English: The presequence pathway into the mitochondrial inner membrane (IM) and matrix. The translocase of the outer membrane (TOM) consists of three receptor proteins, the channel-forming protein Tom40 and three small Tom proteins. Presequence-carrying preproteins are preferentially recognized by the receptors Tom20 and Tom22 and are translocated across the outer membrane (OM) through Tom40. The receptor Tim50 of the presequence translocase of the inner membrane (TIM23) binds the preproteins and transfers them to the channel-forming protein Tim23, which is closely associated with Tim17. The membrane potential (�ψ) activates the TIM23 channel and drives translocation of the positively charged presequences.

Protein translocation into the matrix is driven by the presequence translocase-associated motor (PAM) with the ATP-dependent mitochondrial heat-shock protein 70 (mtHsp70). Tim44 couples mtHsp70 to the TIM23 channel. The chaperone Zim17 (Hep1) prevents aggregation of mtHsp70 (184). The dimeric mitochondrial processing peptidase (MPP) removes the presequences. Additional proteases, the intermediate cleaving peptidase (Icp55) and the octapeptidyl aminopeptidase (Oct1; also termed mitochondrial intermediate peptidase, MIP), can remove destabilizing N-terminal amino acid residues from the imported proteins. The chaperonin, consisting of Hsp60 and Hsp10, promotes the folding of a number of matrix proteins. Preproteins with a hydrophobic sorting signal can be laterally released into the IM, controlled by the lateral gatekeeper Mgr2. For some preproteins, the sorting signal is removed by the inner-membrane peptidase (IMP, with the subunits Imp1, Imp2, and Som1), and the proteins are released into the intermembrane space (IMS). (Inset) For lateral sorting of preproteins, the TIM23 complex can associate via Tim21 with the bc1 complex (complex III) and cytochrome c oxidase (COX, complex IV) of the respiratory chain, promoting the energy-dependent step of preprotein translocation. The respiratory chain complexes

also serve as assembly platforms for some PAM subunits.