Membrane receptors are important proteins
Our cells have an outside and an inside, and more compartments within. These spaces are divided by membranes studded with proteins that control the traffic across them.
Membrane-bound proteins are numerous and hard to study. Many straddle the membrane, sticking out on both sides. The membrane itself is made of phospholipid, so membrane proteins fold to put hydrophobic (water-hating) amino acids in the middle and hydrophilic (water-loving) ones at each side of the membrane. Take these proteins out of the membrane and they tend to unravel.
Some proteins are pores, channels for specific molecules. Some are pumps; they use chemical energy to move molecules across the membrane. Some sense what is going on outside the cell and pass a signal to the inside. Huge enzyme complexes that transfer the chemical energy from food to chemicals such as adenosine triphosphate (ATP) are found in the mitochondria of our cells (see our ‘Focus Protein: ATP synthase’ article). The complexes only work when embedded in the membrane because they set up an electrochemical gradient across it.
Researchers are exploring a large group of proteins known as G-protein-coupled receptors. These share a similar basic sequence, which crosses the membrane seven times, but are each tailored to a specific molecule.
The receptor reads an outside signal, usually a molecule that binds to the protein. This causes a change in shape that leads to the binding and activation of a previously inactive protein – a G protein – inside the cell. This basic system is very versatile and is involved in many things, including sight, taste and smell, and passing on the messages from hormones and neurotransmitters.
About half of all drugs used in medicine act on G-protein-coupled receptors. The Nobel Prize in Chemistry for 2012 was awarded for research on these proteins.