Figure 2.

Electron tomographic reconstruction of frozen-hydrated specimens. A and B are two 12-nm slices extracted from a reconstruction of the archaebacterium, Pyrodictium abyssi (Baumeister et al. 1999). The distance between the slices is 90 nm. The boundary of the cell is evident, including both the plasma membrane and the “S-layer,” which is composed of ordered protein subunits. There are also crystalline structures in the cytoplasm (square arrays in A and B). Vesicles within the cell are evident. The dark circle in each image is a 250-nm latex sphere, which is apparently being internalized by endocytosis. Reprinted with permission from the author and Trends in Cell Biology. C displays a slice from a tomographic reconstruction of isolated, frozen-hydrated thermosomes, which are chaperonins with eightfold rotational symmetry. The proteins are arranged in a top view orientation. Image processing methods have been used to search for sites of correlations between the tomogram shown in C and a template generated from high resolution cryo-EM micrographs of the thermosome. Positions of strong correlation are marked by white crosses in D. This method for feature recognition holds promise as a way to identify the location of specific proteins in tomographic images of cytoplasm. Reprinted with permission of the authors and the Proceedings of the National Academy of Sciences. Bar, (B) 200 nm.