Electron Microscopy in the Context of Structural Systems Biology

As modern molecular biology moves from single molecules toward more complex multimolecular machines, the need for structural information about these assemblies grows. Nuclear magnetic resonance (NMR) spectroscopy (see Chapter 5) and X-ray crystallography (see Chapter 4) are well-established approaches for obtaining atomic structures of biological macromolecules, but it has become increasingly clear that the structures of individual components of assemblies can only be a first step to understanding a biological phenomenon. Based on the analysis of genome sequences, it was suggested that life depends on about 200–300 core biological processes (Martin andDrubin, 2003). Each of these processes involvesmultiple proteins, often organized into large heterogeneous assemblies,with awide range of morphologies and complexity. The ‘‘parts list’’ that has emerged from the genome project and from structural genomics is far from a ‘‘wiring diagram’’ that we need to understand these processes. Detailed knowledge of the parts of a system usually provides only limited insight into the dynamics and function of the system as a whole. A complete understanding requires not only knowledge of the transient and steady-state structures at near-atomic details; it also requires knowledge of structural pathways and ligand interactions in a cellular context. Due to dramatic improvements in experimentalmethods and computational techniques, electron microscopy has matured into a powerful and diverse collection of methods that allow the visualization of the structure and the dynamics of an extraordinary range of biological assemblies at resolution spanning frommolecular (about 2–3 nm) to near atomic