Role of membrane contact sites in protein import into mitochondria.

Mitochondria import more than 1,000 different proteins from the cytosol. The proteins are synthesized as precursors on cytosolic ribosomes and are translocated by protein transport machineries of the mitochondrial membranes. Five main pathways for protein import into mitochondria have been identified. Most pathways use the translocase of the outer mitochondrial membrane (TOM) as the entry gate into mitochondria. Depending on specific signals contained in the precursors, the proteins are subsequently transferred to different intramitochondrial translocases. In this article, we discuss the connection between protein import and mitochondrial membrane architecture. Mitochondria possess two membranes. It is a long-standing question how contact sites between outer and inner membranes are formed and which role the contact sites play in the translocation of precursor proteins. A major translocation contact site is formed between the TOM complex and the presequence translocase of the inner membrane (TIM23 complex), promoting transfer of presequence-carrying preproteins to the mitochondrial inner membrane and matrix. Recent findings led to the identification of contact sites that involve the mitochondrial contact site and cristae organizing system (MICOS) of the inner membrane. MICOS plays a dual role. It is crucial for maintaining the inner membrane cristae architecture and forms contacts sites to the outer membrane that promote translocation of precursor proteins into the intermembrane space and outer membrane of mitochondria. The view is emerging that the mitochondrial protein translocases do not function as independent units, but are embedded in a network of interactions with machineries that control mitochondrial activity and architecture.

Horvath SE ，Rampelt H ，Oeljeklaus S ，Warscheid B ，van der Laan M ，Pfanner N ... -  《-》

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 ... -  《-》

Mitochondrial preprotein translocases as dynamic molecular machines.

van der Laan M ，Rissler M ，Rehling P 《FEMS YEAST RESEARCH》

Unlocking the presequence import pathway.

Translocation of presequence-containing precursor proteins into the inner mitochondrial membrane and matrix is an essential process that is facilitated by the translocase of the outer membrane (TOM) together with the presequence translocase of the inner membrane (TIM23). After initial recognition by receptors of the TOM complex followed by transport across the outer membrane, the precursor emerges into the intermembrane space (IMS). Recognition of the presequence by Tim50 triggers rearrangements of the presequence translocase, priming it for inner membrane translocation. Subsequently, the precursor can be released into the membrane or translocated into the mitochondrial matrix aided by the import motor. This heat-shock protein 70 (Hsp70)-based motor drives precursor unfolding and translocation and is subject to dynamic remodelling. Here, we review recent advances in understanding of the mechanisms underlying protein transport along the presequence pathway.

Schulz C ，Schendzielorz A ，Rehling P 《-》

Cooperation of translocase complexes in mitochondrial protein import.

Kutik S ，Guiard B ，Meyer HE ，Wiedemann N ，Pfanner N ... -  《-》