the translocase of the outer membrane (tom) forms a complex made of tom70, tom22, 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. the n-terminal end of tom22 extends into the cytosol and is involved in preprotein binding.