Translocase of the outer membrane
Encyclopedia
The translocase of the outer membrane (TOM) is a complex of proteins found in the outer mitochondrial membrane
of the mitochondria. Its function is to allow movement of proteins through this barrier and into the intermembrane space
of the mitochondrion. Most of the proteins needed for mitochondrial function are encoded by the nucleus
of the cell
. The outer membrane of the mitochondrion is impermeable to large molecules greater than 5000 Daltons. The TOM works in conjunction with the translocase of the inner membrane
(TIM) to translocate proteins into the mitochondrion
. Many of the proteins in the TOM complex, such as TOMM22
, were first identified in Neurospora crassa
and Saccharomyces cerevisiae
.
The complete mitochondrial protein translocase complex includes at least 19 proteins: several chaperones, four proteins of the outer membrane translocase (Tom) import receptor, five proteins of the Tom channel complex, five proteins of the inner membrane translocase (Tim) and three "motor" proteins.
aids the delivery of the mitochondrial preprotein to the TOM complex in an ATP-dependent process. Many precursor proteins (those that are destined for the matrix) contain amino-terminal presequences that carry information required for the targeting of proteins to the mitochondrial matrix
These matrix targeting signals generally contain 10-80 amino acid residues that take on the conformation of an amphipathic-α helix and contain one positive and hydrophobic face. Once the precursor reaches the matrix, the presequence is typically cleaved off by the matrix processing peptidase. Proteins targeted to other sub-compartments of the mitochondria such as the intermembrane space
and inner mitochondrial membrane
, contain internal targeting signals, these signals have an indefinable nature and are inconsistent in their pattern. Proteins targeted to the outer membrane also contain internal targeting signals, not all of which have been identified, and include proteins that take on a β-barrel structure, such as Tom40. Some proteins however, that are targeted to the outer mitochondrial membrane contain a hydrophobic tail domain that anchors the protein to the membrane.
, and Tom20
, along with Tom40, Tom7, Tom6, and Tom5. Tom20 and Tom22 are preprotein receptors, which are responsible for recognition of the cleavable presequence possessed by mitochondrial-targeted proteins. Tom70 is also a preprotein receptor and may recognise some cleavable presequence proteins, however it is mainly responsible for the recognition of non-cleavable preproteins and acts as a point for chaperone binding. Tom22 is anchored to the outer membrane by a single transmembrane segment and also plays a role in stabilizing the TOM complex. Tom40 is the core element of the translocase complex and complexes with Tom22 with a mass of approximately 350k Daltons
. It forms the central protein-conducting channel with a diameter of approximately 2.5 nm. The human Tom22 is approximately 15.5k Daltons
and complexes with Tom20. It N-terminal end of Tom22 extends into the cytosol and is involved in preprotein binding.
Outer mitochondrial membrane
thumb|300px|Mitochondria structure :1) [[Inner membrane]]2) Outer membrane3) [[Crista]]4) [[Matrix |Matrix]]The outer mitochondrial membrane, which encloses the entire organelle, has a protein-to-phospholipid ratio similar to the eukaryotic plasma membrane...
of the mitochondria. Its function is to allow movement of proteins through this barrier and into the intermembrane space
Intermembrane space
The intermembrane space also known as IMS is the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast. The main function of the intermembrane space is oxidative phosphorylation....
of the mitochondrion. Most of the proteins needed for mitochondrial function are encoded by the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
of the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. The outer membrane of the mitochondrion is impermeable to large molecules greater than 5000 Daltons. The TOM works in conjunction with the translocase of the inner membrane
Translocase of the inner membrane
The translocase of the inner membrane is a complex of proteins found in the inner mitochondrial membrane of the mitochondria. Components of the TIM complex facilitate the translocation of proteins across the inner membrane and into the matrix...
(TIM) to translocate proteins into the mitochondrion
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
. Many of the proteins in the TOM complex, such as TOMM22
TOMM22
Mitochondrial import receptor subunit TOM22 homolog is a protein that in humans is encoded by the TOMM22 gene.-Further reading:...
, were first identified in Neurospora crassa
Neurospora crassa
Neurospora crassa is a type of red bread mold of the phylum Ascomycota. The genus name, meaning "nerve spore" refers to the characteristic striations on the spores. The first published account of this fungus was from an infestation of French bakeries in 1843. N...
and Saccharomyces cerevisiae
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...
.
The complete mitochondrial protein translocase complex includes at least 19 proteins: several chaperones, four proteins of the outer membrane translocase (Tom) import receptor, five proteins of the Tom channel complex, five proteins of the inner membrane translocase (Tim) and three "motor" proteins.
Protein targeting to the mitochondria
There are various mitochondrial import pathways that exist to facilitate the import of precursor proteins to their destined mitochondrial subcompartments. HSP90Hsp90
Hsp90 is a molecular chaperone and is one of the most abundant proteins expressed in cells. It is a member of the heat shock protein family, which is upregulated in response to stress...
aids the delivery of the mitochondrial preprotein to the TOM complex in an ATP-dependent process. Many precursor proteins (those that are destined for the matrix) contain amino-terminal presequences that carry information required for the targeting of proteins to the mitochondrial matrix
Mitochondrial matrix
In the mitochondrion, the matrix contains soluble enzymes that catalyze the oxidation of pyruvate and other small organic molecules.The mitochondrial matrix also contains the mitochondria's DNA and ribosomes. The word "matrix" stems from the fact that this space is viscous, compared to the...
These matrix targeting signals generally contain 10-80 amino acid residues that take on the conformation of an amphipathic-α helix and contain one positive and hydrophobic face. Once the precursor reaches the matrix, the presequence is typically cleaved off by the matrix processing peptidase. Proteins targeted to other sub-compartments of the mitochondria such as the intermembrane space
Intermembrane space
The intermembrane space also known as IMS is the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast. The main function of the intermembrane space is oxidative phosphorylation....
and inner mitochondrial membrane
Inner mitochondrial membrane
The mitochondrial inner membrane forms internal compartments known as cristae, which allow greater space for the proteins such as cytochromes to function properly and efficiently. The electron transport chain is located on the inner membrane of the mitochondria...
, contain internal targeting signals, these signals have an indefinable nature and are inconsistent in their pattern. Proteins targeted to the outer membrane also contain internal targeting signals, not all of which have been identified, and include proteins that take on a β-barrel structure, such as Tom40. Some proteins however, that are targeted to the outer mitochondrial membrane contain a hydrophobic tail domain that anchors the protein to the membrane.
Members of the complex
The translocase of the outer membrane (TOM) forms a complex made of Tom70, Tom22TOMM22
Mitochondrial import receptor subunit TOM22 homolog is a protein that in humans is encoded by the TOMM22 gene.-Further reading:...
, and Tom20
TOMM20
Mitochondrial import receptor subunit TOM20 homolog is a protein that in humans is encoded by the TOMM20 gene.-Further reading:...
, along with Tom40, Tom7, Tom6, and Tom5. Tom20 and Tom22 are preprotein receptors, which are responsible for recognition of the cleavable presequence possessed by mitochondrial-targeted proteins. Tom70 is also a preprotein receptor and may recognise some cleavable presequence proteins, however it is mainly responsible for the recognition of non-cleavable preproteins and acts as a point for chaperone binding. Tom22 is anchored to the outer membrane by a single transmembrane segment and also plays a role in stabilizing the TOM complex. Tom40 is the core element of the translocase complex and complexes with Tom22 with a mass of approximately 350k Daltons
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
. It forms the central protein-conducting channel with a diameter of approximately 2.5 nm. The human Tom22 is approximately 15.5k Daltons
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
and complexes with Tom20. It N-terminal end of Tom22 extends into the cytosol and is involved in preprotein binding.