Different states of assembly of mitochondrial porin in various lipid bilayer membranes

In natural membranes the large variety of proteins and lipids is correlated with different membrane functions. Although there is evidence that specific non-bilayer-forming lipids are needed for the correct functioning of certain cell proteins, it is not known how lipid properties influence protein functions. Several recent experiments have shown that lipid variations induce changes in the configuration of intrinsic membrane proteins. In this work, we describe various different assemblies of mitochondrial porin into black lipid membranes. Bilayers, composed of a single type of lipid (phosphatidylinositol in n-decane or oxidized cholesterol in octane-n-decane), are formed in a medium of variable KCl concentration. In phosphatidylinositol membranes, the kinetics of porin incorporation up to the steady state showed sigmoidal curves, indicating cooperative phenomena. The steepness of the curves decreased with a reduction in the ionic strength of the membrane medium. From the best fit to data point with a four- parameter logistic equation, we obtained slope factors or Hill coefficients indicating the formation of different protein assemblies. In oxidized cholesterol membranes, mitochondrial porin seems to be present as a monomer. This configuration was found at both high and low ionic strengths. Although there is much evidence that the aggregation of protein is related to salt concentration (Lin S. et al., Anal Chim. Acta, 383:101-107,1999), given that solubility increases at low salt concentrations (>0.5M), and B-sheet proteins in particular predominantly increase their stability at salt concentrations of >1.5 M (Bandyopadhyay A.K. and Sonawat H.M., Biophys. J., 790.501- 510,2000), our results could indicate a coupling of the protein and lipid polar head.