Figure 4.

An illustration of the effect upon the PPI network topology of an exon shuffling event(A) Protein X and its interaction partners. The self-interacting red domain mediates two PPIs: a self-interaction of protein X and an interaction between proteins X and A. (B) Protein Y and its interaction partners. Protein Y has just one PPI, namely, with protein B. (C) Exon shuffling event and evolution of a new protein. Hypothetical exon shuffling event promotes the evolution of a new protein Z, merging together domains of proteins X and Y. (D) Protein Z and its interaction partners. The new protein Z may be able to interact with several proteins just by reuse of the interaction properties that its domains already possessed in proteins X and Y. In particular, protein Z has in this case become a self-interacting protein. "d" stands for vertex degree.