text: G proteins contain an alpha-subunit that is critical to the functioning of receptors. These subunits can take a number of forms. There are four broad classes of form of G-protein: G, G, G, and G. Muscarinic receptors vary in the G protein to which they are bound, with some correlation according to receptor type. G proteins are also classified according to their susceptibility to cholera toxin (CTX) and pertussis toxin (PTX, whooping cough). G and some subtypes of G (G and G) are susceptible to CTX. Only G is susceptible to PTX, with the exception of one subtype of G (G) which is immune. Also, only when bound with an agonist, those G proteins normally sensitive to PTX also become susceptible to CTX.
  The various G-protein subunits act differently upon secondary messengers, upregulating Phospholipases, downregulating cAMP, and so on.
  Because of the strong correlations to muscarinic receptor type, CTX and PTX are useful experimental tools in investigating these receptors.