Figure 2

Frequencies of iPfam domain pairs in E. coli, S. cerevisiae and H. sapiens protein interactions. Each point in this graph represents a set of protein interactions. The abscissa reflects the number of interactions in each set that contain the same i Pfam domain pair. The ordinate shows the number of distinct such sets, each defined by a different i Pfam domain pair. In both H. sapiens (blue) and S. cerevisiae (green) a small number of i Pfam domain pairs covers a large fraction of the interactome, whereas in E. coli, no i Pfam domain occurs in more than 4 experimental interactions at a time. Dotted lines denote fitted monomial functions, showing that the distributions follow a power law.