Signal Transduction

The signals that cells use to communicate with one another are often small amino acid chains, called peptides. Depending on the cell type that releases them and the effect they have on the target cell, they may be called hormones, growth factors, neuropeptides, neurotransmitters, or cytokines. Other small molecules can also be signals, such as amino acids and steroids such as testosterone. External signals such as odorants and tastes can be carried to us in the atmosphere or in the fluids of our food and drinks. Stretch, pressure, and other mechanical effects as well as heat, pain, and light can also act as signals.

Given the huge variety of signals to which a cell is exposed, how does it know which to respond to? The answer is that signals are received by protein receptors made by the cell, and a cell is sensitive only to those signals for which it has made receptors. For instance, every cell in the body is Signal transduction converts an environmental signal into a cellular response. A cascade allows rapid response, amplification and control. exposed to estrogen circulating in the blood, but only a subset of them make estrogen receptors, and are therefore sensitive to its influence.

Chemical signals such as hormones bind to their receptors, usually at the surface of the cell (the plasma membrane), but sometimes within the cell. This causes a conformation (shape) change in the receptor. The conformation change typically alters the ability of the receptor to bind to another molecule in the cell, modifying that molecule's conformation, or triggering other actions.

This sequence of events triggered by the signal-receptor interaction is called a transduction cascade. A transduction cascade involves a network of enzymes that act on one another in specific ways to ultimately generate precise and appropriate responses. These responses may include alterations in cell motility or division, induction of the expression of specific genes, and the regulation of apoptosis. The molecular details of several such cascades are known, but many more undoubtedly remain to be discovered.

The value of this complex cascade of events is severalfold. First, the network of interactions provides many levels of control, so that the cell can tailor the magnitude and timing of its response very finely. Second, the many levels of interaction allow amplification of the original signal to quickly produce a strong response to a small stimulus. For example, there may be only a few hundred copies of a specific receptor on the surface of a typical cell. Activation of even a small percentage of them, acting through these amplifying enzyme cascades, can result in activation of millions of downstream target molecules. This explains how even very small amounts of signals such as growth factor can have such profound effects on appropriately receptive cells.