FIGURE 7-18 Structural model of one subunit of OmpX, a porin found in the outer membrane of E. coli. All porins are trimeric transmembrane proteins. Each subunit is barrel shaped, with β strands forming the wall and a transmembrane pore in the center. A band of aliphatic (hydrophobic and noncyclic) side chains (yellow) and a border of aromatic (ring-containing) side chains (red) position the protein in the bilayer. See G. E. Schulz, 2000, Curr. Opin. Struct. Biol. 10:443.
[Data from J. Vogt and G. E. Schulz, 1999, Structure 7:1301, PDB ID 1qj8.]
The porins are a class of transmembrane proteins whose structure differs radically from that of other integral membrane proteins based on α-helical transmembrane domains. Several types of porins are found in the outer membranes of gram-negative bacteria such as E. coli and in the outer membranes of mitochondria and chloroplasts. The outer membrane protects an intestinal bacterium from harmful agents (e.g., antibiotics, bile salts, and proteases), but permits the uptake and disposal of small hydrophilic molecules, including nutrients and waste products. Different types of porins in the outer membrane of an E. coli cell provide channels for the passage of specific types of disaccharides or other small molecules as well as for ions such as phosphate. The amino acid sequences of porins contain none of the long, continuous hydrophobic segments typical of integral membrane proteins with α-helical membrane-spanning domains. Rather, it is the entire outer surface of the fully folded porin that displays its hydrophobic character to the hydrocarbon core of the lipid bilayer. X-ray crystallography shows that porins are trimers of identical subunits. In each subunit, 16 β strands form a sheet that twists into a barrel-shaped structure with a pore in the center (Figure 7-18). Unlike a typical water-soluble globular protein, a porin has a hydrophilic interior and a hydrophobic exterior; in this sense, porins are inside out. In a porin monomer, the outward-facing side chains on each of the β strands are hydrophobic and form a nonpolar ribbonlike band that encircles the outside of the barrel. This hydrophobic band interacts with the fatty acyl groups of the membrane lipids or with other porin monomers. The side chains facing the inside of a porin monomer are predominantly hydrophilic; they line the pore through which small water-soluble molecules cross the membrane. (Note that the aquaporins discussed above, despite their name, are not porins and contain multiple transmembrane α helices.)