<prompt> Action of a a subunit of G protein is ?Break down of GTP to GDPConversion of GDP to GTPInternalization of receptorsBinding of agonists  <response>: a subunit of G protein has intrinsic GTPase activity to breakdown GTP to GDP terminating the effector activation. G protein can be either stimulatory Gs or inhibitory Gi. Stimulatory or innhibitory activity resides in the a subunit. Cholera toxin and peussis toxin acts on a subunit by ADP ribosylation inhibiting its GTPase activity - cAMP - pathological effects. Cholera toxin acts on a subunit of Gs. Peussis toxin acts on a a subunit of Gi. REF: Ganong;s Medical physiology 26th edition Pgno: 58.62 