RECEPTORS FOR CELL SIGNALLING

Cells within multicellular organisms need to be able to communicate with each other in order to coordinate essential functions such as growth and differentiation and to respond to changes in their external environment. Cells in physical contact with each other can interact by the exchange of small molecules via gap junctions but cells physically distant from each other, with the extreme examples being found in plants and animals, need an effective communication system. This is achieved by the release of ligand signalling molecules by the signalling cells and the specific recognition of these ligands by protein receptors either embedded within and spanning the cell membrane or located in the cytoplasm of the target cells. Most commonly the ligand is water soluble and therefore incapable of diffusing across the cell membrane. It therefore binds to a ligand-binding site exposed on the extracellular side of the receptor. The binding initiates a sequence of events, in many cases involving protein–protein interactions at the membrane interface, which result in the cellular response. Examples of such ligands include amines, amino acids, peptides and proteins. However, some ligands are lipid soluble and can diffuse freely across the membrane and bind to cytosolic receptors. The receptor–ligand complexes subsequently diffuse across the nuclear membrane and accumulate in the nucleus where they modulate DNA transcription. For this reason, the receptors are referred to as nuclear receptors.