**4. Production of signals**

For a cell to use a molecule in a cell signalling pathway it needs to firstly be made. Many of the components are constitutively produced and are present to partake in the required activity when called upon to do so. Examples would be large proteins such as kinases. However, there are many situations where a molecule needs to be present, or released, in a rapid manner. Cyclic AMP (cAMP) for example is needed in response to adrenaline (for an example of a signalling pathway in which cAMP is involved see Figure 2) while insulin is released in to the blood stream when required. One of the main underlying principles of signalling is that the system is able to convey the message when and where required, in a temporal manner appropriate to the required response. Therefore molecules need to be able to partake in such signalling when called upon to do so.

There are two main ways to make a signal. Either the molecule is produced when required, or it is made and stored, to be released when required (Figure 3). It a similar manner it could be argued that there are two main ways to listen to music. You either go to a concert and in the presence of the musical instruments you listen to the sounds being made, or you let the band record the music, store it until required and then play it. In this scenario the instruments are the enzymes, producing the message. At the concert instruments make the signal as needed, to the required amount, for the required time. This is just like an enzyme such as adenylyl cyclase which is turned on, generates cyclic AMP (cAMP) for a set period of time, and then turns off. Just like the person at the concert, the protein responding to cAMP can perceive its presence and when it is all over revert back to a quiescent state – concert over.

Fig. 3. Signals can be generated using different scenarios. In (A) there is no signal produced until an enzyme is activated. At that point a compound X and be converted to compound Y, so generating a signal. That is Y can now be recognised and a response or effect produced due to its presence. In (B) the signalling molecule can be pre-made but sequestered into vesicles. On arrival of the appropriate stimulus the vesicles will translocate to the plasma membrane for example. The signalling molecule will then be released and be able to move to its site of recognition and action. In the case of hormones the site of perception may be a different organ or tissue, with the signalling compound being carried by the vascular system of the organism.

The production of insulin on the other hand is more akin to a recording. Insulin is encoded for by a single gene, giving rise to a single protein, referred to as pre-proinsulin. This is heavily modified, primarily through cleave events, to produce the active insulin molecule which comprises of two polypeptides. This "ready to use" insulin is then stored in vesicles in the cell until required. This is like a musical recording, the music is created and then stored, sat in its CD case, or as an MP3 file, awaiting to be played. On demand insulin is released by the fusion of vesicles to the plasma membrane in the islets of Langerhans and is released to the outside.
