Essential Cell Biology 5th edition

General Principles of Cell Signaling
541
plasma membrane
CYTOSOL
receptor protein
scaffold
small intracellular
messenger molecules
extracellular signal molecule
INTEGRATE
RELAY
PRIMARY
TRANSDUCTION
TRANSDUCE AND
AMPLIFY
FEEDBACK
Figure 16–9 Intracellular signaling
proteins can relay, amplify, integrate,
distribute, and modulate via feedback
an incoming signal. In this example, a
receptor protein located on the cell surface
transduces an extracellular signal into an
intracellular signal, which initiates one or
more intracellular signaling pathways that
relay the signal into the cell interior. Each
pathway includes intracellular signaling
proteins that can function in one of the
various ways shown; some, for example,
integrate signals from other intracellular
signaling pathways. Many of the steps in
the process can be modulated via feedback
by other molecules or events in the cell.
Note that some proteins in the pathway
may be held in close proximity by a scaffold
protein, which allows them to be activated
at a specific location in the cell and with
greater speed, efficiency, and selectivity
(discussed in Chapter 4; see Figure 4−52).
We review the production and function of
small intracellular messenger molecules,
more commonly called second messenger
molecules, later in the chapter.
DISTRIBUTE
ALTERED
METABOLISM
ALTERED CELL
SHAPE OR
MOVEMENT
ALTERED
GENE
EXPRESSION
Some Intracellular Signaling Proteins Act as Molecular
Switches
Many intracellular signaling proteins behave as molecular switches:
receipt of a signal causes them to toggle from an inactive to an active
state. Once activated, these proteins can stimulate—or in some cases
suppress—other proteins in the signaling pathway. They then persist in
an active state until some other ECB5 process e16.13-16.09 switches them off again.
The importance of the switching-off process is often underappreciated:
imagine the consequences if a signaling pathway that boosts your heart
rate were to remain active indefinitely. If a signaling pathway is to recover
after transmitting a signal and make itself ready to transmit another,
every activated protein in the pathway must be reset to its original,
(A)
signaling
pathway 1
T
Y
positive feedback
(B)
signaling
pathway 2
+ –
T
Y
negative feedback
Figure 16–10 Feedback regulation within an intracellular signaling pathway can adjust the response to an extracellular signal.
(A) In this simple example, a downstream protein in a signaling pathway, protein Y, acts to increase the activity of the protein that
activated it—a form of positive feedback. Positive feedback loops can ignite an explosive response, such as the activation of the
proteins that trigger cell division (discussed in Chapter 18). (B) In a simple example of negative feedback, protein Y inhibits the
protein that activated it. Negative feedback loops can generate oscillations, similar to the way that populations of predators and prey
can seesaw: an increase in prey (here, protein T) would promote the expansion of predators (protein Y); as the number of predators
increases, the availability of prey will fall (via negative feedback), which will ultimately cause the predator population to decline. As the
predators disappear, the prey populations will recover and multiply, providing food for more predators, and so on.
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