Ribbon diagram of a porin from Rhodobacter capsulatus, refined at 1.8. Reference. This is a good example of the b-barrel folding motif.
Porins are a widespread class of bacterial outer membrane proteins and most are involved in transport of small molecules across the outer membrane. Porin-like proteins have also been identified in mitochondria and chloroplasts.
The 3D structures are known for of porin-like molecules having 8,16, 18 and 22 membrane-spanning b-strands. This porin has 16 strands.
Top view of the same molecule in a ribbon diagram and in CPK form. Notice
the pore through the middle of the structure.
PDB acess code 2POR