Early steps in mitochondrial protein import

The process of insertion of precursor proteins into mitochondrial
membranes was investigated using a hybrid protein (pSc1-c) that
contains dual targeting information and, at the same time, membrane
insertion activity. pSc1-c is composed of the matrix-targeting
domain of the cytochrome c1 presequence joined to the amino
terminus of apocytochrome c. It can be selectively imported along
either a cytochrome c1 route into the mitochondrial matrix or via
the cytochrome c route into the intermembrane space. In contrast to
cytochrome c1, pSc1-c does not require the receptor system/GIP for
entry into the matrix. The apocytochrome c in the pSc1-c fusion
protein appears to exert its membrane insertion activity in such a
manner that the matrix-targeting sequence gains direct access to
the membrane potential-dependent step. These results attribute an
essential function to the receptor system in facilitating the
initial insertion of precursors into the mitochondrial membranes.