Information Technology Reference
In-Depth Information
Exterior
Ligand
Receptor
Membrane
Effector
Enzyme
Transducer
Interior
Secondary
Messenger
Target
A
ACTIVATION
PHOSPHORYLATION
Target
B
Primary
Protein Kinase
Target
C
ACTIVATION
Secondary
Protein Kinase(s)
PHOSPHORYLATION
Target
D
Target
E
Target
F
Figure 2.1
Classic second messenger system.
Intracellular signaling pathways all share important information-processing
features; for example, the generation of a secondary messenger and occur-
rence of cascades, or hierarchies, of protein kinases permits a considerable
degree of amplification to be introduced. The classical model for intracellular
signaling therefore presents a highly sensitive mechanism for relaying small
changes in the external environment to the interior of the cell. The system is
highly flexible and is easily adapted to respond to a diverse range of primary
messenger/receptor interactions. A key feature of many signaling pathways, not
initially apparent in this model, is the ability to deliver receptor-derived informa-
tion to unique sub-sets of target phosphoproteins. For example, although many
different hormones and effectors use a common cyclic AMP-based pathway
to activate the cyclic AMP-dependent protein kinase (PKA), the consequences
of PKA activation can be very different. Specificity is built into the signaling
pathway through the activation of spatially distinct subsets of PKA molecules.
Spatial organization of PKA is therefore a major regulatory mechanism for
ensuring the selectivity and specificity of cyclic AMP-mediated responses.
