Kinetic paremeters of motochondrial porin incorporation into phospholipid membranes with different surface charge

The kinetics of pore formation by mitochondrial porin in lipid bilayer membranes with different polar heads was investigated. Pore formation was estimated by the total current flowing through the membrane. The time course of the current revealed a "pore activation process" or "cooperative process" and was well described by a novel mathematical model in which two of the four parameters, K₁ and K₂, are directly related to the processes of pore insertion into and disappearance from the membrane, respectively. At 1M of KCl, for fixed porin concentration, the K₁ value decreases in the following order: phosphatidylserine, phosphatidylcholine membrane, indicating a different affinity of protein to membranes bearing a different polar head. According to the Guy-Chapman theory, at 0.1 M of KCI, the surface charge of negatively-charged membranes is -137 mV, as compared to -80 mV at 1 M of KCI, and the corresponding K₁ value decreased considerably under these experimental conditions; on the other hand, in zwitterionic phospholipid membranes of phosphatidylcholine, the value of K₁ remains unmodified. When diluting the negative charge of phosphatidylserine by the addition of phosphatidylethanolamine (1: 1 w: v), the K₁ value increased strongly. The K₂ parameter seems to be less influenced by the magnitude of surface potential present on negatively-charged membranes or by the nature of the lipid head. These results indicate that the electrostatic coupling between porin and the polar head of lipid membranes could play an important role in porin incorporation.