Question

There are seven events when G-protein coupled receptor is activated. What are those seven steps.

Answer 1

Introduction to G Protein Coupled Receptors

Heterotrimeric G proteins (Gα, Gβ/Gγ subunits) constitute one of the most important components of cell signaling cascade. G Protein Coupled Receptors (GPCRs) perceive many extracellular signals and transduce them to heterotrimeric G proteins, which further transduce these signals intracellular to appropriate downstream effectors and thereby play an important role in various signaling pathways. GPCRs exist as a superfamily of integral membrane protein receptors that contain seven transmembrane α-helical regions, which bind to a wide range of ligands. Upon activation by a ligand, the GPCR undergoes a conformational change and then activate the G proteins by promoting the exchange of GDP/GTP associated with the Gα subunit. This leads to the dissociation of Gβ/Gγ dimer from Gα. Both these moieties then become free to act upon their downstream effectors and thereby initiate unique intracellular signaling responses. After the signal propagation, the GTP of Gα-GTP is hydrolyzed to GDP and Gα becomes inactive (Gα-GDP), which leads to its re-association with the Gβ/Gγ dimer to form the inactive heterotrimeric complex. The GPCR can also transduce the signal through G protein independent pathway. GPCRs also regulate cell cycle progression. Till to date thousands of GPCRs are known from animal kingdom with little homology among them, but only single GPCR has been identified in plant system. The Arabidopsis GPCR was reported to be cell cycle regulated and also involved in ABA and in stress signaling. Here I have described a general mechanism of signal transduction through GPCR/G proteins, structure of GPCRs, family of GPCRs and plant GPCR and its role.

A General Mechanism of Signal Transduction through GPCR and G Proteins

The regulatory cycle of G proteins i.e., activation/inactivation through GPCR is shown in Figure 1. In the inactive state, Gα is bound to Gβγ dimer and GDP. G protein mediated signaling starts by binding of an agonist molecule that leads to activation of GPCR. GPCR is also a guanine nucleotide exchange factor that promotes the exchange of guanosine disphosphate (GDP)/guanosine triphosphate (GTP) associated with the Gα subunit.12 Therefore, the activated GPCR catalyzes exchange of GTP for GDP on the Gα subunit, as a result conformational changes takes place in the GPCR, which leads to dissociation of Gβγ dimer from Gα and thus activates multiple molecules of G proteins (Fig. 1). The G proteins activated in this way constitute an amplified representation of the activated GPCR. Activated Gα and Gβγ proteins in turn binds to various effectors and thereby switches it either on or off in different systems, and effectors continue to pass the signal to different kinds of second messengers. Here intrinsic GTPase activity of Gα comes into play, that leads to conversion of bound GTP into GDP and hence the inactivation of G proteins cascade. GTPase activity of the Gα subunits may also be regulated by regulators of G proteins signaling (RGS proteins) as well as effectors. Moreover, effector enzymes such as adenylyl cyclases may also regulate the activation of G proteins by receptors (Fig. 1).

The activated Gα interacts and regulates many effector molecules such as calcium, potassium channels, adenylyl cyclase, phospholipase C (PLC), PLD and protein kinases. Initially it was hypothesized that the βγ dimer acts as negative regulator and can block activation of adenylyl cyclase by this mechanism.But subsequently this hypothesis was changed with the discovery that the βγ subunit could activate the muscarinic K+ channel and βγ subunits positively regulate effectors.Finally, the βγ subunit was shown to be a positive regulator of a large number of effectors in addition to the K+ channel, including adenylyl cyclase, phospholipase C-β (PLC-β), phospholipase A2 (PLA2), phosphoinositide 3-kinase (PI3-kinase), and β-adrenergic receptor kinase. Also, Gβγ can activate Gα subunit. It is now clear that many effectors are regulated both by α and βγ subunits.