DIFFERENTIAL RECOGNITION OF PREPROTEINS BY THE PURIFIED CYTOSOLIC DOMAINS OF THE MITOCHONDRIAL IMPORT RECEPTORS TOM20, TOM22, AND TOM70

The preprotein translocase of the outer mitochondrial membrane (Tom) i s a multi-subunit complex required for specific recognition and membra ne translocation of nuclear-encoded preproteins. We have expressed and purified the cytosolic domains of three postulated import receptors, Tom20, Tom22, and Tom70. Each receptor domain is able to bind mitochon drial preproteins but with different specificity, Tom20 binds both pre proteins with N-terminal presequences and preproteins with internal ta rgeting signals; the binding is enhanced by the addition of salt. Tom2 2 selectively recognizes presequence-carrying preproteins in a salt-se nsitive manner, Tom70 preferentially binds preproteins with internal t argeting information, A chemically synthesized presequence peptide com petes with preproteins for binding to Tom20 and Tom22 but not to Tom70 . We conclude that each of the three import receptors binds preprotein s independently and by a different mechanism, Both Tom20 and Tom22 fun ction as presequence receptors.