Translocase of the outer membrane

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File:PBB Protein TOMM20 image.jpg
A structural depiction of a translocase of the outer mitochondrial membrane (TOMM20)

The translocase of the outer membrane (TOM) is a protein found in the outer mitochondrial membrane of the mitochondria. Its function is 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.[1] 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.[2]

Protein targeting to the mitochondrion

Proteins destined for the mitochondrion have several characteristics. For example, HSP90 aids the delivery of the mitochondrial protein to the mitochondrion in an ATP-dependent process.[3] The N-terminal end of the protein encodes a mitochondrial targeting sequence of 10-80 amino acids which can form amphipathic helices.[4][5] Further, not all mitochondrial proteins have defined N-terminal targeting sequences. Some have "internal" sequences which lack consistent patterns.[4]

Members of the complex

The translocase of the outer membrane (TOM) forms a complex made of Tom70, Tom22, and Tom20, along with Tom40, Tom7, Tom6, and Tom5. Tom40 is the core element of the translocase complex and Tom40 complexes with Tom22 with a mass of approximately 350k Daltons.[6] It forms the central protein-conducting channel with a diameter of approximately 2.5 nm.[6] The human Tom22 is approximately 15.5k Daltons and complexes with Tom20.[7] It N-terminal end of Tom22 extends into the cytosol and is involved in preprotein binding.[7]


  1. Alberts, Bruce (1994). Molecular Biology of the Cell. New York: Garland Publishing Inc. ISBN 0815332181. Unknown parameter |coauthors= ignored (help)
  2. Seki N, Moczko M, Nagase T; et al. (1996). "A human homolog of the mitochondrial protein import receptor Mom19 can assemble with the yeast mitochondrial receptor complex". FEBS Lett. 375 (3): 307–10. PMID 7498524.
  3. Humphries AD, Streimann IC, Stojanovski D, Johnston AJ, Yano M, Hoogenraad NJ, Ryan MT (2005 Mar). "Dissection of the mitochondrial import and assembly pathway for human Tom40". J Biol Chem. 280 (12): 11535–43. doi:10.1074/jbc.M413816200. PMID 15644312. Check date values in: |date= (help)
  4. 4.0 4.1 Neupert W, Herrmann JM (2007). "Translocation of Proteins into Mitochondria". Ann Rev Biochem. 76: 723–749. doi:10.1146/annurev.biochem.76.052705.163409. PMID 17263664.
  5. Herrmann JM, Neupert W (2000 Apr). "Protein transport into mitochondria". Curr Opin Microbiol. 3 (2): 210&ndash, 214. doi:10.1016/S1369-5274(00)00077-1. Check date values in: |date= (help)
  6. 6.0 6.1 Ahting U, Thieffry M, Engelhardt H, Hegerl R, Neupert W, Nussberger S (2001). "Tom40, the Pore-forming Component of the Protein-conducting TOM Channel in the Outer Membrane of Mitochondria". J. Cell Biol. 153: 1151–60. doi:10.1083/jcb.153.6.1151.
  7. 7.0 7.1 Yano M, Hoogenraad N, Terada K, Mori M (2000). "Identification and functional analysis of human Tom22 for protein import into mitochondria". Mol Cell Biol. 20 (19): 7205–13. PMID 10982837.

See also

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