Each of the following statements concerning cholera toxin is correct EXCEPT:

  •   Cholera toxin acts by adding ADP-ribose to a G protein.
  •  Cholera toxin inhibits elongation factor-2 in the mucosal epithelium.
  •  Cholera toxin activates the enzyme adenylate cyclase in the enterocyte.
  •  Binding of cholera toxin to the mucosal epithelium occurs via interaction of the B subunit of the toxin with a ganglioside in the cell membrane.