Porins

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 sugar maltose and phosphorus, respectively).

Since the discovery of porins in the 1970s in Escherichia coli, these proteins have been shown to be a general feature of the Gram-negative outer membrane. Much of the early work on porins came from the laboratories of Hiroshi Nikaido and Robert Hancock. Some examples of the bacteria known to possess porins are Pseudomonas aeruginosa, many other species of Pseudomonas, Aeromonas salmonicida, Treponema pallidum, and Helicobacter pylori.

A bacterium typically contains a variety of porins. Possession of porins of different sizes and chemistries is very advantageous for a bacterium. The various channels allow for the inward diffusion of a variety of nutrients required by the bacterium for survival and growth. Moreover, the diffusional nature of the molecule's entry means that a bacterium is able to acquire some needed nutrients without having to expend energy.

Another example of porin importance is found in Escherichia coli. In this bacterium, a duo of porins, which are designated OmpF and OmpC, function in response to changes in osmolarity. The production of these porins is under the control of a protein that senses the osmotic character of the environment. Depending on the ionic conditions, the amounts of OmpF and OmpC in the outer membrane can be altered so as to control the types of ions that enter the bacterium.

Porins share the same function in these bacteria from various habitats. This similar function is mirrored by the similarity in the three-dimensional structure of the proteins. Each porin is visually reminiscent of a barrel that is open to both ends. The slats of the barrel are arrangements of the constituent amino acids of the protein (beta sheets). The sequence of amino acids that makes up a beta sheet region allows the region to assume a zigzag configuration of the amino acids in one plane. The result is a narrow, flat strip of amino acids. When such strips are linked together, the barrel shape can be created. The outer surface of the porin barrel is more hydrophobic (water-hating) and so the partitioning of this surface into the hydrophobic interior of the membrane will be favored. The inner surface of the porin barrel contains side groups of the amino acids that prefer to interact with water.

The function of porin proteins was discovered by isolating the particular protein and then inserting the protein into model systems, that consisted either of lipid membranes floating in solution (liposomes) or floating as a sheet on the surface of a liquid (black lipid bilayer membranes). The passage of radioactive sugars of various sizes out of the liposomes or across the black lipid bilayer membranes could be measured, and the various so-called exclusion limits for each porin could be determined.

Porins also have relevance in the antibiotic resistance of bacteria, particularly Pseudomonas aerugionsa, which is the cause of lung infections in those afflicted with cystic fibrosis, and can cause so-called "opportunistic infections" in those whose immune system is impaired. For example, the porin designated OprD is specifically utilized for the diffusion of the antibiotic imipenem into the bacterium. Imipenem resistance is associated with an alteration in the three-dimensional structure of OprD such that imipenem is excluded from entering the bacterium. The resistance of a number of clinical isolates of Pseudomonas aeruginosa is the result of porin alteration. Knowledge of the molecular nature of the alterations will help in the design of antibiotics that overcome the channel barrier.

See also Bacterial membranes and cell wall; Protein export