Linearity, Branching and Crosstalk

Crosstalk refers to a situation where a signaling enzyme from one pathway activate (E4) or inhibit (E ) signaling components involved in signal 

Signaling pathways are often formulated as linear pathways where the various signaling molecules are arranged in a hierarchical order and a linear, unequivocal flow of information takes place. This is however a simplistic view of the situation in the cell and is mostly born of an incomplete knowledge of the pathway. 

The use of second messengers is a further important tool for distributing information to different effector molecules. A second messenger like cAMP (see Chapter 6) can bind to and activate a multitude of substrates, among which the cAMP-dependent protein kinases constitute the most important and divergent class. 

The coupling and linkage of different signaling pathways also enables the cell to use different signals to trigger the same biochemical reaction. Furthermore, the same signal can be delivered to different pathways and thus trigger distinct reactions. 

Network formation and crosstalk endows the cell with a great plasticity and variability in signal transmission. Each cell receives at a time a large number of signals and the responses on these signals must be fine tuned and orchestrated in a way that is optimal for the growth and development of the organism. 

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