A kinetic and structural study of two-step aggregation and fusion of neutral phospholipid vesicles promoted by serum albumin at low pH.
The addition of bovine serum albumin (BSA) to 25 +/- 5 nm diameter single bilayer phosphatidylcholine (PC) vesicles (SBV) (pH 3.5) gives rise to readily visible transient turbidity. Studies of this system, employing a series of techniques, including time-dependent turbidity changes, membrane filtration, centrifugation, Sepharose chromatography and freeze fracture electron microscopy demonstrated that the process involves aggregation and fusion of the vesicles. At least three distinct time-dependent steps have been characterized: (1) the rapid initial formation (in approx. 5 min) of large aggregates (responsible for the visible turbidity) composed of SBV interconnected by BSA in its F form. The formation of these aggregates may be reversed by raising the pH or adding excess BSA to the system at this stage; (2) spontaneous collapse of these large aggregates, in an irreversible step, to form a heterogeneous population of vesicles; (3) fusion produces as the final product of the process, a relatively homogeneous population of larger (50 +/- 10 nm diamter) vesicles. This system serves as a convenient and simple model system for the detailed study of protein-mediated aggregation and fusion of membranes at the molecular level.
D000818:Animals; D002417:Cattle; D002459:Cell Fusion; D005614:Freeze Fracturing; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008051:Lipid Bilayers; D008854:Microscopy, Electron; D008954:Models, Biological; D008968:Molecular Conformation; D010743:Phospholipids; D011485:Protein Binding; D011487:Protein Conformation; D012710:Serum Albumin, Bovine
D008051:Lipid Bilayers; D010743:Phospholipids; D012710:Serum Albumin, Bovine