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, t he N-terminal presequence initially binds to a surface-exposed site, t ermed cis site, of the protein translocation complex of this membrane (the TOM complex). The presequence then moves into the translocation p ore and becomes exposed at the intermembrane space side. Membrane pass age is driven by specific interaction of the presequence with the tran s site, We have used chemical cross-linking to identify components in the vicinity of the translocating presequence. Preproteins bound to th e surface-exposed cis site can be cross-linked via their N-terminal pr esequence to Tom20 and Tom22, demonstrating their direct association w ith this part of the preprotein, In addition, the presequence establis hes an early contact to Tom40, a membrane-embedded protein of the TOM complex. Upon further entry of the preprotein into the translocation p ore, the presequence loses its contact with Tom20/Tom22, but remains i n firm association with Tom40, Our study suggests that Tom40 plays an important function in guiding the presequence of a preprotein across t he mitochondrial outer membrane, We propose that Tom40 forms a major p art of the traits presequence binding site.