Mitochondrial import receptor TOM70
Saccharomyces cerevisiae (yeast)
Mitochondrion is a membrane-bounded organelle, about the size of a bacterium, that carries out oxidative phosphorylation and produces most of the adenosine triphosphate (ATP) in eukaryotic cells. Both of metabolites, pyruvates from sugars and fatty acids from fats, are converted to the crucial metabolic intermediate acetyl CoA by enzymes located in the mitochondrial matrix. Next, acetyl CoA are completely oxidized by O2 to CO2 and H2O via special subsequent reaction called citric acid cycle. This allows 15 times more ATP to be made than that produced by glycolysis alone.
There are two subcompartments in mitochondria: the internal matrix space and the internal membrane space. These components are formed by the two mitochondrial membranes: the inner membrane, which forms extensive invaginations and encloses the matrix space, and the outer membrane, which is in contact with the cytosol. Most of the enzymes (ex. for citric acid cycle reactions) used in mitochondrial matrix space are imported from the cytosol via outer and inner membranes because mitochondrial DNAs cannot synthesize all the compounds they need. There are protein translocators across the outer and inner membrane, called translocase of the outer membrane complex (TOM) and translocase of the inner membrane complex (TIM), respectively. These complexes contain some components that act as receptors for proteins having special amino acid sequences direct to mitochondria, and other components that forms the translocation channel.
The structure shown here is a major surface receptor in TOM complex, Tom 70 of yeast. All the helices tightly packed together to constitute a superhelix-like architecture and forms a homodimer. It contains two domains, a short N-terminal transmembrane domain anchored in the mitochondrial outer membrane and a large C-terminal domain located in cytosol, are connected by a linker. A large pocket for mitochondrial precursor proteins are located in the middle of the helix bundles in C-terminal domain, and C-terminal EFVD motif of Hsp 70 may interact at N-terminal domain.
Protein Data Bank (PDB)
Wu, Y. Sha, B.; "Crystal structure of yeast mitochondrial outer membrane translocon member Tom70p."; Nat.Struct.Mol.Biol.; (2006) 13:589-593 PubMed:16767096.
author: Sachiyo Nomura