Fig. 3. Functional cooperation of the TOM and TIM complexes. (A) A presequence-containing precursor protein is transferred from the TOM complex to the TIM23 complex. (1) The N-terminal domain of Tim23 tethers the TIM23 complex to the outer membrane. (2) The presequence of the precursor protein reaches the presequence-binding site on the IMS side of the TOM complex (trans site) and is close to Tim50 of the TIM23 complex. (3) facilitates transfer of the presequence from the TOM complex to the TIM23 complex via Tim50 and translocation of the presequence across the inner membrane. (4) Translocation of the entire precursor proteins through the TIM23 complex is facilitated by mHsp70 (Ssc1p), in most cases, at the expense of matrix ATP. (B) Polytopic inner membrane proteins including AAC are transferred from the TOM complex to the TIM22 complex. (1) AAC enters the import pathway via Tom70. (2) ATP drives translocation of AAC through the TOM channel probably in a loop conformation to bind to the Tim9-Tim10 complex. (3) AAC is transferred to the TIM22 complex via the Tim9/10/12 complex with the aid of . (4) AAC is inserted into the inner membrane by the TIM22 complex and , and forms a dimer. OM, the outer membrane; IM, the inner membrane.

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