Integration of porin into the outer mitochondrial membrane through the common import machinery located in contact sites.

Although no binding of porin-precursor to trypsin-pretreated rat liver mitochondria was detectable, its integration into these mitochondria was observed to some extent, possibly by the bypass-import system first demonstrated in fungus mitochondria. A topographical study of this bypass-import system demonstrated that import of porin occurs at contact sites between the outer and inner membranes. Furthermore, antibodies to 29 kDa outer membrane protein, a component of the import machinery in contact sites for precursors of ornithine aminotransferase and sulfite oxidase, inhibited the integration of porin, suggesting that the 29 kDa protein is involved in the imports of most mitochondrial precursor proteins.