  We employed the random graph theory approach to analyze the protein-protein
interaction database DIP (Feb. 2004), for seven species (S. cerevisiae, H.
pylori, E. coli, C. elegans, H. sapiens, M. musculus and D. melanogaster).
Several global topological parameters (such as node connectivity, average
diameter, node connectivity correlation) were used to characterize these
protein-protein interaction networks (PINs). The logarithm of the connectivity
distribution vs. the logarithm of connectivity study indicated that PINs follow
a power law (P(k) ~ k-\gamma) behavior. Using the regression analysis method we
determined that \gamma lies between 1.5 and 2.4, for the seven species.
Correlation analysis provides good evidence supporting the fact that the seven
PINs form a scale-free network. The average diameters of the networks and their
randomized version are found to have large difference. We also demonstrated
that the interaction networks are quite robust when subject to random
perturbation. Average node connectivity correlation study supports the earlier
results that nodes of low connectivity are correlated, whereas nodes of high
connectivity are not directly linked. These results provided some evidence
suggesting such correlation relations might be a general feature of the PINs
across different species.
