Bacterial protein interaction networks: connectivity is ruled by gene conservation, essentiality and function

Protein-protein interaction (PPI) networks are the backbone of all processes in living cells. In this work we study how conservation, essentiality and functional repertoire of a gene relate to the connectivity k of the corresponding protein in the PPI networks. Focusing on a set of 42 mostly distantly related bacterial species, we investigate three issues: i) whether the distribution of connectivity values changes between PPI subnetworks of essential and nonessential genes; ii) how gene conservation, measured both by the evolutionary retention index (ERI) and by evolutionary pressure (as represented by the ratio Ka/Ks) is related to the the connectivity of the corresponding protein; iii) how PPI connectivities are modulated by evolutionary and functionaly relationships, as represented by the Clusters of Orthologous Proteins (COGs). We show that conservation, essentiality and functional specialization of genes control in a universal way the topology of the emerging bacterial PPI networks. Noteworthy, a structural transition in the network is observed such that, for connectivities k > 40, bacterial PPI networks are mostly populated by genes that are conserved, essential and which, in most cases, belong to the COG cluster J, related to ribosomal functions and to the processing of genetic information.