Graphical Analysis and Visualization Tools for Protein Interaction Networks

Introduction

In recent years, a predominant theme in biological research has been a shift from the reductionist approach to an integrative systems approach (Kitano et al., 2002; Oltvai & Barabási, 2002; Han, 2008), which views the components of a cell as acting together in a network of reactions and interactions. This change has been aspired to for many years, as it is widely understood that observed cell behaviour is rarely attributed to one component acting alone. The continued growth in the processing power of computers and the increase in availability of biological data from high throughput technologies have allowed such an approach to become realistic.

This chapter describes software that uses algorithms taken from the field of graph theory to draw biological conclusions about protein-protein interaction (PPI) networks. It contains background on the subject, a method for the representation of networks, including bio-molecular networks, as a set of nodes connected by edges. In PPI networks, the nodes represent proteins, while the edges represent the interactions between them. Very often, PPI networks aim to represent the whole proteome of a species. As an example, Figure 1 shows a holistic PPI network for the budding yeast, Saccharomyces cerevisiae (Schwikowski, Uetz & Fields, 2000). Holistic PPI networks are usually constructed from two-hybrid screening (Fields & Song, 1989) and other experimental data, much of which is available in public databases (see Background).

Figure 1.

An S. cerevisiae holistic PPI network, containing 1548 proteins and 2358 interactions, from a study carried out by Schwikowski, Uetz and Fields, (2000). (Permission to reproduce figure kindly provided by Peter Uetz, Institute of Genetics, University of Karlsruhe.)

Two-hybrid technology is particularly powerful as it identifies large numbers of putative interactions. To test if two proteins interact, the DNA-binding domain from a transcription factor is spliced onto the end of the gene for one, and the activating domain from the transcription factor is spliced onto the other. When the genes are transformed into a cell (usually in S. cerevisiae or E. coli) and expressed, two hybrid proteins are produced. The protein with the DNA-binding domain binds to an upstream activating sequence (UAS) of a reporter gene and the protein with the activating domain binds the remaining transcriptional machinery. If the two proteins bind each other then the transcriptional machinery will be brought into close proximity with the UAS, and the reporter gene will be expressed. Whole libraries of genes can be transformed into a cell suspension, to create a population with a very diverse set of combinations of hybrid genes. If a reporter gene is used that is essential for survival under particular conditions, then those conditions can be used to select only those cells that contain a pair of interacting hybrid proteins, and the genes from these can then be sequenced to identify them. The artificial context in which the interactions take place can result in both false positive and false negative results. For example, animal proteins may fold incorrectly in a yeast cell, or proteins that can interact but would never come into contact in vivo (because they are found in different cellular compartments or at different times) are reported as interacting.

It is often desirable to visualise PPI networks in order to see their structure and therefore provide further insight into the system. Visualizations of large networks are often complicated and dense, making it impossible to discern structure by eye. However, the use of analysis techniques can help elucidate structure and therefore function. The results of such analyses may be interesting in their own right, or may lead to further visualization steps on selected subsets or simplifications of the network. For these reasons, the software for visualising networks usually contains various network analysis functions.