The biogenesis of the mitochondrial outer membrane – molecular mechanisms and involvement in human pathogenesis

The mitochondrial outer membrane (MOM) contains a diverse set of proteins including enzymes, components of the preprotein translocation machinery, pore-forming proteins, regulators of programmed cell death, and those that control the morphology of the organelle. All these proteins, like the vast majority of mitochondrial proteins, are nuclear-encoded, synthesized in the cytosol and harbor signals that are essential for their subsequent import into mitochondria. A functional mitochondrial protein import system is essential for cell viability.

Mitochondrial defects have been implicated in a wide variety of degenerative diseases, aging, and cancer. Hence, understanding the mechanism of mitochondrial biogenesis is of considerable importance to our ability to deal with mitochondria-related diseases. Furthermore, as many regulators of programmed cell death reside in the mitochondrial outer membrane, gaining an insight into their insertion process will improve our understanding of the apoptotic mechanism in general.

We investigate the molecular mechanisms by which the various mitochondrial outer membrane proteins are targeted to mitochondria, inserted into the outer membrane and assembled into functional complexes within the membrane. We study in addition the involvement of lipids in the biogenesis of the MOM. For our studies we use both yeast and mammalian tissue cultures as experimental systems.

Specific research topics:

Biogenesis of single span mitochondrial membrane proteins

In the last two years we obtained initial hints regarding the mechanisms by which single span proteins are targeted to the mitochondria, recognized by the mitochondrial import machinery and inserted into the outer membrane. Surprisingly, our results suggest that these proteins are inserted into the mitochondrial outer membrane by a process that is not dependent on additional proteins but is rather facilitated by the distinct lipid composition of this membrane. The future goals are to understand the role of the lipid molecules in the membrane integration process and to study the involvement of cytosolic factors in the biogenesis of these proteins.

Membrane insertion of multispan mitochondrial outer membrane proteins

Very recently we found that integration of multispan proteins into the mitochondrial outer membrane occurs via a novel pathway. This pathway involves initial docking of chaperone-associated substrate protein to the import receptor Tom70. The precursor protein is then inserted into the membrane in a process that is facilitated by the membrane-embedded protein Mim1. We intend to characterize the molecular mechanism of this pathway and to investigate the structure-function relationships of the Mim1-containing complex.

Mitochondria related neurodegenerative diseases

To maintain their multitude of different functions mitochondria have to respond to changes in the physiological conditions by adaptation of copy number, size, morphology and intracellular positioning. In a BMBF supported project, we are currently trying to identify abnormalities of mitochondrial fission and fusion in mitochondrial cytopathies in humans and to assess the relevance of such alterations for human disease. For that end we analyze mitochondrial morphology and motility in cell cultures from mitotic and post-mitotic tissues.

Membrane assembly of beta-barrel proteins

The outer membrane of mitochondria, chloroplasts and Gram-negative bacteria are distinguished by the presence of beta-barrel proteins. We identified a complex (the TOB/SAM complex) which specifically mediates the insertion of beta-barrel protein into the MOM. Recently, we expressed bacterial beta-barrel proteins in eukaryotic cells and eukaryotic beta-barrels in bacteria and could demonstrate that the basic mechanism of beta-barrel assembly in bacteria and mitochondria is conserved. We intend to study the evolutionary conservation of this pathway among the three aforementioned systems and thus to obtain a comprehensive understanding of the underlying molecular mechanism.