Melanosomes

In animals

Melanosomes found in certain fish species contain pigments that control the color of the fish's scales. Molecular motors, when signaled, will either carry melanosomes containing pigments out to the periphery of the cell, or concentrate them at the center. The motor protein dynein is responsible for concentrating the melanosomes toward the center of the cell, or the "minus end" of microtubules. Conversely, the protein kinesin is responsible for dispersing the melanosomes to the periphery of the cell, and are plus end directed motors. Because the plus ends of microtubules are oriented towards the periphery, kinesin will carry melanosomes to the periphery. Dispersing melanosomes to the periphery causes the cell to appear darker; concentrating melanosomes towards the center will cause the cell to appear lighter color. This is how a photoprotective system works for the fish on a molecular level.