Some herbivorous insects can derive protection from predators by sequestration of toxins from the plants they feed upon. For example, milkweeds contain ouabain that is toxic to most animals. Ouabain is an inhibitor of an important ion pump in nerves, the Na+/K+ ATPase. Animal nerves require a high concentration of K+ (potassium ion) and a low concentration of Na+ (sodium ion) inside the cell. The Na+/K+ ATPase uses energy from ATP to pump Na+ out of the cell and K+ into the cell. Oaubain inhibits this pump and leads to inhibition of signaling in the nervous system. Sequestration of ouabain is an effective defense against predators.
If ouabain is toxic to nerves of animals, why are monarch caterpillars able to withstand ouabain? Ouabain inhibits the Na+/K+ ATPase by binding to an area of the protein that contains a conserved sequence of 12 amino acids. Most animals have the exact same amino acids in this region of the protein, so ouabain affects these animals in the same manner. In monarchs, this site is altered by mutation. Monarchs have a mutant Na+/K+ ATPase that contains a histidine in place of an asparagine in the ouabain binding site. THe mutant Na+/K+ ATPase of the monarch will continue to function in the presence of ouabain. This allows the monarch to feed on milkweed without suffering from its toxic effects on other animals.