Effects of temperature and lipid composition on the serum albumin-induced aggregation and fusion of small unilamellar vesicles |
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Authors: | Sergio Schenkman Pedro S. Araujo Ruud Dukman Frank H. Quina Hernan Chaimovich |
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Affiliation: | 1. Instituto de Quimica da Universidade de São Paulo, Group for Interfacial Studies (GIST), Caixa Postal 20, 780 São Paulo Brasil;2. Laboratory of Biochemistry, Rijksuniversiteit Utrecht, Padualaan 8, Pb 80054, 3508 TB, Utrecht The Netherlands |
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Abstract: | Small unilamellar vesicles of egg phosphatidylcholine (PC) or dimyristoylphosphatidylcholine, mixed with small unilamellar vesicles labelled with 2-(10-(1-pyrene)decanoyl)phosphatidylcholine, exhibit a constant average size and excimer to monomer (E/M) ratio for several hours when incubated at pH 3.6 at a temperature higher than the phase transition temperature (Tc) of the lipids. Addition of bovine serum albumin to this system produces a transient turbidity increase, a fast decrease in the E/M ratio, a partial loss of vesicle-entrapped [14C]sucrose and a measurable leak-in of externally added sucrose. Sepharose 4B filtration of the system demonstrates that the E/M ratio decrease is strictly paralleled by the formation of liposomes which exhibit a low E/M ratio and a hydrodynamic radius larger than that of small unilamellar vesicles. These data demonstrate that the E/M ratio decrease can be unequivocally ascribed to a vesicle-vesicle fusion process induced by serum albumin. The rate of serum-albumin induced fusion of small unilamellar vesicles is: (a) maximal at a stoichiometric ratio of approx. 2 albumins per vesicle: (b) sensitive to the nature of the lipid and; (c) not altered when human serum albumin replaces bovine serum albumin. The rate of albumin-induced fusion of dimyristoylphosphatidylcholine small unilamellar vesicles is higher below the Tc of the lipid and increases with temperature above the Tc. The formation of protein-bound aggregates with defined stoichiometries and a high local vesicle concentration, as well as changes in the local degree of hydration, are proposed to be the driving forces for the protein-induced vesicle fusion in this system. |
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Keywords: | Temperature effect Albumin Lipid composition Vesicle fusion Aggregation Fluorescent probe |
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