Mitochondria contain the translocator of the outer mitochondrial
membrane (TOM) for protein entry into the organelle, and its subunit Tom40
forms a protein-conducting channel. Here we report the role of Tom40 in protein
translocation across the membrane. The site-specific photocrosslinking
experiment revealed that translocating unfolded or loosely folded precursor
segments of up to 90 residues can be associated with Tom40. Purified Tom40
bound to non-native proteins and suppressed their aggregation when they are
prone to aggregate. A denatured protein bound to the Tom40 channel blocked the
protein import into mitochondria. These results indicate that, in contrast to
the nonstick tunnel of the ribosome for polypeptide exit, the Tom40 channel
offers an optimized environment to translocating non-native precursor proteins
by preventing their aggregation.
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