Question

You are interested in cell size regulation and discover that signaling through a GPCR is important in controlling cell size in rat white blood cells. The G protein downstream of this receptor activates adenylyl cyclase, which ultimately leads to the activation of PKA. You find that cells that lack the GPCR are 15% smaller than normal cells, whereas cells that express a mutant, constitutively activated version of PKA are 15% larger than normal cells. Furthermore, the normal blood cells become smaller when treated with cholera toxin, which has been shown to inhibit certain subclasses of the a subunit of the G protein. Given these results, explain what you predict would happen to the cell’s size (bigger, smaller, or no change) if cells were treated in the following fashion.

A) You add pertussis toxin.

B) You add a drug that increases the activity of cyclic AMP phosphodiesterase.

C) You add a drug that inhibits adenylyl cyclase.

D) You mutate the cAMP-binding sites in the regulatory subunits of PKA, so that the PKA binds more tightly to cAMP.

Answer 1

A) You add pertussis toxin

Cell size increases: Pertusis toxin is specifically increase intracellular accumulation of cAMP         by ADP-ribosylation procees of uncoupling of the α subunit of heterotrimeric G_i/oproteins. This is result in inhibiting the communication between recptor and intracellular effector molecules such as adenyl cyclase.

B) You add a drug that increases the activity of cyclic AMP phosphodiesterase

A drug that increases the activity of cyclic AMP phosphodiesterase is going to reduce the cAMP intracellular levels finally leading to low level of protein kinase activity result in “smaller WBC cell”

C) You add a drug that inhibits adenylyl cyclase

Irrespective of activation of GPCR or not, if you add adenyl cyclase is not leading to rise in intracellular cAMP levels finally result in “smaller WBCell”. In case if the cAMP levels are not raised with respect to GPCRs stimulation then it is leading to low PKA activity   

D) You mutate the cAMP-binding sites in the regulatory subunits of PKA, so that the PKA binds more tightly to cAMP.

Mutated version fo cAMP binding to the PKA is leading to act “to induce more tighter affinity” for cAMP finally the regulatory subunit is going to dissociate from PKA in response to G-protein cpoup[led receptor activation result in more cAMP in WBc with “larger size”