Signal transduction

Signal transduction is how a cell converts a message from outside, such as a hormone, neurotransmitter or growth factor, into action inside. Most signaling molecules cannot cross the plasma membrane, so they bind to a receptor protein on the cell surface. Binding changes the receptor's shape, and the change is relayed inward through a series of molecules, a signaling pathway or cascade, until the cell responds, for instance by switching genes on or off, changing its metabolism or dividing. Steroid hormones are an exception: they cross the membrane and bind receptors inside the cell.

Some relay methods recur. Enzymes called kinases add phosphate groups to proteins, switching them on or off, and phosphatases remove them. Many receptors pass the signal to G proteins, which are active while bound to GTP. Small molecules called second messengers, such as cyclic AMP and calcium ions, spread the signal through the cytoplasm. Because one activated receptor can activate many enzyme molecules, each acting on many more, a weak signal can be greatly amplified.

Faulty signal transduction underlies many diseases. Mutations that leave growth-factor pathways stuck in the on position, such as overactive forms of the RAS protein or the EGF receptor, can drive the uncontrolled division of cancer cells, and many modern drugs are designed to block single steps in these pathways.