16.13: Porin Insertion in the Outer Mitochondrial Membrane
Porins are beta-barrel proteins translocated to the mitochondrial outer membrane through the TOM complex into the intermembrane space. Porin precursors bind TIM chaperones within the intermembrane space and are guided to the Sorting and Assembly Machinery complex or SAM complex on the outer mitochondrial membrane.
Three models describe the assembly of porins by the SAM complex and their insertion into the outer membrane. Model 1 suggests that porins are assembled outside the SAM channel as the hydrophilic beta-hairpins get inserted into the hydrophobic lipid bilayer by an unknown mechanism. In models 2 and 3, precursors are threaded through the SAM channel and released laterally into the membrane.
In model 2, assembled beta-barrel precursors are lifted over the lowest point of the SAM50 beta-barrel rim. Model 3 suggests a gap between the first and last beta-strands of the SAM50 that allows reversible hydrogen bond formation and disruption to help open the SAM50 beta-barrel. Porin precursors are assembled into beta-barrels within the open SAM50 and then laterally released into the outer membrane.
During beta-barrel assembly, accessory proteins SAM35 and SAM37 also cooperate with SAM50. SAM35 prevents the precursor from passing into the cytosol by interacting with the beta signals of the precursors while they are being folded in the lumen of the SAM50 channel. In addition, SAM37 stabilizes the SAM complex and helps develop TOM-SAM supramolecular complexes that allow membrane compression to facilitate precursor release.