Question

A cell contains a mutation that prevent its' Gs proteins from being able to hydrolyze GTP. What is another likely way in which the cell could turn OFF the Gs signaling pathway?

a) Remove all G-proteins from the cells

b) Phosphorylate the beta-gamma complex

c) Degrade cGMP

d) Activate a Gi protein

Answer 1

Ans: 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. 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. These free subunits are now competent to interact with the downstream enzymes or channels to drive second messenger generation or changes in membrane potential that modulate cell physiology. 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. Now given in the question that because of the mutation the Gs protein is not able to hydrolyze GTP to GDP and not able to turn off GPCR. So as we know that the subunits of Gα coupled with ATP and Gβ/Gγ dimer interacts with several downstream enzymes to drive secondary messangers for example the Gβ/Gγ dimer activates  phosphoinositide 3-kinase (PI3K), phospholipase Cβ (PLCβ) and  GPCR kinase 2 (GRK2). So if we phosphorylate the beta-gamma complex then this complex is not able to activate such enzymes and the signal is not transduced through the cell but phosphorylating the beta-gamma complex will result only in the blocking of the enzymes linked to the beta-gamma complex and not those enzymes which are linked with Gα coupled with ATP subunit.

The Gα coupled with ATP subunit basically activates adenylyl cyclase enzyme which catalyzes the conversion of ATP into cyclic AMP and cyclic AMP further activates protein kinase A which will phosphorylates further proteins so this is the whole pathway. Now Gαi or Gi protein basically inactivates the adenylyl cyclase so that it is not able to catalyze the conversion of ATP into cAMP and hence protein kinase A will not be activated so Gi protein is reversing the mecahnism caused by the Gα coupled with ATP subunit so in order turn off the pathway initiated by the mutated Gα coupled with ATP subunit we have to activate a Gi protein because this protein will inhibit the enzyme catalyzed by the muatated protein.