Biology Reference
In-Depth Information
least one signal corresponding to the induction of each of these cell
types is required. However, cells must also be subject to a vast
number of signals corresponding to other cell functions such as mul-
tiplication and apoptosis, or physiological functions controlled by
hormones. We might therefore estimate that the minimum number
of specific signals necessary in a mammal is 1000, and there again,
this is still a considerable underestimate. Even if this were the case
however, if these 1000 signals each activate a cascade of four inter-
actions, that means that they must activate a total of 1000
×
2401
=
2.4
10
6
specific targets. Now we know that in one cell there are
about 10
4
genes corresponding to roughly the same number of pro-
teins. In terms of specific regulation, we once again come up
against impossibility. Far too many specific targets are necessary
in relation to the possibilities offered by one cell. Some signalling
pathways are bound to be used by several signals. The second
example of interconnected interaction cascades shows that this is
indeed the case.
There are indeed relatively few signalling pathways in a cell,
compared with the enormous number of signals that the cell can
receive and situations with which it is confronted. Because of the
multiplicity of molecular interactions, a single cascade of signals can
produce different effects. The same pathways are used by different
signals for transporting their information and achieving responses
appropriate for the cell. The yeast
Sacharomyces cerevisae
uses
three kinases, Fus3, Hog1 and Kss1 to respond to the sex
pheromone, to osmotic pressure and to induce filament growth. The
three pathways which activate these kinases share several portions
made of the same proteins and yet, depending on which signal it is
that activates them, only one of the three responses is produced
(Schwartz and Madhani, 2004).
However, this once again raises the question of the specificity of
the signal. The problem can be simply represented in a diagram.
Three signals A, B, C converge to use the same signalling pathway
in a non-specific manner, then diverge and induce three specific
responses A
×
, respectively (Fig. 9). Why does each signal
induce a unique response instead of the three responses possible?
′
, B
′
, C
′
