active
Adenylate Cholera
cyclase .GTp toxin

c
8 § 5
- —_
o o~
& E 7 o
2 ° o
.
c = oa
Q a
= o9°
~_
9 EF
<
inactive
GTP Adenylate GDP +P;
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).