In this particular signaling mechanism, the ligand binds to the extracellular domain of a transmembrane receptor protein
The receptor in turn stimulates a GTP-binding signal transducer protein (G protein
) located on the cytoplasmic face of the plasma membrane.
The G protein then activates an effector (E)
(an enzyme or ion channel) that is responsible for modulating the production of an intracellular second messenger
The family of G proteins contains a number of diverse subfamilies which mediate effects of receptors to effectors. Receptors coupled to G proteins belong to a family of proteins known as ‘Serpentine Receptors
’ or ‘7-Transmembrane Receptors
’; all serpentine receptors transduce signals across the plasma membrane in the same fashion.
A large number of extracellular ligands act by increasing the intracellular concentrations of second messengers
, such as cAMP, calcium ion, or the phosphoinositide
s, via this G protein-coupled signaling pathway.
For example, for cAMP, the effector enzyme is adenylyl cyclase
, a transmembrane protein that converts intracellular ATP to cAMP. The corresponding G protein, Gs
, stimulates adenylyl cyclase after being activated by ligands that act via a specific receptor; examples of these ligands include catecholamines
, parathyroid hormone
G proteins use a molecular mechanism that involves binding and hydrolysis of GTP to amplify
the transduced signal.
Amplification of the original signal (which is the result of binding of the ligand to its membrane receptor) is attributed to the fact that the active GTP-bound G protein remains in its active state for a relatively long time
(tens of seconds
). The duration of activation of adenylyl cyclase, for example, depends on the duration of activation of the G protein (not on the receptor’s affinity for binding to the ligand or the duration of that binding).