Structure and function of colicin A and N
摘要
The lipid bilayer membrane which surrounds living cells is a very effective barrier in preventing diffusion of small molecules and macromolecules into the cytoplasm. In order to reach their site of action inside the target cells, from their site of synthesis within the producing cell, toxins must cross two to four membranes. How these macromolecules are able to cross these barriers is a central question which is still far from being understood. One large class of toxins exerts its lethal effect by inserting into the target cell membranes and forming a pore which perturbs the ionic and osmotic balance across the membrane, inducing a wide diversity of biological effects such as loss of solute, inhibition of active transport, depolarization of the membrane, and arrest of protein synthesis. These toxins are produced in a water soluble form which is able to diffuse towards the target cell in an aqueous environment. But in their final state, the pore, they become integral membrane proteins embeded in the hydrophobic core of a lipid bilayer. How a single polypeptide chain can first cross a membrane and then insert into a second membrane to form a voltage gated channel is the question we have been addressing for many years by studying the pore-forming colicins A and N.