The G protein coupled receptor mechanism represents the most versatile transmembrane signaling mechanisms.
To see why, take a look at the overview of G protein signaling in the movie to the right .
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(Suggestion: step through the movie, a single step at a time, by repeatedly using the single arrow to right button; review the uninterupted movie using the double arrow to right button)
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Versatility is achieved because there are a number of different G proteins (~ 20 have currently been described) activating a number of 2nd messenger-producing effector enzymes.
The best known of the effector enzymes are adenylyl cyclase and phospholipase C.
These enzymes produce 2nd messenger molecules e.g. cyclic AMP, inositol triphosphate and diacyl glycerol (the ligand which activtes the receptor, i.e. either the homone or neurotransmitter, is considered the first messenger.
The second messengers in turn can activate different kinases (e.g. Protein kinase A, protein kinase C).
The kinases regulate the activity of metabolic enzymes and, in doing so, regulate cell function.
The response of a cell to a message molecule (e.g. a hormone) depends on which G proteins, which effectors and which metabolic enzymes the cell expresses....thus the system is highly versatile in giving a wide range of cellular responses.
Studies on excitatory cells i.e. cells that can depolarize such as neurons and endocrine cells, has revealed an even greater degree of versatility in the operation of this signaling mechanism.
First, it was discovered that the kinases could phosphorylate ion channels on the membrane and, in doing so, effect membrane excitability.
Then it was discovered that some second messengers, namely the cyclic nucleotides cyclic GMP and cyclic AMP, can bind directly to ion channels and activate them.
Such channels are called "cyclic nucleotide-gated channels".
This mode of signaling, originally described for the sensory nervous system (signal transduction in the retina and in the olfactory system) is now known to also be important in neurons in the brain.
Finally, it has been discovered that some G proteins can directly interact with ion channels to regulate their function.
In other words, not only kinases but also ion channels can be effector proteins for the G proteins!
Add to this the fact that a transcription factor (called CREB) is also a target for kinases means that G protein coupled receptors are involved in regulating virtually every aspect of cell function, from cell membrane ion channel activity to metabolic pathways in the cell and gene expression in the nucleus.
G protein signaling is indeed a highly versatile signaling system.