Essential Cell Biology 5th edition

General Principles of Cell Signaling
543
Cell-Surface Receptors Fall into Three Main Classes
All cell-surface receptor proteins bind to an extracellular signal molecule
and transduce its message into one or more intracellular signaling molecules
that alter the cell’s behavior. Most of these receptors belong to
one of three large classes, which differ in the transduction mechanism
they use.
1. Ion-channel-coupled receptors change the permeability of the plasma
membrane to selected ions, thereby altering the membrane potential
and, if the conditions are right, producing an electrical current (Figure
16–13A).
2. G-protein-coupled receptors activate membrane-bound, trimeric
GTP-binding proteins (G proteins), which then activate (or inhibit)
an enzyme or an ion channel in the plasma membrane, initiating an
intracellular signaling cascade (Figure 16–13B).
3. Enzyme-coupled receptors either act as enzymes or associate with
enzymes inside the cell (Figure 16–13C); when stimulated, the enzymes
can activate a wide variety of intracellular signaling pathways.
The number of different types of receptors in each of these three classes
is even greater than the number of extracellular signals that act on them.
This is because for many extracellular signal molecules there is more
than one type of receptor, and these may belong to different receptor
classes. The neurotransmitter acetylcholine, for example, acts on skeletal
muscle cells via an ion-channel-coupled receptor, whereas in heart cells
it acts through a G-protein-coupled receptor. These two types of receptors
generate different intracellular signals and thus enable the two types
(A)
(B)
ION-CHANNEL-COUPLED RECEPTORS
closed channel
open channel
G-PROTEIN-COUPLED RECEPTORS
ions
CYTOSOL
signal molecule
signal molecule
plasma
membrane
Figure 16–13 Cell-surface receptors fall
into one of three main classes. (A) An
ion-channel-coupled receptor opens in
response to binding an extracellular signal
molecule. These channels are also called
transmitter-gated ion channels. (B) When
a G-protein-coupled receptor binds its
extracellular signal molecule, the activated
receptor signals to a trimeric G protein on
the cytosolic side of the plasma membrane,
which then turns on (or off) an enzyme (or
an ion channel; not shown) in the same
membrane. (C) When an enzyme-coupled
receptor binds its extracellular signal
molecule, an enzyme activity is switched on
at the other end of the receptor, inside the
cell. Many enzyme-coupled receptors have
their own enzyme activity (left), while others
rely on an enzyme that becomes associated
with the activated receptor (right).
CYTOSOL
inactive
receptor
inactive
G protein
inactive
enzyme
activated
receptor binds
to G protein
activated
enzyme
activated G protein
(C)
ENZYME-COUPLED RECEPTORS
signal molecule
in form of a dimer
signal molecule
CYTOSOL
OR
inactive receptor
inactive catalytic
domains
active catalytic
domains
active receptor
activated
associated
enzyme