The New Era of Microcrystallography

The function of a protein dictates its physical state in a cell. Evolution has imparted selection pressure on proteins to maximize their function and minimize cell death. Most of the proteins exist in their soluble form inside or outside the cells. However, a small fraction of proteins in the total protein pool crystallizes with functional consequence. These in vivo-grown protein crystals perform a diversity of functions, ranging from food storage to defense. Sometimes limited by the volume of the cells and the cellular concentration of proteins, these crystals are very small in size. Hence, it has been difficult to carry out conventional X-ray crystallography on these crystals. With the advent of microcrystallography, it is now possible to study the structures of these tiny crystals. In this review, some of the diverse examples of in vivo crystals and the new approaches towards microcrystallography are summarized.