Role of ATP in mitochondrial protein import

The role of nucleoside triphosphates (NTPs) in the import of porin
into the mitochondrial outer membrane was investigated with two
forms of the porin precursor: the in vitro synthesized biosynthetic
precursor (bs- porin) and a water-soluble form of porin (ws-porin)
obtained by subjecting the membrane-derived porin to an acid-base
treatment (exposure to trichloroacetic acid, followed by alkali and
rapid neutralization). The import of ws-porin into mitochondria did
not require NTPs, whereas the import of bs-porin required NTPs. In
other characteristics, such as binding to a specific receptor
protein on the mitochondrial surface, two-step insertion into the
outer membrane, and formation of specific membrane channels,
ws-porin was indistinguishable from bs-porin. Thus, the acid-base
treatment applied in the preparation of ws-porin can substitute for
the NTP-requiring step in mitochondrial protein import. We conclude
that NTPs are required for unfolding mitochondrial precursor
proteins ("translocation competent folding").