Porins are proteins that are located in the outer membrane of Gram-negative bacteria. They function to form a water-filled pore through the membrane, from the exterior to the periplasm, which is a region located between the outer and inner membranes. The porin channel allows the diffusion of small hydrophilic (water-loving) molecules through to the periplasm. The size of the diffusing molecule depends on the size of the channel.

A porin protein associates with two other porin proteins of the same type in the outer membrane. This may act to stabilize the three-dimensional structure of each porin molecule. Each porin contains a pore, so that there are three pores in the triad of porins.

The size of the water-filled channel that is created by a porin depends on the particular porin protein. For example, in the bacterium Escherichia coli, the so-called maltoporin and phosphoporin have different specificities (for the...