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.

Tom20 and Tom22 are preprotein receptors, which are responsible for recognition of the cleavable presequence possessed by mitochondrial-targeted proteins.

[6][10] Tom22 is anchored to the outer membrane by a single transmembrane segment and also plays a role in stabilizing the TOM complex.