Semi-automated icosahedral particle reconstruction at sub-nanometer resolution.

Electron cryomicroscopy of large macromolecular complexes is becoming an increasingly powerful tool for revealing three-dimensional structures without the need for crystallization. The execution of image processing, however, requires experience and is error-prone due to the need for a human operator to carry out interactive and repetitive processes. We have designed an approach which is intended to make image processing simple and rapid, both for experts and for novice users. We demonstrate this approach using the well-established reconstruction scheme for icosahedral particles. Finally, we implement semi-automated virus reconstruction (SAVR), an expert system that integrates the most CPU intensive and iterative steps using the scripting language Python. SAVR is portable across platforms and has been parallelized to run on both shared and distributed memory platforms. SAVR also allows the incorporation of new algorithms and facilitates the management of the increasingly large data sets needed to achieve higher resolution reconstructions. The package has been successfully applied to several data sets and shown capable of generating icosahedral reconstructions to sub-nanometer resolutions (7-10 A ).