active

        Adenylate
        Adenylate       Cholera
                     Cholera
         cyciase
         cyciase   -GTP
                -GTP   toxin
                       toxin
6        aI
         aI
.-
i
E  p p
-x x
5   ro ro

.-
9
.-
5 x x
            inactive
            inactive
  GTP
  GTP      Adenylate
         Adenylate    GDP + Pi
                 GDP + Pi
           cyclase
           cyclase

Fig. 5:  1977 model of hormonal stimulation of an adenylyl cyclase system in
which the activating GTP is deactivated by an intimately associated cholera
toxin sensitive GTPase (From Cassel and Selinger, 1977). Hormone (receptor)
was later proposed to act by facilitating the release of GDP and entry of
GTP (Cassel and Selinger, 1978).