Protein-Protein Interaction Networks
A protein-protein interaction (PPI) network is a network or graph representation of physical interactions between protein pairs in an organism. In the network, each node/vertex represents a protein and each edge/link represents an interaction. The network is undirected, usually simple (which means maximally only one edge is allowed between two proteins), and allows self-loops (which indicates interactions between the same protein). PPIs can be determined by small- or large-scale experiments such as the yeast-two-hybrid screens or tandem affinity purification experiments, computational predictions such as interolog mapping, multimeric threading or machine learning, and expert curations (Xia et al. 2004).