Biology Reference
In-Depth Information
several organisms such as yeast,
Drosophila
and humans and all the
interactions which may occur in a cell have thus been mapped
(Bork
et al.,
2004). These large scale proteome studies are not yet
absolutely complete but the results they have already provided are
significant. Protein interaction networks are constructed around a
central hub where connection density is the strongest. This area is
composed of proteins which can bind to approximately a hundred
other partners and constitute about 10% of the network. Their
number is therefore in the order of 10
3
. The connectivity of other
proteins located on the periphery of the network is much lower, but,
overall, the average connectivity of proteins over the whole of the
network is between five and ten. These data suggest that all pro-
tein interaction pathways implicated in metabolism, signalling or
gene transcription are potentially interconnected, with a very great
number of contact points (Barabasi and Oltvai, 2004; Albert, 2005).
These wide-ranging studies confirm the results obtained from
those restricted to particular proteins, and demonstrate that an
immense number of combinations of potential molecular interac-
tions must exist in cells. Indeed, the play of multiple interactions
very rapidly provokes a 'combinatorial explosion'. In his topic
The
Music of Life,
Denis Noble estimated the number of potential inter-
actions between the 25000 genes of a mammalian genome and
reached similar conclusions (Noble, 2006). To give an idea, let us do
a very simple calculation and consider a cascade of 20 sequential
protein interactions. The first protein can interact with one of seven
other proteins (first interaction), each of these seven potential pro-
teins can in turn interact with one of seven others (second interac-
tion), each of the 49 proteins of the second interaction can in turn
interact with one of seven other proteins (third interaction), and so
on up to the 20th interaction. The number of possible combinations
for producing this cascade of 20 interactions is in the order of 10
17
.
In comparison, one must remember that in a mammal the number
of cell types is in the order of 10
2
and the total number of cells,
around 10
12
. One should also keep in mind here that this is only a
rough calculation leading to an underestimate. Obviously there are
more than 20 interactions in a cell!
