You decide to complete a summer undergraduate research project in a lab that studies G-Protein coupled receptor (GPCR) signaling. The laboratory you join has identified two new GPCRs, GPCR-A and GPCR-B. The same ligand binds each receptor, but two different heterotrimeric G-proteins are activated that regulate adenylyl cyclase. A. You find that activation of GPCR-A causes an increase in adenylyl cyclase activity, while activation of GPCR-B causes a decrease in adenylyl cyclase activity.
B. You have a cell line that expresses both GPCR-A, GPCR-B, the corresponding G proteins and adenylyl cyclase. There is a basal level of adenylyl cyclase activity that produces a baseline cAMP concentration. Your project is to characterize a series of mutations in these components. Will each mutation increase or decrease the intracellular levels of cAMP upon ligand addition (remember, both GPCR-A and GPCR-B bind the same ligand)?
a. A mutation in GPCR-A that prevents G-protein activation
b. A mutation in GPCR-B that prevents G protein activation
c. A mutation in Gsa that prevents release of bound GDP
d. A mutation in Gia that prevents GTP hydrolysis
EXPLAIN your thought process and why each one was answered as increase or decrease