Mitochondria in Health and Disease

Known as the power plants of eukaroytic cells, mitochondria are highly dynamic and versatile organelles of endosymbiotic origin that play crucial roles in cellular energy metabolism and many biosynthetic pathways. Moreover, mitochondria are intimately involved in large-scale regulatory circuits, like intracellular Ca2+ homeostasis, and cell fate decisions, like programmed cell death (apoptosis). Recent studies have highlighted the emerging role of mitochondria as signaling organelles that trigger, execute and control developmental programs, cellular stress responses and metabolic adaptations. Mitochondria are engulfed by an outer membrane and an inner membrane that define two aqueous compartments, the intermembrane space and the matrix. A number of proteomic studies have revealed that even in simple unicellular model organisms, like the baker's yeast (Saccharomyces cerevisiae) mitochondria contain more than 1,000 different proteins.

Functional Organization of Mitochondrial Membranes

The inner mitochondrial membrane is characterized by a very peculiar morphology. It consists of an inner boundary membrane that is closely opposed to the outer mitochondrial membrane and large tubular invaginations termed cristae. Whereas the inner boundary membrane is enriched in protein complexes devoted to transport of metabolites and preproteins, the cristae are the main sites of oxidative phosphorylation. The border regions between inner boundary and cristae membranes are morphologically well defined and have been named crista junctions. We aim to answer the question, how the characteristic morphology of the inner mitochondrial membrane is generated and maintained. We have identified an evolutionary conserved inner membrane protein complex that was termed Mitochondrial Contact Site and Cristae Organizing System (MICOS) that is required for the formation and/or stability of crista junctions. MICOS consists of the core subunits Mic10 and Mic60 together with at least four other subunits and forms large oligomeric structures of several Megadalton in size. Mutants defective in MICOS assembly show dramatically altered inner membrane morphology with extended stacks of sheet-like cristae that are detached from the inner boundary membrane. Moreover, MICOS components are engaged in multiple interaction sites between inner and outer mitochondrial membranes (contact sites). We are studying the molecular mechanisms of MICOS function and dynamics. Moreover, we aim to understand the structural and functional relationship between MICOS and other machineries involved in mitochondrial architecture, metabolite transport and protein biogenesis.

Sorting and Assembly of Mitochondrial Membrane Proteins

More than 99% of mitochondrial proteins are encoded by genes in the nuclear DNA. Thus, these proteins are synthesized on cytosolic ribosomes as precursors containing signal sequences that target them to the mitochondrial surface and guide their sorting into the outer membrane, intermembrane space, inner membrane, or matrix compartment. Various protein translocation machineries of the inner and outer mitochondrial membranes accomplish the complicated task of selective protein sorting into and across lipid bilayers.

The presequence translocase of the inner mitochondrial membrane (TIM23 complex) mediates transport of proteins with N-terminal cleavable presequences. Presequence proteins are directly transferred from the general outer membrane translocase (TOM complex) to the TIM23 machinery. Depending on further signal information within presequence proteins, they are either inserted into the inner mitochondrial membrane or imported across the membrane into the matrix. Selective sorting of proteins to different destinations requires dynamic switches in the TIM23 machinery and the reversible association with distinct partner protein complexes at the inner membrane. For matrix translocation, TIM23 is coupled to the ATP-driven presequence translocase-associated import motor (PAM). Membrane insertion by a stop-transfer mechanism is facilitated through the association of TIM23 with the proton-pumping respiratory chain complexes via the regulatory Tim21 subunit. We are investigating, how these large-scale dynamic rearrangements in the TIM23 machinery are regulated by import signals of incoming presequence proteins. Our specific questions are: How and where are these import signals perceived and transmitted? Which mechanisms trigger the signal-dependent switches in the TIM23 system? How are transmembrane segments recognized by the receptor domain and/or the protein-conducting channel of the machinery and how are they laterally released into the inner mitochondrial membrane? How is the energy derived from the membrane potential across the inner mitochondrial membrane coupled to protein transport? How are the dynamic transitions modulated by auxiliary factors and the overall metabolic state of mitochondria?