Structure of Porins

Porins are barrel-shaped proteins of the outer mitochondrial membrane that allow the free diffusion of ions and metabolites through a central channel.

Porins are formed of beta-sheets arranged into a circular hollow structure with hydrophobic amino acid residues facing the lipid bilayer and the hydrophilic amino acid residues lining the inner channel.

Porin precursors contain conserved sequence motifs called beta signals at their C-terminal end. These signals are crucial for precursor recognition and assembly.

Porin precursors are translocated through the TOM complex into the intermembrane space. TIM chaperones bind and guide these precursors to the sorting and assembly machinery complex or the SAM complex on the outer mitochondrial membrane.

The SAM complex is a membrane protein comprising a beta-barrel core and two receptors on the cytosolic side of the membrane.

SAM recognizes the beta signals within the precursor peptide and guides them to fold into beta-hairpins. With successive folding, the beta-hairpins begin to assemble into a beta-barrel structure to form the porin.