title: Cholesterol is required for the fusion of single unilamellar vesicles with Mycoplasma capricolum.
abstract: Small unilamellar vesicles (SUV) were prepared from the total lipid extract of Mycoplasma capricolum. The SUV were labeled with the fluorescent probe octadecylrhodamine B chloride (R18) to a level at which the R18 fluorescence was self-quenched. At pH 7.4 and 37 degrees C, and in the presence of 5% polyethylene glycol, an increase in the R18 fluorescence with time was observed when the R18-labeled SUV were introduced to a native M. capricolum cell suspension. The fluorescence dequenching resulting from dilution of the R18 into the unlabeled membranes of M. capricolum, was interpreted as a result of lipid mixing during fusion between the SUV and the mycoplasma cells. The presence of cholesterol in the SUV was found to be obligatory to allow SUV-mycoplasma fusion to occur. Adaptation of M. capricolum cells to grow in a medium containing low cholesterol concentration provided cells in which the unesterified cholesterol content was as low as 17 micrograms/mg cell protein. The fusion activity of the adapted cells was very low or nonexistent. Nonetheless, when an early exponential phase culture of the adapted cells was transferred to a cholesterol-rich medium, the cells accumulated cholesterol and regained their fusogenic activity. The cholesterol requirement for fusion in the target mycoplasma membrane was met by a variety of planar sterols having a free beta-hydroxyl group, but differing in the aliphatic side chain, e.g., beta-sitosterol or ergosterol, even though these sterols, having a bulky side chain, are preferentially localized in the outer leaflet of the lipid bilayer. It is suggested that the role of cholesterol in mycoplasma-SUV fusion is not at the level of bulk bilayer viscosity but rather, affecting local lipid-lipid or lipid-protein interactions that are relevant to the fusion event.
authors: Department of Membrane and Ultrastructure Research, Hebrew University-Hadassah Medical School, Jerusalem, Israel.
authors: M
authors: M
authors: Tarshis
authors: M
authors: M
authors: Salman
authors: S
authors: S
authors: Rottem
mesh_terms: D000222:Adaptation, Physiological; D055592:Biophysical Phenomena; D001703:Biophysics; D002784:Cholesterol / Q000378:metabolism*; D003470:Culture Media; D008081:Liposomes; D008561:Membrane Fusion / Q000502:physiology*; D009174:Mycoplasma / Q000378:metabolism*
chemical_list: D003470:Culture Media; D008081:Liposomes; D002784:Cholesterol