A protein complex is a set of proteins appearing at the same time and in the same location with certain structure and certain function. Studies on protein complexes can enhance our under- standing of biological processes and contribute to the prediction of protein function (Li et al., 2008). Considering that many essential
proteins are involved in forming protein complexes, the in-degree introduced by Radicchi et al. (2004) is used to describe the importance of a protein from the perspective of protein complex. The in-degree (degreein(u,C)) of a protein u in a protein complex C is defined as the number of interactions between u and other proteins in C. This definition is described in Function (3).