Porins are beta barrel proteins that cross a cellular membrane and act as a pore, through which molecules can diffuse.

[1] Unlike other membrane transport proteins, porins are large enough to allow passive diffusion, i.e., they act as channels that are specific to different types of molecules.

[2] The amino acid composition of the porin β strands are unique in that polar and nonpolar residues alternate along them.

[5] Hydrogen bonding and dipole-dipole interactions between each monomer in the homotrimer ensure that they do not dissociate, and remain together in the outer membrane.

Porins are primarily involved in passively transporting hydrophilic molecules of various sizes and charges across the membrane.

[9] Additionally, porins can regulate permeability and prevent lysis by limiting the entry of detergents into the cell.

General porins have no substrate specificities, though some exhibit slight preferences for anions or cations.

[9] They exhibit similar transport functions but have a more limited variety of porins, compared to the distribution found in gram-negative bacteria.

[9] Gram-positive bacteria lack outer membranes, so these porin channels are instead bound to specific lipids within the cell walls.

They have similar characteristics as bacterial and mitochondrial porins, indicating physiological overlaps over all three domains of life.

[7] Many porins are targets for host immune cells, resulting in signaling pathways that lead to bacterial degradation.

[5] The mutations may lead to a loss of porins, resulting in the antibiotics having a lower permeability or being completely excluded from transport.