As inferred from the yeast TOM40:TOM70 complex, the human TOMM40:TOMM70 complex transports precursor proteins from the cytosol, across the outer membrane of the mitochondrion, and into the intermembrane space from where they may be targeted to all locations within the mitochondrion. As inferred from yeast, TOMM40, TOMM22, TOMM5, TOMM6, and TOMM7 probably form the general import pore across the membrane. On the cytosolic side TOMM20 and TOMM22 interact with presequences on mitochondrial precursors while TOMM70 interacts with hydrophobic sequences in mature internal regions of mitochondrial proteins.
In yeast, experimentally verified substrates of the TOM40:TOM70 complex include ATP1 (ATP5A1 in human), ATP2 (ATP5B in human), ATP9 (ATP5G1 in human), TOM40 (TOMM40 in human), SSC1 (mtHsp70, HSPA9 in human), CIT1 (CS in human), ACO1 (ACO2 in human), IDH1 (IDH3G in human), BCS1 (BCS1L in human), CYT1 (CYC1 in human), TIM54 (TIMM54 in human), TIM22 (TIMM22 in human), AAC (ADP/ATP translocase 1, ANT, SLC25A4 in human), HSP60, and CYB2. In humans, TOMM40 has been shown to be a substrate (Humphries et al. 2005). In yeast some proteins such as ACO1, ATP1, CIT1, IDH1, and ATP2 contain both presequences that interact with TOM20 and mature regions that interact with TOM70 (Yamamoto et al. 2009). Most proteins imported into mitochondria are anticipated to be transported through the TOMM40:TOMM70 complex.