The role of sterols in the functional reconstitution of water-soluble mitochondrial porins from plants

Water-soluble porins were prepared from native mitochondrial porins isolated from different plants (pea and corn). In the water-soluble form the porins have lost their channel-forming properties. The water-soluble porins were investigated for the influence of different sterols on their membrane activity and their channel-forming properties in lipid bilayer membranes. Our experiments demonstrated that the water-soluble porins regained channel forming activity when the protein was preincubated with different sterols in the presence of a detergent. The channels formed in lipid bilayer membranes after this procedure regain in many but not all cases the original properties of the native mitochondrial porins. Preincubation with other sterols led to a change in the single-channel conductance or to a complete loss of the voltage dependence. The sterols had also a strong influence on the channel-forming activity of the porins. Preincubation of water-soluble pea porin with the plant sterol -sitosterol resulted in a considerable higher channel-forming activity than with all the other sterols used for preincubation. The role of the sterols in the channel-forming complex is discussed.