Abstract
Mitochondria fulfill central functions in cellular energetics, metabolism, and signaling. The outer membrane translocator complex (the TOM complex) imports most mitochondrial proteins, but its architecture is unknown. Using a cross-linking approach, we mapped the active translocator down to single amino acid residues, revealing different transport paths for preproteins through the Tom40 channel. An N-terminal segment of Tom40 passes from the cytosol through the channel to recruit chaperones fromthe intermembrane space that guide the transfer of hydrophobic preproteins. The translocator contains three Tom40 beta-barrel channels sandwiched between a central alpha-helical Tom22 receptor cluster and external regulatory Tom proteins. The preprotein-translocating trimeric complex exchanges with a dimeric isoform to assemble new TOM complexes. Dynamic coupling of alpha-helical receptors, beta-barrel channels, and chaperones generates a versatile machinery that transports about 1000 different proteins.