Question

The CFTR protein is a membrane Cl- transport protein and has a cytosolic regulatory domain that is phosphorylated by protein kinase A (PKA). PKA phosphorylation has been found to activate the Cl- channel activity of CFTR in lung cells. Secretion of chloride by CFTR is enhanced by binding of epinephrine to the β-adrenergic receptor that couples to G_s, and is inhibited by binding of serotonin to a G-protein coupled receptor (GPCR) that couples to G_i. However, epinephrine and serotonin added together leads to net activation of adenylyl cyclase. Determine how the following conditions would affect Cl- secretion by the CFTR protein when both epinephrine and serotonin are added together to the cells, and give a brief explanation. Assume that when mutant proteins are present, there is no wild-type copy (this is not an overexpression experiment) and that for the sake of this problem these signals are the only inputs for adenylyl cylcase and PKA.

A.        A mutation in the regulatory domains of PKA that prevents binding of cAMP.

B.        A mutation in the catalytic domain of PKA that prevents binding the regulatory domains.

C.        A mutation in the β-adrenergic receptor that disables binding of the receptor protein to G_s.

D.        A mutation of the CFTR regulatory domain within the PKA phosphorylation site that changes the PKA-phosphorylated serine residue to an alanine residue.

E.         Infection of lung epithelial cells by a bacterium that secretesa toxin which prevents the interaction of Gi with CFTR_i.

Answer 1