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

PRINCIPLES OF CELL SIGNALING
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receptor
protein
signal
molecule
plasma
membrane
intracellular
signaling protein
inhibitory
protein
endosome
lysosome
RECEPTOR
SEQUESTRATION
RECEPTOR
DOWN-REGULATION
RECEPTOR
INACTIVATION
INACTIVATION OF
SIGNALING PROTEIN
PRODUCTION OF
INHIBITORY PROTEIN
Figure 15–20 Some ways in which target cells can become adapted (desensitized) to an extracellular signal molecule.
The mechanisms shown here that operate at the level of the receptor often involve phosphorylation or ubiquitylation of the
receptor proteins.
the vast number of signals that impinge upon animal cells. The whole molecular
control network, leading from the receptors at the cell surface to the genes in
the nucleus, can be viewed as a computing device; and, like that other biological
computing device, the brain, it presents one of the
MBoC6
hardest
m15.29/15.20
problems in biology.
We can identify the components and discover how they work individually. We can
understand how small subsets of components work together as regulatory modules,
noise filters, or adaptation mechanisms, as we have seen. However, it is a
much more difficult task to understand how the system works as a whole. This
is not only because the system is complex; it is also because the way in which it
behaves is strongly dependent on the quantitative details of the molecular interactions,
and, for most animal cells, we have only rough qualitative information. A
major challenge for the future of signaling research is to develop more sophisticated
quantitative and computational methods for the analysis of signaling systems,
as described in Chapter 8.
Summary
Each cell in a multicellular animal is programmed to respond to a specific set of
extracellular signal molecules produced by other cells. The signal molecules act by
binding to a complementary set of receptor proteins expressed by the target cells.
Most extracellular signal molecules activate cell-surface receptor proteins, which
act as signal transducers, converting the extracellular signal into intracellular ones
that alter the behavior of the target cell. Activated receptors relay the signal into
the cell interior by activating intracellular signaling proteins. Some of these signaling
proteins transduce, amplify, or spread the signal as they relay it, while others
integrate signals from different signaling pathways. Some function as switches that
are transiently activated by phosphorylation or GTP binding. Large signaling complexes
form by means of modular interaction domains in the signaling proteins,
which allow the proteins to form functional signaling networks.
Target cells use various mechanisms, including feedback loops, to adjust the
ways in which they respond to extracellular signals. Positive feedback loops can
help cells to respond in an all-or-none fashion to a gradually increasing concentration
of an extracellular signal and to convert a short-lasting signal into a longlasting,
or even irreversible, response. Negative feedback allows cells to adapt to a
signal molecule, which enables them to respond to small changes in the concentration
of the signal molecule over a large concentration range.