Electron Microscopy Reconstruction of Helical Assemblies

Abstract

The cryo-electron microscopy reconstruction technique has been developed rapidly in recent years and becoming an important approach for structural studies of macromolecular complexes,and it includes three different methods,cryo-electron tomography,electron crystallography,and single particle analysis(SPA).Among them,the SPA technique is now becoming a powerful tool to determine near-atomic resolution structures of macromolecular complexes.However,for those biological macromolecular assemblies with helical symmetry,such as tobacco mosaic virus(TMV),microtubules,microfilament,human immunodeficiency virus 1 capsid protein and etc.,it is not easy to obtain their three dimensional structures by using SPA because there are lots of math to deal with.In this paper,the authors described the helical reconstruction technique in details,including the basic mathematical description of helical assemblies,helical diffraction and its indexing,the relationship between Fourier transform of helical points array and that of real helical assemblies,and how to determine the helical parameters.Based on the above introduction,the authors reviewed two main helical reconstruction algorithms,the Fourier-Bessel reconstruction and the iterative helical real space reconstruction(IHRSR).At the end,they selected the helical assembly of Par-3 NTD(Par3 is a kind of protein related to cell polarity control and Par-3 NTD is its N-terminal domain) as an example to show a detailed helical reconstruction protocol using IHRSR algorithm.