Question

D. Please state whether you agree or disagree with the following, and provide your reasoning:

Treating cells with cholera toxin (which activates Gs proteins) or with pertussis toxin (which inhibits Gi proteins) leads to similar relative changes in cAMP levels in the cell.

Circle: AGREE or DISAGREE Reasoning:

Answer 1

I agree with the above statement.

Pertusis toxin released from B. pertussis is an inactive form. When this toxin bind to a cell membrane receptor, it is taken up in an endosome, after which it undergoes retrograde transport to the trans-Golgi network and endoplasmic reticulum. During this process, the A subunit becomes activated, in the presence glutathione and ATP. Pertusis toxin catalyzes the ADP-ribosylation of the ?_i subunits of the heterotrimeric G protein. This process inhibits G proteins from interacting with GPCR on the cell membrane, thus interfering with intracellular communication. The Gi subunits remain locked in their GDP-bound, inactive state, thus unable to inhibit adenylate cyclase activity, leading to increased cellular concentrations of cAMP, which affects normal biological signaling.

When cholera toxin is released from the bacteria, it binds to the intestinal cells known as enterocytes through the interaction of the pentameric B subunit of the toxin with the GM1 ganglioside receptor on the intestinal cell, triggering endocytosis of the toxin. A/B cholera toxin undergo cleavage of the A1 domain from the A2 domain in order for A1 to become an active enzyme.  On entering the enterocyte, the enzymatic A1 fragment of the toxin A subunit enters the cytosol, where it activates the G protein G_sa through an ADP-ribosylation reaction that acts to lock the G protein in its GTP-bound form, thereby continually stimulating adenylate cyclase to produce cAMP.  The high cAMP levels activate the CFTR causing a dramatic efflux of ions and water from intestinal enterocytes, leading to watery diarrhoea.