Essential Cell Biology 5th edition

540 CHAPTER 16 Cell Signaling
Figure 16–8 Many extracellular signals
activate intracellular signaling pathways
to change the behavior of the target cell.
A cell-surface receptor protein activates one
or more intracellular signaling pathways,
each mediated by a series of intracellular
signaling molecules, which can be proteins
or small messenger molecules; only one
pathway is shown. Signaling molecules
eventually interact with specific effector
proteins, altering them to change the
behavior of the cell in various ways.
plasma
membrane
EXTRACELLULAR SIGNAL MOLECULE
RECEPTOR PROTEIN
INTRACELLULAR SIGNALING MOLECULES
EFFECTOR PROTEINS
metabolic
enzyme
cytoskeletal
protein
transcription
regulator
ALTERED
METABOLISM
ALTERED CELL
SHAPE OR
MOVEMENT
ALTERED
GENE
EXPRESSION
TARGET-CELL RESPONSES
QUESTION 16–2
In principle, how might an
intracellular signaling protein amplify
a signal as it relays it onward?
The components of these intracellular signaling pathways perform one
or more crucial functions (Figure 16–9):
1. They can relay the signal onward and thereby help spread it through
the cell.
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2. They can amplify the signal received, making it stronger, so that a few
extracellular signal molecules are enough to evoke a large intracellular
response.
3. They can detect signals from more than one intracellular signaling
pathway and integrate them before relaying a signal onward.
4. They can distribute the signal to more than one effector protein,
creating branches in the information flow diagram and evoking a
complex response.
5. They can modulate the response to the signal by regulating the activity
of components upstream in the signaling pathway, a process known
as feedback.
Feedback regulation, although it is last on our list, is actually a very
important feature of cell signaling. It can occur anywhere in the signaling
pathway and can either boost or weaken the response to the signal. In
positive feedback, a component that lies downstream in the pathway acts
on an earlier component in the same pathway to enhance the response to
the initial signal; in negative feedback, a downstream component acts to
inhibit an earlier component in the pathway to diminish the response to
the initial signal (Figure 16–10). Such feedback regulation is very common
in biological systems and can lead to sophisticated responses: positive
feedback can generate all-or-none, switchlike responses, for example,
whereas negative feedback can generate responses that oscillate on and
off as the activities or concentrations of the inhibitory components rise
and fall.