Discovered in the 70s and worth a Nobel Prize for Martin Rodbell in 1994, signal transduction in the cell is still a topic of intense research. In order to respond to environmental stimuli, living organisms have developed a wide variety of complex processes to transmit signals from the outside to the inside to elicit an adequate cellular response. Defects in these molecular pathways may lead to very different disorders such as diabetes, cancer and psychotic diseases.
There are two main steps in the process:
- Firstly, a signalling molecule (a ligand) needs to activate a specific receptor in the membrane or the cytoplasm of the cell. The ligand-receptor union is very specific, like that of a key and a lock.
- Then a secondary messenger transmits the signal to enzymes or gene activators.
In either step, the signal can be amplified leading to different responses. Most of the receptors for a specific signal undergo a change in shape once a ligand binds. This can expose an enzymatic domain or open an ion channel. The enzymatic activity, like phosporylation, initiates a signalling cascade by modifying or releasing secondary messenger molecules.
Steroid hormones like cortisone, testosterone or progesterone are soluble in lipids and can, therefore, pass through the membrane into the interior of the cell without the need of an external receptor. There, they bind a specific receptor and the complex moves into the nucleus. Here, genes can be activated for reproductive functions, to respond to stress or to build up muscles. This kind of signalling may take several minutes, whereas a photon that hits the cells in the retina of the eye has an immediate response.