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First published online July 10, 2003
doi: 10.1242/10.1242/jcs.00667

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Functional cooperation and separation of translocators in protein import into mitochondria, the double-membrane bounded organelles

¹ Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
² Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation, Japan

View larger version (16K): [in a new window]   Fig. 1. Targeting and sorting signals of mitochondrial proteins. N-terminal cleavable presequences are shown in light green, transmembrane segments in purple. Sorting signals to the IMS (the intermembrane space) of IM (inner membrane) consist of a hydrophobic segment (purple) flanked by hydrophilic segments (dark blue). Cleavage sites are indicated by arrowheads. From the top to the bottom, precursor proteins with a presequence that are targeted to the matrix or inserted into the IM from the matrix side, IM proteins with a presequence that are anchored to the IM by a single transmembrane segment with the Nin-Cout orientation (e.g. D-lactate dehydrogenase and subunit Va of cytochrome c oxidase), precursor proteins with a bipartite presequence containing a sorting signal to the IMS (e.g. cytochrome b2 and cytochrome c1), and polytopic IM proteins without a presequence (e.g. AAC) are shown. + indicates positively charged regions.

View larger version (88K): [in a new window]   Fig. 2. Translocator complexes in mitochondria. The TOM complex (pink) in the outer membrane, the TIM23 complex (yellow) and the TIM22 complex (light green) in the inner membrane are shown. Ssc1p and Yge1p (Mge1p), peripheral components assisting the function of TIM23 complex (blue) in the matrix, are also included.

View larger version (63K): [in a new window]   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.

View larger version (73K): [in a new window]   Fig. 4. Translocation across the outer membrane mediated by the TOM complex in the absence of apparent energy. (A) The anchor diffusion mechanism. (B) The folding-driven Brownian ratchet mechanism. OM, the outer membrane; IM, the inner membrane.