The genome is the same in every cell in an organism. Cell differentiation has no effect on this basic genetic template. However, the proteome - the ensemble of proteins found in a cell - differs greatly, according to cell type. The protein content of a cell is constantly changing in response to factors such as environmental conditions, cell cycle, or other stimuli. In many instances the role played by a protein is determined by its cellular location. Consequently, in addition to determining the composition of the proteome at key time points in cellular processes, it is essential that we visualize the spatial location and distribution of individual proteins in the cell. The emerging technique of biological soft x-ray tomography is an excellent method for obtaining this information. The relatively high energy of x-rays gives rise to images with higher spatial resolution than is possible using light based imaging techniques, and with greater penetration depth than is achievable with electron based microscopies. Moreover, by using x-rays in the 'water window' region of the spectrum biological materials are imaged with high contrast. Taken together, this means we can use soft x-ray microscopy to image whole cells, and visualize the internal sub-cellular architecture. Currently, we have used this technique to image yeast, and other cells, at a spatial resolution of 40 nm.

Using soft x-ray microscopy we can image an entire cell, in its natural hydration state. Determining the location of specific protein-protein interactions requires that we label these particular proteins with readily detectable markers. Consequently, we also have a research program for the development of novel methods for labeling proteins, both for x-ray and optical microscopy. As with any technique, soft x-ray microscopy is only one piece of the puzzle. We are also working on developing new optical microscopy methods, and the computational methods to correlate the images produced by these techniques with images from the x-ray microscope.