Integrating signals through complex assembly
Figure 1: We employ a number of different resolution techniques to visualise cellular structures
Figure 2: Atomic model of the INF-b enhanceosome
The Panne group looks to understand important signalling processing pathways in the cell, which could help in the discovery of anti-viral drugs.
Previous and current research
Most cellular processes depend on the action of large multi-subunit complexes, many of which are assembled transiently and change shape and composition during their functional cycle. The modular nature of the components, as well as their combinatorial assembly, can generate a large repertoire of regulatory complexes and signalling circuits. Characterisation and visualisation of such cellular structures is one of the most important challenges in molecular biology. Characterisation requires expertise in a number of techniques including molecular biology, biochemistry, biophysics, structural biology and bioinformatics. Visualisation uses low-resolution imaging techniques such as electron microscopy and small angle X-ray scattering , or high-resolution techniques such as NMR and macromolecular X-ray crystallography (figure 1).
The systems we have been studying are involved in transcriptional regulation. This is mediated by transcription factors which bind to their cognate sites on DNA, and through their interaction with the general transcriptional machinery, and/or through modification of chromatin structure, activate or repress the expression of a nearby gene. The so-called ‘cis-regulatory code’, the array of transcription factor binding sites, is thought to allow read-out and signal processing of cellular signal transduction cascades. Transcriptional networks are central regulatory systems within cells and in establishing and maintaining specific patterns of gene expression. One of the best-characterised systems is the interferon-β promoter. Three different virus-inducible signalling pathways are integrated on the 60 base pair enhancer through coassembly of eight ‘generic’ transcription factors to form the so-called ‘enhanceosome’, which is thought to act as a logic AND gate. The signal transducing properties are thought to reside in the cooperative nature of enhanceosome complex assembly.
To understand the signal transducing properties of the enhanceosome, we have determined co-crystal structures that give a complete view of the assembled enhanceosome structure on DNA (figure 2). The structure shows that association of the eight proteins on DNA creates a continuous surface for the recognition of the enhancer sequence. For the first time, we have detailed insights into the structure of an enhanceosome, and the design and architecture of such higher-order signalling assemblies.
Future projects and goals
We are particularly interested in understanding the signal processing through higher order assemblies. The enhanceosome has served as a paradigm for understanding signal integration on higher eukaryotic enhancers. The interferon (IFN) system is an extremely powerful anti-viral response and central to innate immunity in humans. Most serious viral human pathogens have evolved tools and tricks to inhibit the IFN response. Many viruses do so by producing proteins that interfere with different parts of the IFN system. Therefore, our studies are of fundamental interest in understanding important signal processing pathways in the cell and may also point to better methods of controlling virus infections; for example, novel anti-viral drugs might be developed which prevent viruses from circumventing the IFN response. Misregulation of IFN signalling pathways is also involved in inflammation and cancer and is therefore of fundamental importance for human health. We will expand our multiprotein crystallisation strategies to complexes involved in modification of chromatin structure.