16.12 : Structure of Porins

Mitochondria, chloroplasts, and gram-negative bacteria have transmembrane, beta-barrel proteins called porins to mediate the free diffusion of ions and metabolites across the membrane. Mitochondrial porin precursors contain conserved amino acid sequences called beta signals at their C-terminal. Beta signals have a motif of PoXGXXHyXHy (Po-Polar, X-Any amino acid, G-Glycine, Hy-LargeHydrophobic), which are crucial for precursor recognition to initiate precursor assembly. Beta-barrel precursors move through the TOM complex on the outer mitochondrial membrane into the intermembrane space. The precursors interact with TIM chaperones inside the intermembrane space and are guided to a trimeric complex called the Sorting and Assembly machinery complex or SAM complex.

SAM complex has three components: a beta-barrel core called the SAM50 and accessory proteins SAM35 and SAM37. SAM50 comprises 16-beta-strands that span the outer membrane and expose an N-terminal polypeptide transport-associated domain or POTRA domain in the intermembrane space. The POTRA domain functions as a receptor for beta-barrel precursors and facilitates beta-strand assembly. In contrast, SAM50 interacts closely with accessory proteins SAM35/SAM37 towards the cytosolic side to help fold the beta-barrel precursors entering the intermembrane space.