Translocase of the outer membrane

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. 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.

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. The N-terminal end of the protein encodes a mitochondrial targeting sequence of 10-80 amino acids which can form amphipathic helices. Further, not all mitochondrial proteins have defined N-terminal targeting sequences. Some have "internal" sequences which lack consistent patterns.

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. 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.