Combining experimental data for structure determination of flexible multimeric macromolecules by molecular replacement

A major effort has been made by the structural biology community to develop user-friendly software for the use of biologists. However, structural projects become more and more challenging and their solution often relies on a combination of information from various sources. Here, it is described how X-ray data, normal-mode analysis (NMA) and electron-microscopy (EM) data can be successfully combined in order to obtain a molecular-replacement (MR) solution for crystal structures containing multimeric molecules. NMA is used to simulate computationally the inherent internal flexibility of the monomer and thus enhance, together with the crystal noncrystallographic symmetry (NCS), the MR capabilities. NCS is also used to obtain a reliable EM reconstruction, which is then employed as a filter to construct oligomers starting from monomers. The feasibility of the direct use of EM reconstructions as a template for MR when the X-­ray and EM data resolutions overlap is also discussed.