Translocation of proteins across the mitochondrial inner membrane, but not into the outer membrane, requires nucleoside triphosphates in the matrix.

Work in several laboratories has established that import of proteins across the mitochondrial inner membrane requires an electrochemical potential across that membrane and cleavage of nucleoside triphosphate. We now show that nucleoside triphosphate must be present inside the inner membrane. In contrast, the potential-independent insertion of porin into the outer membrane requires ATP only outside the inner membrane.