Porin superfamily. These outer membrane channels share a beta-barrel structure that differ in strand and shear number. Classical (gram-negative ) porins are non-specific channels for small hydrophillic molecules and form 16 beta-stranded barrels (16,20), which associate as trimers. Maltoporin-like channels have specificities for various sugars and form 18 beta-stranded barrels (18,22), which associate as trimers. Ligand-gated protein channels cooperate with a TonB associated inner membrane complex to actively transport ligands via the proton motive force and they form monomeric, (22,24) barrels. The 150-200 N-terminal residues form a plug that blocks the channel from the periplasmic end.