Reality → Life → Cell → Communication
To regulate the myriads of molecular interactions that take place throughout an organism's life, cells maintain highly integrated and dynamic ways of communicating. Transmembrane receptor proteins play a pivotal role in controlling the exchange of chemical messages between cells [1] . A huge number of different external signal molecules (hormones, neurotransmitters, small protein and peptide molecules), when received by matching receptors, trigger cascades of intracellular signaling events that regulate a cell's function [2] . Other processes of cell signaling include enzymatic interactions [3] , cell adhesion [4] , and electric pulses [5] .
The most common transmembrane receptors are GPCRs (G-Protein-Coupled Receptors) that bind to G-proteins of the cell membrane and activate these after receipt of an external signal molecule. GPCRs are highly specific (about 1000 different human GPCRs are known).
A signal molecule changes the conformation of its matching receptor which, in turn, activates a G-protein on the inner surface of the cell membrane. The activated G-protein can cause the production of hundreds or thousands of small intracellular secondary messenger molecules which regulate the cell's function in many different pathways (see also transduction and GPCR).
RTKs (Receptor Tyrosine Kinases) are transmembrane protein receptors that, after binding of a signal molecule, act as enzymes that catalyze phosphorylation of other proteins. The enzymatic reactions ultimately alter the gene transcription in the cell's nucleus.
Cells that build tissues stick together through special transmembrane proteins that provide adhesion and communication channels at the same time. One type of such proteins is integrin, which mainly connects the cell's cytoskeleton to the extracellular matrix. Other connections, from cell to cell, are provided by tight junctions and gap junctions.
Nerve cells, muscle cells, and the skin's touch cells can generate, conduct, or receive electrical signals. Chemical or mechanical (pressure) signals are converted into electrical pulses in ion channels, special multi-proteinic receptors in the membrane of such cells. The pulses are caused by a transmembrane potential. Cell responses to electrical signals are in the order of milliseconds, whereas responses to chemical signals normally take several minutes (about 10,000 times longer).