Distance constraints from site-directed spectroscopy as a tool to study membrane protein structure

A membrane protein is a large (typically several nm in diameter) molecule that is attached to or incorporated into a phospholipid membrane of a cell, or a cell organelle. If a protein is permanently incorporated into the membrane, the protein is called an integral membrane protein. Integral membrane proteins can be classified into two classes: transmembrane membrane proteins and monotopic membrane proteins. Transmembrane membrane proteins span the membrane, whereas monotopic membrane proteins are permanently attached to one side of the membrane.

Integral membrane proteins play a crucial role in the life cycle of all organisms, including plants, fungi, bacteria, and animals. To fully understand the complexity and diversity of the structure and functioning of membrane proteins, one should first grasp the concept and the organization of a living cell. A cell is the smallest metabolically functional unit of life; for this reason, the cell is sometimes called the ‘building block of life’. The word cell is derived from the Latin word ‘cella’ or ‘cellula’, meaning storage room or little room, respectively.