Essential Cell Biology 5th edition

Essential Concepts
569
signal A
plasma
membrane
kinase 1
signal B
signal C
kinase 2
P
P
target protein
CELL RESPONSE
signal D
EXTRACELLULAR
SPACE
CYTOSOL
Figure 16–43 Intracellular signaling
proteins serve to integrate incoming
signals. Extracellular signals A, B, C, and D
activate different receptors in the plasma
membrane. The receptors act upon two
protein kinases, which they either activate
(black arrow) or inhibit (red crossbar). The
kinases phosphorylate the same target
protein and, when it is fully phosphorylated,
this target protein triggers a cell response.
It can be seen that signal molecule B
activates both protein kinases and therefore
produces a strong output response. Signals
A and D each activate a different kinase and
therefore produce a response only if they
are simultaneously present. Signal molecule
C inhibits the cell response and will
compete with the other signal molecules.
The net outcome will depend both on the
numbers of signaling molecules and the
strengths of their connections. In a real cell,
these parameters would be determined by
evolution.
network of signaling pathways, it will remain a major challenge to figure
out exactly how they all work together to allow cells—and organisms—to
ECB5 e16.43/16.43
integrate the diverse array of information that inundates them constantly
and to respond in a way that enhances their ability to adapt and survive.
ESSENTIAL CONCEPTS
• Cells in multicellular organisms communicate through a huge variety
of extracellular chemical signals.
• In animals, hormones are carried in the blood to distant target cells,
but most other extracellular signal molecules act over only a short
distance. Neighboring cells often communicate through direct cell–
cell contact.
• For an extracellular signal molecule to influence a target cell it must
interact with a receptor protein on or in the target cell. Each receptor
protein recognizes a particular signal molecule.
• Most extracellular signal molecules bind to cell-surface receptor
proteins that convert (transduce) the extracellular signal into different
intracellular signals, which are usually organized into signaling
pathways.
• There are three main classes of cell-surface receptors: (1) ionchannel-coupled
receptors, (2) G-protein-coupled receptors (GPCRs),
and (3) enzyme-coupled receptors.
• GPCRs and enzyme-coupled receptors respond to extracellular signals
by activating one or more intracellular signaling pathways,
which, in turn, activate effector proteins that alter the behavior of
the cell.
• Turning off signaling pathways is as important as turning them on.
Each activated component in a signaling pathway must be subsequently
inactivated or removed for the pathway to function again.
• GPCRs activate trimeric GTP-binding proteins called G proteins; these
act as molecular switches, transmitting the signal onward for a short
period before switching themselves off by hydrolyzing their bound
GTP to GDP.
• G proteins directly regulate ion channels or enzymes in the plasma
membrane. Some directly activate (or inactivate) the enzyme adenylyl