Protein-protein interaction (PPI) networks are complex entities generally containing thousands of interactions acquired through a variety of experimental and computational techniques. An interesting issue in biological networks is the connection  between topological properties of proteins and their biological relevance. I will discuss the link between centrality properties such as degree and betweenness  and the evolutionary conservation of proteins across different species. I will present results obtained when centrality properties are computed for a restricted subset of proteins corresponding  to relevant functional modules in human and yeast interactomes.