1.B.33.3.1
The mitochondrial Sorting and Assembly Machinery (SAM) includes SAM50, Tom37 (Mas37; Sam37), Tom13 (Mim1), Mim2, and porin; see 3.A.8 (Paschen et al., 2005). The MIM complex can assemble N- and C-terminal α-helical anchor proteins. SAM and TOM insert β-barrel proteins in the outer mitochondrial membrane (Stojanovski et al., 2007). Mim1 (Tom13) is required for the biogenesis of the beta-barrel protein Tom40 and also for membrane insertion and assembly of signal- and C-terminally-anchored Tom receptors (Becker et al., 2008; 2011). It has cation-selective ion transport activity (Checchetto and Szabo 2018; Krüger et al. 2017). Tom7 regulates Mdm10-mediated assembly of the mitochondrial import channel protein Tom40 (Yamano et al., 2010). Homologous Omp85 proteins are essential for membrane insertion of β-barrel precursors. Precursors are apparently threaded through the Omp85-channel interior and exit laterally. Höhr et al. 2018  mapped the interaction of a precursor in transit with the mitochondrial Omp85-channel Sam50 in the native membrane environment. The precursor is translocated into the channel interior, interacts with an internal loop, and inserts into the lateral gate by β-signal exchange. Transport through the Omp85-channel interior followed by release through the lateral gate into the lipid phase represents a basic mechanism for membrane insertion of β-barrel proteins (Höhr et al. 2018). The TOM and SAM complexes cooperate in the import of beta-barrel proteins, whereas the mitochondrial import  (MIM) complex (Mim1/Mim2/porin) inserts precursors of multi-spanning alpha-helical proteins. Single-spanning proteins constitute more than half of the integral outer membrane proteins. Doan et al. 2020 reported that the yeast MIM complex promotes the insertion of proteins with N-terminal (signal-anchored) or C-terminal (tail-anchored) membrane anchors. The MIM complex exists in three dynamic populations. MIM interacts with TOM to accept precursor proteins from the receptor Tom70. Free MIM complexes insert single-spanning proteins that are imported in a Tom70-independent manner. Finally, coupling of MIM and SAM promotes early assembly steps of TOM subunits. Thus, the MIM complex is a major and versatile protein translocase of the mitochondrial outer membrane (Doan et al. 2020).