Mitochondrial protein import. Tom40 plays a major role in targeting and translocation of preproteins by forming a specific binding site for the presequence.

During preprotein transport across the mitochondrial outer membrane, the N-terminal presequence initially binds to a surface-exposed site, termed cis site, of the protein translocation complex of this membrane (the TOM complex). The presequence then moves into the translocation pore and becomes exposed at the intermembrane space side. Membrane passage is driven by specific interaction of the presequence with the trans site. We have used chemical cross-linking to identify components in the vicinity of the translocating presequence. Preproteins bound to the surface-exposed cis site can be cross-linked via their N-terminal presequence to Tom20 and Tom22, demonstrating their direct association with this part of the preprotein. In addition, the presequence establishes an early contact to Tom40, a membrane-embedded protein of the TOM complex. Upon further entry of the preprotein into the translocation pore, the presequence loses its contact with Tom20/Tom22, but remains in firm association with Tom40. Our study suggests that Tom40 plays an important function in guiding the presequence of a preprotein across the mitochondrial outer membrane. We propose that Tom40 forms a major part of the trans presequence binding site.