The outer mitochondrial membrane has the unique property of being freely permeable to small water soluble substrate molecules. The molecular basis for this biologically important property has been investigated. For this purpose a channel forming protein in the outer membrane of rat liver mitochondria was for the first time purified and characterized by biochemical and immunological methods. Furthermore its physiological role and biogenesis was also studied. It was demonstrated that a single polypeptide of Mr 30 000 is the component mediating passive diffusion across the outer membrane. This pore forming polypepide, named porin, was further characterized by determining its amino acid composition, isoelectric point (pl 7.6) and hydrodynamic properties, which indicate that the functional pore is a dimer. Quantification of the amount of porin in the outer membrane and in whole mitochondria as well as the subcellular distribution of porin was performed by means of immunoblotting techniques using porin monospecific antibodies. The amount of porin was estimated to 0.3% of total mitochondrial protein. Furthermore, porin was found to be located exclusively in the outer mitochondrial membrane and was not detected in any other membrane of the cell.

The identity between porin and the hexokinase binding protein was established suggesting that porin might have a regulatory function in carbohydrate metabolism. The biogenesis of porin and the outer mitochondrial membrane was also studied. It was concluded that porin was synthesized on cytoplasmic ribosomes as a precursor with the same molecular weight as the mature protein and that import into the outer membrane was independent of proteolytic processing.