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. 2021 Jul 4;26(13):4087. doi: 10.3390/molecules26134087

Figure 1.

Figure 1

Overview of protein import into mitochondria. The TOM complex recognises most of the mitochondrial pre-proteins via its receptors (i.e., Tom20, Tom22 and Tom70) and imports across the outer membrane through the Tom40 channel (dimer). Small TOM subunits (i.e., Tom5, Tom6 and Tom7) stabilise the TOM complex. β-barrel precursors (i.e., Tom40, Tob55/Sam50, VDAC and Mdm10) are inserted into the outer membrane by the TOB/SAM complex, which consists of the protein channel Tob55/Sam50 (dimer) and receptors Tob38 and Tob37. The mitochondrial contact site and cristae organising system (MICOS) complex supports the insertion of the β-barrel proteins into the OM. The dynamic structure of TOB/SAM and MICOS is called mitochondrial intermembrane space bridging (MIB). α-helical precursors are inserted into the OM by the MIM complex (Mim1 and Mim2). The ERMES complex links the mitochondria and endoplasmic reticulum (ER) and supports the topogenesis of the mitochondrial outer membrane β-barrel proteins/sorting and assembly machinery (TOB/SAM) complex. The precursor targeted into the intermembrane space engages the MIA complex. TIM23 is responsible for import across the IM or into the mitochondrial matrix. TIM22 inserts carrier proteins into the IM. Voltage-dependent anion-selective channel (VDAC) is responsible for metabolite transport. Ayr1, OMC7 and OMC8 are the channels of unknown substrates. OM—outer membrane; IM—inner membrane; IMS—intermembrane space; ER—endoplasmic reticulum.