ABSTRACT
The biogenesis of mitochondria, chloroplasts, and Gram-negative bacteria requires the insertion of ß-barrel proteins into the outer membranes. Homologous Omp85 proteins are essential for membrane insertion of ß-barrel precursors. It is unknown if precursors are threaded through the Omp85-channel interior and exit laterally or if they are translocated into the membrane at the Omp85-lipid interface. We have mapped the interaction of a precursor in transit with the mitochondrial Omp85-channel Sam50 in the native membrane environment. The precursor is translocated into the channel interior, interacts with an internal loop, and inserts into the lateral gate by ß-signal exchange. Transport through the Omp85-channel interior followed by release through the lateral gate into the lipid phase may represent a basic mechanism for membrane insertion of ß-barrel proteins.
Subject(s)
Mitochondria/metabolism , Mitochondrial Membrane Transport Proteins/metabolism , Mitochondrial Membranes/metabolism , Mitochondrial Proteins/metabolism , Porins/metabolism , Saccharomyces cerevisiae Proteins/metabolism , Saccharomyces cerevisiae/metabolism , Voltage-Dependent Anion Channel 1/metabolism , Mitochondrial Membrane Transport Proteins/genetics , Mitochondrial Proteins/chemistry , Mitochondrial Proteins/genetics , Porins/genetics , Protein Conformation, beta-Strand , Protein Folding , Protein Transport , Saccharomyces cerevisiae Proteins/chemistry , Saccharomyces cerevisiae Proteins/genetics , Voltage-Dependent Anion Channel 1/geneticsABSTRACT
Mitochondria evolved through endosymbiosis of a Gram-negative progenitor with a host cell to generate eukaryotes. Therefore, the outer membrane of mitochondria and Gram-negative bacteria contain pore proteins with ß-barrel topology. After synthesis in the cytosol, ß-barrel precursor proteins are first transported into the mitochondrial intermembrane space. Folding and membrane integration of ß-barrel proteins depend on the mitochondrial sorting and assembly machinery (SAM) located in the outer membrane, which is related to the ß-barrel assembly machinery (BAM) in bacteria. The SAM complex recognizes ß-barrel proteins by a ß-signal in the C-terminal ß-strand that is required to initiate ß-barrel protein insertion into the outer membrane. In addition, the SAM complex is crucial to form membrane contacts with the inner mitochondrial membrane by interacting with the mitochondrial contact site and cristae organizing system (MICOS) and shares a subunit with the endoplasmic reticulum-mitochondria encounter structure (ERMES) that links the outer mitochondrial membrane to the endoplasmic reticulum (ER).
