A method for counting and localizing protein complexes in the pathogen Leptospira interrogans is reported in a paper published online this week in Nature Methods.
Methods to map the locations and compositions of cellular protein complexes could lead to new insights about how they function in the context of the whole biological system. Quantitative mass spectrometry approaches can identify which proteins are present at a given time and in what abundances, but mass spectrometry is not a visualization technique. Cryo-electron tomography (cryoET), in contrast, is suited for visually mapping the location of sub-cellular structures. In a process called template matching, protein complexes in a cell can be localized by comparing their cryoET signals to those of the reference structure. However, turning the technique into a general method has met substantial limitations because the template matching approach is highly subject to making false positive matches.
Ruedi Aebersold and colleagues have now advanced the visualization of protein complexes in a cell by combining the two methods. First the researchers used mass spectrometry to select and quantify suitable protein complexes for template matching, and then used cryoET to map the locations of these complexes. To avoid false positives, the researchers developed a statistical method to score the template matches and distinguish true from false positives. This allowed them to map the location of nine different protein complexes in L. interrogans with different abundance levels and molecular weights.
Though the approach is currently limited to very thin organisms such as L. interrogans, and to detecting only relatively abundant protein complexes, further advances in cryoET technology should broaden the applicability of this 'visual proteomics' approach.