Porin Insertion in the Outer Mitochondrial Membrane

Model 1 proposes beta-barrel precursors fold outside the SAM channel and insert simultaneously into the membrane. Model 2 suggests unfolded precursors pass through the SAM channel, assemble, then lift over the SAM channel's lowest rim for insertion. Model 3 describes SAM50 undergoing conformational change, unfolding its beta-barrel to allow successive assembly of precursor beta-hairpins between its strands using hydrogen bonds.

During precursor assembly in Model 3, multiple hydrogen bonds form between beta-hairpins and the beta-strands of SAM50. Disruption of these hydrogen bonds provides the energy necessary for complete assembly and insertion of beta-barrel structures. This reversible bonding mechanism allows controlled assembly and lateral release of mature porins into the outer membrane.

SAM50 is a beta-barrel subunit of the SAM complex that undergoes conformational changes during porin assembly. It unfolds its beta-barrel structure to allow precursor beta-hairpins to assemble between its strands. After assembly completes, SAM50 returns to its closed barrel conformation, releasing newly assembled porins into the outer mitochondrial membrane.

SAM35 prevents precursor escape into the cytosol by interacting with beta signals during folding within the SAM50 channel lumen. SAM37 stabilizes the entire SAM complex and helps develop TOM-SAM supramolecular complexes. These complexes enable membrane compression, facilitating efficient lateral release of assembled porins into the outer membrane.

Porin precursors bind TIM chaperones in the intermembrane space and are guided to the SAM complex on the outer mitochondrial membrane. In Models 2 and 3, precursors thread through the SAM channel where they assemble into beta-barrels. They are then released laterally into the outer membrane through conformational changes in SAM50 or by lifting over the channel rim.

Model 2 involves lifting fully assembled beta-barrel precursors over the lowest point of the SAM50 rim for insertion. Model 3 proposes a gap between the first and last beta-strands of SAM50 that allows reversible hydrogen bond formation and disruption. This gap enables precursor assembly within the open SAM50 channel before lateral release into the membrane.

In Model 1, hydrophilic beta-hairpins insert into the hydrophobic lipid bilayer of the outer membrane as porins assemble outside the SAM channel. This insertion mechanism contrasts with Models 2 and 3, where assembly occurs within or through the SAM channel. The specific interaction between hydrophilic precursor structures and the hydrophobic membrane environment drives insertion in this model.