Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

846 Chapter 15: Cell Signaling
to Na + , so that when they close, the normal influx of Ca 2+ is inhibited, causing
the Ca 2+ concentration in the cytosol to fall. The decrease in Ca 2+ concentration
stimulates guanylyl cyclase to replenish the cyclic GMP, rapidly returning its level
to where it was before the light was switched on. A specific Ca 2+ -sensitive protein
mediates the activation of guanylyl cyclase in response to the fall in Ca 2+ levels. In
contrast to calmodulin, this protein is inactive when Ca 2+ is bound to it and active
when it is Ca 2+ -free. It therefore stimulates the cyclase when Ca 2+ levels fall following
a light response.
Negative feedback mechanisms do more than just return the rod to its resting
state after a transient light flash; they also help the rod to adapt, stepping down
the response when the rod is exposed to light continuously. Adaptation, as we
discussed earlier, allows the receptor cell to function as a sensitive detector of
changes in stimulus intensity over an enormously wide range of baseline levels
of stimulation. It is why we can see faint stars in a dark sky, or a camera flash in
bright sunlight.
The various trimeric G proteins we have discussed in this chapter fall into four
major families, as summarized in Table 15–3.
Nitric Oxide Is a Gaseous Signaling Mediator That Passes
Between Cells
Signaling molecules like cyclic nucleotides and calcium are hydrophilic small
molecules that generally act within the cell where they are produced. Some signaling
molecules, however, are hydrophobic enough, small enough, or both, to
pass readily across the plasma membrane and carry signals to nearby cells. An
important and remarkable example is the gas nitric oxide (NO), which acts as a
signal molecule in many tissues of both animals and plants.
In mammals, one of NO’s many functions is to relax smooth muscle in the walls
of blood vessels. The neurotransmitter acetylcholine stimulates NO synthesis by
Table 15–3 Four Major Families of Trimeric G Proteins*
Family
Some family
members
Subunits that
mediate action
Some functions
I G s α Activates adenylyl cyclase;
activates Ca 2+ channels
G olf α Activates adenylyl cyclase in
olfactory sensory neurons
II G i α Inhibits adenylyl cyclase
βγ
Activates K + channels
G o βγ Activates K + channels; inactivates
Ca 2+ channels
α and βγ
Activates phospholipase C-β
G t (transducin) α Activates cyclic GMP
phosphodiesterase in vertebrate
rod photoreceptors
III G q α Activates phospholipase C-β
IV G 12 / 13 α Activates Rho family monomeric
GTPases (via Rho-GEF) to regulate
the actin cytoskeleton
*Families are determined by amino acid sequence relatedness of the α subunits. Only selected
examples are included. About 20 α subunits and at least 6 β subunits and 11 γ subunits have
been described in humans.