Targeting and Import of Mitochondrial Precursor Proteins

We are studying protein sorting processes into and within mitochondria. These are complex reactions, as every protein has to be routed to its specific destination within the organelle (Annu Rev Biochem. 76, 723-749).

We employ biochemical, genetic, cell biological and immunological techniques with Saccharomyces cerevisiae as model systems to explore the function of mitochondrial translocases which, in a concerted fashion, mediate import and sorting of proteins into the mitochondrial subcompartments.

One main focus of our group is the biogenesis of proteins of the mitochondrial inner membrane which belongs to the protein-richest membranes of the eukaryotic cell. It accommodates a large number of different integral membrane proteins which, typically assembled in multiprotein complexes, perform a variety of functions like the transport of molecules or generation of ATP. We analyze how these proteins are inserted into the inner membrane, and identify and characterize components that mediate this process (J Cell Biol. 214, 417-431; J Cell Biol. 217, 1369-1382).

The Cytosolic Life of Mitochondrial Precursor Proteins

Owing to their predominantly post-translational mode of import, precursors of mitochondrial proteins explore the cytosol before they reach the mitochondrial import pore. These early steps in mitochondrial protein targeting are poorly understood. Thereby, different types of precursor proteins show different characteristics. Most matrix proteins seem to be very rapidly imported. However, if their synthesis exceeds the capacity of the mitochondrial import system these proteins are handed over to the ubiquitin-proteasome system for degradation (FEBS Lett. 595, 1223-1238) or stored in specific stress granules called MitoStores (EMBO J e112309). Cytosolic precursors also influence the aggretation of non-mitochondrial proteins in the cytosol, presumably because of their competion for the same chaperones (EMBO J 40, e107913).

The accumulation of precursors triggers a cellular response program which increases the capacity of the folding and degradation systems in the cytosol and in mitochondria, and which reduces the synthesis of proteins on cytosolic ribosomes (Nat Cell Biol. 21, 442-451). We explore how this mitoprotein-induced stress program is wired and study its relevance for cellular proteostasis using a wide range of methods, including protein biochemistry, yeast genetics, fluorescence microscopy and proteomics.

Precursors on the ER Surface: ER-SURF Targeting

The ER surface is the professional sorting platform for newly synthesized proteins. We recently discovered that also mitochondrial precursor proteins associate with the ER surface. In yeast, a dedicated machinery exists to recognize ER-bound precursors and to hand them over to mitochondria (Science 361, 1118-1122). How many mitochondrial precursors embark on this ER-SURF targeting route is not known. We identified the one critical factor of the ER-SURF targeting system, Djp1, and search currently for components that are relevant for ER-binding of mitochondrial precursors or for their subsequent transfer to the mitochondrial import machinery.

Import of Proteins into the Intermembrane Space

Many proteins of the intermembrane space do not contain mitochondrial presequences. Instead they contain conserved patterns of cyteine residues which are essential for targeting. We identified a system in the intermembrane space that introduces disulfide bonds into newly imported proteins (Cell 121, 1059-69; Science 324, 1284-1287; Mol Cell 37, 516-552) . This system consists of the sulfhydryl oxidase Erv1 and of Mia40 . Erv1 is a sulfhydryl oxidase, thus an enzyme that can form disulfide bonds de novo. Mia40 serves as an oxidoreductase which transfers these disulfide bonds into its substrate proteins. During protein import into the IMS, Mia40 traps its substrates via hydrophobic interactions and drives their translocation across the outer membrane (EMBO J. 31, 4348-58; eLIFE pii: e16177). During this import reaction, the substrate proteins are oxidized and stably folded, which prevents them from subsequent degradation by proteases.

Biogenesis of Mitochondrial Ribosomes

Mitochondria contain ribosomes which are distinct from cytosolic ribosomes (Nat Commun. 6, 6019; Science 348; 288-289). Owing to the hydrophobic nature of their translation products, mitochondrial ribosomes are tightly bound to the inner membrane (EMBO J. 22, 6448-6457). About 80 different mitochondrial ribosomal proteins (MRPs) need to be imported into mitochondria where they associate with the rRNA molecules for ribosome assembly (Mol Biol Cell. 27, 3031-3039). This assembly process is only poorly understood and an exciting project of our lab.

We Thank our Funding Agencies!

The projects in our lab are financially supported by several research grants. We are grateful to our funders for supporting our work! We receive funding from the European Union (ERC advanced grant) for a project about the biology of mitochondrial precursor proteins (MitoCyto) and from the Deutsche Forschungsgemeinschaft DFG for several reserach projects and for the research training group (Graduertenkolleg) STRESSistance. We receive also support from BioComp, a local research initiative of the federal state Rheinland-Pfalz.