Courtesy of Sidec Technologies
Researchers looking for a three-dimensional view of interesting molecules typically turn to high-resolution structural determination methods such as nuclear magnetic resonance (NMR) or X-ray crystallography. But not all applications require the level of resolution obtainable through these methods. Stockholm-based Sidec Technologies has developed a novel imaging method for such applications; the company's SET technology combines a proprietary optimization algorithm (COMET) with low-dose cryoelectron tomography to reconstruct images of individual molecules in solution, cells, or tissues.
Malin Engström, marketing director, uses an aquatic analogy to describe how the technology works: "It is like taking an aquarium of fish and flash-freezing it, and then you can zoom in and study all the individual fish that are in the aquarium." In Sidec's case, the fish are macromolecules such as proteins, and each of the proteins in a client's sample can be reconstructed three-dimensionally at a resolution of two nanometers.
SET employs electron tomography, in which the sample is first frozen in a thin layer onto a copper grid and subjected to electron microscopy at a series of +/- 60° angles. The resulting data are processed and refined to reconstruct a 3-D image of the sample. "It's an iterative refinement process to find the optimal solution to the structure that fits the original microscope data," explains Engström. The refinement procedure allows Sidec to use low-dose electron tomography, which preserves the sample and yet offers sufficient resolution to examine individual molecules, Engström adds.
Engström notes that the technique is not intended to replace higher-resolution structural methods. Rather, it complements them, offering researchers a view of individual proteins at the molecular level, rather than an average structure of thousands of macromolecules at the atomic level. "We can see domains, subunit assembly of molecular complexes, larger conformational changes in the molecules, and how the surface areas or domains of two proteins interact," says Engström. "SET can visualize intact proteins in their biological surroundings--for example, proteins that are located in the cell membrane," says Engström.
Stockholm-based Biovitrum has collaborated with Sidec in the analysis of hormone-receptor interactions for lead optimization in drug discovery. SET, says Martin Norin, head of structural chemistry for Biovitrum, "has great potential" for drug-discovery applications in which proteins need to be screened for conformational changes.