Beta-strand Membrane Proteins

A class of integral membrane proteins is emerging in which the protein spans the lipid bilayer region via a series of beta-strands. These 14-18 strands form a continuous beta-sheet that creates a pore through the membrane. The first examples were porins: pore-forming proteins found in the outer membrane of some bacteria. They are found as trimers of identical subunits, and permeabilize the membrane to certain small molecules such as sugars. Similar proteins may also be found in the outer membranes of mitochondria. More recently, the crystal structure of a pore-forming toxin, alpha-hemolysin, from Staphylococcus aureus has been solved. It is composed of seven identical subunits, each of which contributes two beta-strands to form a pore through the membrane. The opening of this pore leads to cell death, as ion gradients cannot be maintained.