Exploring the
inner space of cells by cryoelectron-tomography
Wolfgang Baumeister
Max-Planck-Inst. of Biochemistry, Am Klopferspitz 18,
82152
Electron tomography is uniquely suited to obtain three-dimensional images
of large pleiomorphic structures, such as supramolecular assemblies, organelles or even whole cells.
With the advent of automated data acquisition, facilitated by technological
advances (computer-controlled electron microscopes and large area CCD cameras),
it has become possible to examine frozen-hydrated samples in a close-to-life
state under non-critical electron dose conditions and to attain resolutions
which allow the docking of high resolution component structures.
High-resolution tomograms of organelles or cells are essentially 3-D images of
the cell's entire proteome and should ultimately enable us to map the spatial
relationships of macromolecules in a functional cellular context. However, it
is no trivial task to retrieve this information because of the poor
signal-to-noise ratio of such tomograms and the crowded nature of the cytoplasm
and many organelles. Denoising procedures can help to
combat noise and to facilitate visualization, but advanced pattern recognition
methods are needed for detecting and identifying with high fidelity specific
macromolecules based on their structural signature (size and shape).
Experiments with phantom cells, i.e. lipid vesicles encapsulating a known set
of proteins have shown that such a template-matching approach is feasible. Once
the challenges of obtaining sufficiently good resolution and of creating
efficient data-mining algorithms are met, and comprehensive libraries of
template structures become available, we will be able to map the supramolecular landscape of cells systematically and
thereby provide a new perspective for analyzing the molecular interaction
networks underlying higher cellular functions.