cytochalasin B (CB) had no effect on detectability of cholesterol in these structures. On no occasion were cholesterol-filipin complexes formed in coated pits. The present findings call for caution when interpretating data on absence of filipincholesterol complexes in those membrane domains that are characterized by the presence of closely associated proteins.