Properties of PPINs: small world effect

Protein-protein interaction networks show a small world effect meaning that there is great connectivity between proteins (Figure 17). In other words, it can be said that the network's diameter (the maximum number of steps separating any two nodes) is small, no matter how big the network is. This usually means that any two nodes are separated by less than six steps, more or less, reflecting the now widely popularised “six degrees of separation” theory used in social sciences.

The small world effect.

Figure 17 The small world effect.

This level of connectivity has important biological consequences, since it allows for an efficient and quick flow of signals within the network. However, it also poses an interesting question: if the network is so tightly connected, why don't perturbations in a single gene or protein have dramatic consequences for the network?

Biological systems are extremely robust and can cope with a relatively high amount of perturbations in single genes/proteins. In order to explain how can this happen, we need to have a look at another fundamental property of PPINs: they are scale-free networks.