Determination of protein structures in situ: Electron tomography of intact viruses and cells

Electron tomography is a powerful tool for investigation of the three-dimensional structures of large sub-cellular assemblies at resolutions one to two orders of magnitude higher than what is currently achieved using light microscopy. This field has seen a significant burst of activity in the last few years with the availability of tools for automated data acquisition using modern computerized microscopes. Efforts at imaging complex assemblies at room temperature using stained specimens, as well as cryogenic temperatures using unstained specimens have rapidly begun to provide many new insights into the 3D architecture of cells and viruses. In our laboratory, we are using three-dimensional electron microscopy as a tool to discover and analyze the spatial architectures of receptors and membrane assemblies in viruses, cells and tissues. The electron microscopic studies bridge the gap between cellular and molecular structure, and provide a foundation to integrate the structural information with biochemical and genetic analyses. Here, I focus on challenges in using electron tomography to extract structural information on proteins and protein complexes by molecular averaging (text is based largely on material presented in Subramaniam 2006 [1] and Zhang et al 2006 [2]).