Figure 23.12 depicts the subunit structure of G proteins
and their interactions, as affected by GTP
and GDP. Several different
forms of each G protein are known. In most forms, the 
subunit is prenylated (covalently bound to a 20 carbon
isoprenoid moiety at the C-terminal cysteine). The 
subunit is
covalently linked to myristic acid
and is the subunit that contains both the guanine nucleotide-binding
site and the GTPase activity.
Binding of GTP by the subunit causes
it to dissociate from the other subunits and bind to adenylate
cyclase. When GTP is hydrolyzed in the subunit, however,
it dissociates from adenylate cyclase and reassociates with the
other subunits.
Cholera and pertussis toxins catalyze the covalent
addition of ADP-ribose to specific sites in the subunits of
Gs
and Gi,
respectively. This modification inhibits GTPase action in the
subunits and converts them to irreversible activators
of adenylate cyclase. As a result, cAMP accumulates. In the intestine,
the response to this is an uncontrollable secretion of water and
sodium--causing severe diarrhea and dehydration.
At least four different subunits, five
subunits, and 6 subunits are known, allowing
for a large number of G proteins to be made from these
combinations. Table 23.4 summarizes
some of the properties of different G proteins.
The subunits of G proteins
are part of a family of small GTP-binding proteins that are active
when GTP is bound and inactive in the presence of GDP. This includes
the Ras oncogene proteins (see here)
and the GTP-binding elongation factors in protein synthesis (see
here).
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