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Cell Signalling Biology Michael J. Berridge Module 2 Cell Signalling Pathways 2 69

of iNOS may account for the fall in blood pressure during a group of single membrane-spanning receptors that use
endotoxic shock. an enzyme to transduce information. In this case, it is the
guanylyl cyclase region of the cytoplasmic domain that
Neuronal nitric oxide synthase (nNOS) functions both as a transducer and an amplifier to generate
Neuronal nitric oxide synthase (nNOS) was first described the second messenger cyclic GMP.
in neurons, but has since been found in other cell types such
as skeletal muscle, which has an alternatively spliced vari- Cyclic GMP hydrolysis
ant that has a 34-amino-acid insert between exons 16 and The enzyme that reverses the second messenger action of
17. In both neurons and muscle, nNOS is closely associated cyclic GMP by hydrolysing it to GMP is the cyclic GMP-
with the plasma membrane where it binds to various pro- specific phosphodiesterase (PDE5), which can be activated
teins through its PDZ domains. In neurons, nNOS binds by cGKI, thus setting up a negative-feedback loop. Excess-
to the postsynaptic density (PSD) proteins such as PSD- ive signalling through the cyclic GMP signalling pathway
93 and PSD-95 (Module 10: Figure postsynaptic density), will thus be curtailed through this ability of cyclic GMP
whereas in skeletal muscle it interacts with α1-syntrophin to enhance its own hydrolysis. PDE5 is of considerable
(a binding partner of dystrophin). interest as it is the target of Viagra.

Cyclic GMP-dependent protein kinase (cGK)
Cyclic GMP signalling pathway
There are two cyclic GMP-dependent protein kinases
The cyclic GMP signalling pathway (Module 2: Figure NO
(cGKs): cGKI and cGKII. The cGKI comes in two al-
and cGMP signalling) is governed by the second messen-
ternatively spliced forms, cGKIα and cGKIβ. These cGKs
ger cyclic GMP, which is synthesized by guanylyl cyclase
are serine/threonine protein kinases that exist as homodi-
(GC). The latter comes in two different forms: there are
mers that are held together by leucine zippers in their N-
the soluble GC (sGC) and membrane-bound particulate
guanylyl cyclases (pGCs). Many of the signalling func- terminal domains, which fold over to inhibit the catalytic
tions of cyclic GMP are carried out by cyclic GMP-de- domain. The binding of cyclic GMP to a regulatory site
pendent protein kinase (cGK). In addition, cyclic GMP induces a conformational change that relieves this inhibi-
can act directly to open cyclic nucleotide-gated channels. tion, thus enabling the catalytic domain to phosphorylate
Cyclic GMP hydrolysis is carried out by a cyclic GMP- its substrates.
specific phosphodiesterase (PDE5). Through these differ- cGK targets
ent signalling pathways, cyclic GMP functions to regulate These cGKs are targeted to specific sites in the cell. The
a diverse collection of cellular processes. There are those N-terminal domain is responsible for targeting cGKIα and
where cyclic GMP mediates the action of NO: cGKIβ to specific cellular regions as is particularly import-
ant for smooth muscle relaxation mediated by nitric oxide
• NO/cyclic GMP and smooth muscle relaxation (NO) and cyclic GMP (Module 7: Figure smooth muscle
• NO/cyclic GMP and synaptic plasticity cell cyclic GMP signalling).
• NO/cyclic GMP and cardiac hypertrophy
The cGKII has an N-terminal Gly-2 myristic acid
In those cases where cyclic GMP is formed by the pGC, residue that serves to target it to the plasma membrane.
cyclic GMP acts independently of NO to regulate cellular
cGK protein substrates
processes such as phototransduction (Module 10: Figure The cGMP-sensitive target proteins regulated by cGKs
phototransduction overview). Certain strains of Escheri- include inositol 1,4,5-trisphosphate receptor-associated
chia coli, which secrete the STa toxin, increase intestinal se- cGKI substrate (IRAG), large conductance (BK) chan-
cretion and cause diarrhoea by activating the cyclic GMP nels, cGMP-specific phosphodiesterase (PDE5), cerebellar
signalling pathway by stimulating the particulate guanylyl G substrate, vesicle-associated membrane protein (VASP)
cyclase C (pGC-C) receptor that is normally activated by and telokin.
guanylin (Module 7: Figure intestinal secretion).
Inositol 1,4,5-trisphosphate receptor-associated cGKI
NO/cyclic GMP and synaptic plasticity substrate (IRAG)
The enzymes responsible for NO formation and its action Inositol 1,4,5-trisphosphate receptor (InsP 3 R)-associated
are richly expressed in the nervous system. Although the cGKI substrate (IRAG) is located on the endoplasmic
precise function of NO is still debated, there are indica- reticulum (ER), where it appears to associate with the
tions that it might function in synaptic plasticity by con- InsP 3 R. When it is phosphorylated by cGKIβ,IRAG
tributing to cerebellar cell long-term potentiation (LTP) at acts to inhibit channel opening (Module 7: Figure smooth
the parallel fibre/Purkinje cell synapse. muscle cell cyclic GMP signalling).

Guanylyl cyclase (GC) Reactive nitrogen species (RNS) signalling
Guanylyl cyclase (GC) is the enzyme that synthesizes cyc- One of the ways by which NO functions in cells is through
lic GMP from ATP (Module 2: Figure NO and cyclic an S-nitrosylation reaction (Module 2: Figure NO and
GMP signalling). It comes in two main forms, soluble GC cGMP signalling). This covalent modification results from
(sGC), which is activated by NO, and membrane-bound the addition of NO to reactive cysteine residues on spe-
particulate guanylyl cyclases (pGCs). The latter belongs to cific target proteins. NO does not react directly with these

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