title: Evidence for the additions of clustered interacting nodes during the evolution of protein interaction networks from network motifs.
abstract: BACKGROUND
  High-throughput screens have revealed large-scale protein interaction networks defining most cellular functions. How the proteins were added to the protein interaction network during its growth is a basic and important issue. Network motifs represent the simplest building blocks of cellular machines and are of biological significance.
  
  
  RESULTS
  Here we study the evolution of protein interaction networks from the perspective of network motifs. We find that in current protein interaction networks, proteins of the same age class tend to form motifs and such co-origins of motif constituents are affected by their topologies and biological functions. Further, we find that the proteins within motifs whose constituents are of the same age class tend to be densely interconnected, co-evolve and share the same biological functions, and these motifs tend to be within protein complexes.
  
  
  CONCLUSIONS
  Our findings provide novel evidence for the hypothesis of the additions of clustered interacting nodes and point out network motifs, especially the motifs with the dense topology and specific function may play important roles during this process. Our results suggest functional constraints may be the underlying driving force for such additions of clustered interacting nodes.
authors: State Key Laboratory of Proteomics, Beijing Proteome Research Center, Beijing Institute of Radiation Medicine, Beijing, China.
authors: Zhongyang
authors: Z
authors: Liu
authors: Qijun
authors: Q
authors: Liu
authors: Hanchang
authors: H
authors: Sun
authors: Lin
authors: L
authors: Hou
authors: Hao
authors: H
authors: Guo
authors: Yunping
authors: Y
authors: Zhu
authors: Dong
authors: D
authors: Li
authors: Fuchu
authors: F
authors: He
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