Question

Explain the difference between amplification of signal by a tyrosine receptor kinase and a GPCR. Explain what would happen if you increased expression of a phosphatase or phosphodiesterase in each situation respectively.

Answer 1

Answer GPCR (G-protein coupled receptor) forms the largest class of coll surface receptor. The amplification of the signal is mainly the result of the activation of several other Components of the signalling cascade.. . The GPCR is coupled i to a s kimeric hi G-protein that has B subunits tab and r and aest the a- subunit acts as a GoP ase switch proteine . when a ligand bind to GPCR, there is a conformational change in the receptor which activates its CEF actively (Guanine nucleotide exchange faelor activity, that ireplaces the bound GDP to the subunit of ben proteen with a GIP and leadena to the activation of the G-protein t The process of amplification starts from here as the activated G-proteina delaches from the B-Y component and phosphor activates several o copies of the enzyme Adenylate Cyclase which. is involved in the production of huge amount of cAMP molecules from Ar e Amp molecules acts a 2nd messenger in the signalling pathway. Thes a single ligand binding at GPCR activates se & Adenylate Cyclea Cyclase which produces several CAMP molecule causes a huge amplification (several fold) on the signal in GPCR. This amplification is further transferred to the effector the mainly that are Kinases e AMP and the amplifica activated by continues up till this end of the pathway R Stivates several

Signal (lizand) GIPER 1 L Adenglate esclase Amplification Amplifications [] O O - CAMP CZ\ / Ó O OOO0000000 – Kinase Amplification use I dimenzation of i recep sigual amplification ů GPCR- RTK (Receptor Tyrosine kinase.) I binding to the extracellular The ligand binding to the e. domain of RTK, causes dimen zal tor, that brings a catalytie mesmai cuto solie leinase domain on each re on each receptor a close to each other. The catalytie domain first activate themselves by Auto phosphorylation and then phos each other on several Tyrosine residueste te by cross-phosphorylation. These phospho- rolated sites serves as the docking sile for several other signalling protions The amplification mainly takes place because the bending of a single ligand leads to the activation of several signalling pathways which includes - RAS - MAPK pathway @ IP3-DAG pathway 3 PI - 3 / Akt pathway ryo

eaks e hence diereases In the G. PCR signalling? Phosphalare would cancel out the phosphate added by the kinase activated on signalling from the target protein and hence the effector response would not take place. Phosphodiesterase enzyme, has a role e that in antagonistic to that of e Adenylate egelase. Phosphodiesterase down camp to ATP and hence die the celes the amount of choup inside This would lead to the stop of signallera pathway as cAMP is the secondari secondary messenger in the GPCR signalling pathway In Rik signalling The phosphalase is an enzyme that removes phosphate froin a target proten. of the phosphatase is increased then o added on the added the RT on phosphate the RTK catalytic. o and croes - phosphorylation domain by auto and croes - phoen Loremoved easily by the phosphatase and hence signal transduction further would not proceed removed would be hodiesterase breaks down e bimp PAMP and hence it would heheam of and cAMP and he affeet the further downs activated by RTK to here e timp is used as see Rik used here e AMP and secondary messenger. as

Apart from the effect of increased phosphatase in GPCR signaling mentioned in the above pictures,another process called GPCR desensitization,which is a response against a prolonged stimulus by an agonist would be affected because in desensitization the cytosolic domain of GPCR is phosphorylated by a kinase called GRK(GPCR associated kinase) on SER/THR residues and then the binding of a protein called ARRESTIN on these phosphorylated part which detaches the receptor from binding the G - protein and hence stops signaling. But increased phosphatase would remove the phosphorylation of GRK and arrestin would not be able to bind leading to prolonged stimulation of GPCR leading to adverse effects.