A MICOS-TIM22 Association Promotes Carrier Import into Human Mitochondria.

Mitochondrial membrane proteins with internal targeting signals are inserted into the inner membrane by the carrier translocase (TIM22 complex). For this, precursors have to be initially directed from the TOM complex in the outer mitochondrial membrane across the intermembrane space toward the TIM22 complex. How these two translocation processes are topologically coordinated is still unresolved. Using proteomic approaches, we find that the human TIM22 complex associates with the mitochondrial contact site and cristae organizing system (MICOS) complex. This association does not appear to be conserved in yeast, whereby the yeast MICOS complex instead interacts with the presequence translocase. Using a yeast mic10Δ strain and a HEK293T MIC10 knockout cell line, we characterize the role of MICOS for protein import into the mitochondrial inner membrane and matrix. We find that a physiological cristae organization promotes efficient import via the presequence pathway in yeast, while in human mitochondria, the MICOS complex is dispensable for protein import along the presequence pathway. However, in human mitochondria, the MICOS complex is required for the efficient import of carrier proteins into the mitochondrial inner membrane. Our analyses suggest that in human mitochondria, positioning of the carrier translocase at the crista junction, and potentially in vicinity to the TOM complex, is required for efficient transport into the inner membrane.

Callegari S ，Müller T ，Schulz C ，Lenz C ，Jans DC ，Wissel M ，Opazo F ，Rizzoli SO ，Jakobs S ，Urlaub H ，Rehling P ，Deckers M ... -  《-》

Phosphatidylcholine Affects Inner Membrane Protein Translocases of Mitochondria.

Two protein translocases transport precursor proteins into or across the inner mitochondrial membrane. The presequence translocase (TIM23 complex) sorts precursor proteins with a cleavable presequence either into the matrix or into the inner membrane. The carrier translocase (TIM22 complex) inserts multispanning proteins into the inner membrane. Both protein import pathways depend on the presence of a membrane potential, which is generated by the activity of the respiratory chain. The non-bilayer-forming phospholipids cardiolipin and phosphatidylethanolamine are required for the activity of the respiratory chain and therefore to maintain the membrane potential for protein import. Depletion of cardiolipin further affects the stability of the TIM23 complex. The role of bilayer-forming phospholipids like phosphatidylcholine (PC) in protein transport into the inner membrane and the matrix is unknown. Here, we report that import of presequence-containing precursors and carrier proteins is impaired in PC-deficient mitochondria. Surprisingly, depletion of PC does not affect stability and activity of respiratory supercomplexes, and the membrane potential is maintained. Instead, the dynamic TIM23 complex is destabilized when the PC levels are reduced, whereas the TIM22 complex remains intact. Our analysis further revealed that initial precursor binding to the TIM23 complex is impaired in PC-deficient mitochondria. We conclude that reduced PC levels differentially affect the TIM22 and TIM23 complexes in mitochondrial protein transport.

Schuler MH ，Di Bartolomeo F ，Mårtensson CU ，Daum G ，Becker T ... -  《-》

The channel-forming Sym1 protein is transported by the TIM23 complex in a presequence-independent manner.

Reinhold R ，Krüger V ，Meinecke M ，Schulz C ，Schmidt B ，Grunau SD ，Guiard B ，Wiedemann N ，van der Laan M ，Wagner R ，Rehling P ，Dudek J ... -  《-》

Conserved regions of budding yeast Tim22 have a role in structural organization of the carrier translocase.

Mitochondrial biogenesis requires efficient sorting of various proteins into different mitochondrial sub-compartments, mediated by dedicated protein machinery present in the outer and inner membrane. Among them, the TIM22 complex enables the integration of complex membrane proteins with internal targeting signals into the inner membrane. Although the Tim22 protein forms the core of the complex, the dynamic recruitment of subunits to the channel is still enigmatic. In this study, we highlight that the intermembrane space (IMS) and transmembrane 4 (TM4) regions of Tim22 are critically required for interactions with the membrane-embedded subunits, including Tim54, Tim18, and Sdh3, and thereby maintain the functional architecture of the TIM22 translocase. Furthermore, we find that the TM1 and TM2 regions of Tim22 are important for association with Tim18, whereas TM3 is exclusively required for the interaction with Sdh3. Moreover, impairment of TIM22 complex assembly influences its translocase activity, the mitochondrial network, and the viability of cells lacking mitochondrial DNA. Overall, our findings provide compelling evidence highlighting the significance of conserved regions of Tim22 that are important for the maintenance of the TIM22 complex and mitochondrial integrity.

Kumar A ，Matta SK ，D'Silva P 《-》

The mitochondrial carrier pathway transports non-canonical substrates with an odd number of transmembrane segments.

Rampelt H ，Sucec I ，Bersch B ，Horten P ，Perschil I ，Martinou JC ，van der Laan M ，Wiedemann N ，Schanda P ，Pfanner N ... -  《BMC BIOLOGY》