Smallest Structure of Biological Macromolecular Complex Revealed by Single Particle Image Analysis
Abstract: Single particle image analysis by electron cryomicroscopy (cryoEM) is a powerful tool for structural analysis of macromolecular complexes such as ribosomes and viruses. However, because of the extremely low signal/noise ratio and low contrast of cryoEM images, molecules smaller than a molecular weight of 100 kDa are generally difficult to visualize and analyze their structures. We solved the structure of a very small DNA nanostructure whose molecular weight is only 78 kDa by cryoEM and single particle image analysis. Here, we demonstrate that this technique allows us to deduce structural information of sufficient resolution to reveal the absolute 3D configuration of a designed DNA nanostructure. We successfully visualized the DNA helix of a self-assembled DNA tetrahedron, each edge of which consists only of a 7-nm, 20-basepair duplex. Structural analysis at such high resolution by cryoEM image analysis is unprecedented for any biological molecule of such a small size.
Key words: Single particle image analysis, DNA tetrahedron, Electron cryomicroscopy, nanotechnology