trans

CYCLIC NUCLEOTIDES 259
also occur independently of the PKA pathway
as occurs, for example, with the cAMP-gated
cation channels in olfactory neurons. AC
activity is closely coordinated with that of
phosphodiesterases, which breakdown cAMP
by hydrolysis of the 3-ester bond.
Guanylyl cyclases (GCs) exist as two main
forms that are expressed in almost all types of
mammalian cells. The receptor forms have a
single membrane-spanning helix and an extracellular
domain that is able to bind directly
to the corresponding activator (Figure 11.4A).
The activators can include peptide hormones
or bacterial toxins, depending on the isoform.
Binding of the activator to the cyclase leads to
activation of the single intracellular catalytic
domain that functions as a homodimer. The
soluble, cytosolic form of GC exists as heterodimers
of the and subunits, and is activated
by nitric oxide in the presence of heme.
cGMP signaling has a central role in numerous
diverse physiological processes, including
photoreceptor signal transduction, neurotransmission,
and electrolyte homeostasis. In
sea urchins, a membrane-bound GC found
on the surface of sperm can act as a receptor
for peptides released by the egg and regulate
sperm chemotaxis. cGMP binds to and activates
cGMP-dependent protein kinases, which
then phosphorylate and regulate the activity
of a number of specific proteins. In other circumstances
cGMP can act directly, as in the
case of the cGMP-gated ion channels in retinal
cone cells. The intracellular level of cGMP is
tightly regulated by the interplay between the
activities of GC and cGMP-specific phosphodiesterases.
The catalytic domains of GC and AC have
several shared features that reflect their common
evolutionary origin. A small number of
residues in the active site, located within
hydrophobic pockets, are involved in conferring
purine specificity; Lys and Asp in AC and Glu,
Arg and Cys in the case of GC. The purine
specificity of GC can be altered by changing
these key residues (Glu and Cys) to their
counterparts (Lys and Asp) in AC, resulting
in an enzyme with AC activity. In homodimeric
cyclases the polypeptide chains form two
symmetrical active sites. However, with heterodimeric
cyclases, such as mammalian AC,
one of the adenosine binding sites interacts
with allosteric activators such as the diterpene
forskolin, and presumably an endogenous
structurally related molecule.
Trypanosomatid adenylyl cyclases
In Leishmania, T. brucei and T. cruzi, proteins
with AC activity have been predicted to
conform to receptor-type cyclases, and have
structures more typical of the mammalian
membrane-bound GCs (Figure 11.4B). The
trypanosomatid ACs have a large extracellular
domain, a single transmembrane helix and a
cytosolic catalytic domain. In T.brucei at least,
ACs appear to be confined to the flagellum.
No GC genes have been identified in the
trypanosomatids.
The ACs are expressed by gene families that
vary in complexity among trypanosomatid parasites.
In the simplest case, L. donovani, the AC
genes are organized as a cluster of five. Two of
these (designated Rac-A and B) are developmentally
regulated and are expressed only in
the promastigote (insect) stage of the life cycle.
The Rac-A protein functions as an AC, but
Rac-B exhibits no cyclase activity. Interestingly,
the activity of Rac-A was downregulated when
co-expressed with Rac-B in Xenopus oocytes.
The functional relevance of this observation
remains to be determined. Preliminary data
from the T. brucei Genome Project suggest that
several hundred AC-like genes may be present,
and that they fall into two main categories.
Genes belonging to one group, designated
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA