Chemistry Reference
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did not change, suggesting that immobilization depended on the phosphorylation of
individual receptor molecules, but that k252a did not change the global membrane
structure. Phosphorylation of TrkA induces activation of Ras, which in turn induces
translocation of Raf1 from the cytoplasm to the plasma membrane. GFP-Raf1 and
Cy3.5-NGF were simultaneously observed in single molecules. Colocalization of
NGF and Raf1 suggests the formation of signaling complexes including Shc, Grb2,
and Sos. Measurement of the diffusion movement has revealed that colocalization
takes place only during the immobile periods.
Thus, single-molecule tracking of NGF/NGFR dynamics has revealed that the
complex repeats random states of diffusion and immobilization and that the
signaling complexes are formed during the immobilization period. NGF signaling
does not occur continually but occasionally in discrete time periods and positions
(Figure 5.5B). Although the importance of this phenomenon is not fully understood,
this type of discrete signaling seems to be more ef
cient under conditions of high
background noise. If a protein continually emits small signals, all of the signals will
be hidden under the high background noise. However, signals emitted in larger
packets can be distinguished from the high background noise even if the level of the
individual signals is not increased.
5.6
Stochastic Signal Processing and Transduction in Living Cells
Intracellular signal transduction depends on stochastic processes such as associa-
tion/dissociation, enzymatic catalysis, chemical modi
cation, conformational
changes and diffusion of signaling molecules, and thus intracellular signals are
inevitably accompanied by randomnoise. How stochastic signaling systems in living
cells operate reliably to receive, process and transduce signals under the strong
in
uence of thermal and stochastic
fluctuations is an open question. Chemotactic
signaling systems in eukaryotic cells are an ideal model system for elucidating
mechanisms of stochastic signal processing and transduction in living cells.
Chemotaxis is a directional motile response in living cells, in which cells move
in a preferential direction in response to a chemical gradient. Chemotactic cells
are extremely sensitive to chemical gradients. In eukaryotic cells, a difference of
only a few percent in the concentration of the chemoattractant across cells is
suf
cient to trigger chemotactic movements in a wide range of background con-
centrations [15
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17]. Because ligand binding to speci
c receptors is a stochastic
process, receptor occupancy should
fluctuate with time and space, and thereby signal
inputs for chemotaxis should become noisy. The noise resulting from the measure-
ment of chemical concentrations has been studied theoretically, and has revealed
physical limits to a cells sensing ability [18
21]. Theoretical estimations of the signal
inputs suggest that cells receive a faint signal under the strong in
uence of stochastic
noise generated during ligand-binding reactions. Thus, how cells obtain reliable
information regarding the gradient is a critical question for directional sensing in
chemotaxis.
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