The

outer mitochondrial membrane A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used t ...

is made up of two essential proteins, Tom40 and Sam50.

Tom40

Tom40 is a protein import

pore Pore may refer to: Biology Animal biology and microbiology * Sweat pore, an anatomical structure of the skin of humans (and other mammals) used for secretion of sweat * Hair follicle, an anatomical structure of the skin of humans (and other m ...

required for the import of precursor proteins across the outer mitochondrial membrane, and it makes up part of the

translocase of the outer membrane The translocase of the outer membrane (TOM) is a complex of proteins found in the outer mitochondrial membrane of the mitochondria. It allows movement of proteins through this barrier and into the intermembrane space of the mitochondrion. Most ...

Sam50

Sam50 is a subunit of the sorting and assembly machinery (SAM) of the outer membrane. The sorting and assembly machinery is a protein complex, that operates after the translocase of the outer membrane, to mediate insertion of

beta barrel In protein structures, a beta barrel is a beta sheet composed of tandem repeats that twists and coils to form a closed toroidal structure in which the first strand is bonded to the last strand (hydrogen bond). Beta-strands in many beta-barrels are ...

proteins into the outer mitochondrial membrane.

Complex components

The sorting and assembly machinery is made up of three subunits, Sam35, Sam37, and Sam50, of which Sam50 is embedded within the outer mitochondrial membrane.Bolender, N., Sickmann, A., Wagner, R., Meisinger, C., Pfanner, N. (2008) Multiple pathways for sorting mitochondrial precursor proteins. EMBO J 9(1): 42-49. Both Sam35 and Sam37 are located on the cytosolic face of the SAM complex are

peripheral membrane proteins Peripheral membrane proteins, or extrinsic membrane proteins, are membrane proteins that adhere only temporarily to the biological membrane with which they are associated. These proteins attach to integral membrane proteins, or penetrate the periph ...

that are not essential for survival.Chan, N.C, and Lithgow, T. (2008) The Peripheral Membrane Subunits of the SAM Complex Function Codependently in Mitochondrial Outer Membrane Biogenesis. Mol. Biol. Cell 19(1): 126-136 Sam35 is believed to bind to substrate proteins, while Sam37 acts after Sam35 to stimulate release of the substrate protein from the SAM complex.

Protein import and integration

The sorting and assembly machinery is required for the assembly of beta barrel proteins, this includes proteins such as the Tom40 import pore and porin. Like all mitochondrial proteins, beta barrel proteins are transported into the intermembrane space of

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

via the

. Following import, beta barrel proteins are transported to the SAM complex by chaperone complexes, formed by assembly of the

small Tim proteins Tim9 and Tim10 make up the group of essential small Tim proteins that assist in transport of hydrophobic precursors across the intermembrane space in mammalian cells. Both Tim9 and Tim10 form a hexamer, the Tim9-Tim10 complex, that when associated ...

. The complex formed by the three sorting and assembly machinery subunits (SAM core complex), is responsible for the integration of beta barrel proteins into the outer mitochondrial membrane. Mdm10 is another mitochondrial membrane protein, that is responsible for maintaining mitochondrial morphology and distribution. It has been found to interact with the SAM core complex, and may play a role in assembling Tom40 into the translocase of the outer membrane.

Notes

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References

*Bolender, N., Sickmann, A., Wagner, R., Meisinger, C., Pfanner, N. (2008) Multiple pathways for sorting mitochondrial precursor proteins. EMBO J 9(1): 42-49. *Chan, N.C, and Lithgow, T. (2008) The Peripheral Membrane Subunits of the SAM Complex Function Codependently in Mitochondrial Outer Membrane Biogenesis. Mol. Biol. Cell 19(1): 126-136. Membrane proteins Mitochondrial proteins