Coupling electron cryomicroscopy and X-ray crystallography to understand secondary active transport

In the past few years we have seen an amazing increase in the number of high-resolution structures for secondary transporters determined by X-ray crystallography, while 3D data obtained by electron cryomicroscopy (cryo-EM) from two-dimensional (2D) crystals are only available at medium resolutions of about 6–10 Å. Despite their superior resolution, it turned out that the description of a molecular mechanism of secondary transport could not solely rely on high-resolution X-ray structures and have to be supplemented with biochemical and spectroscopic data. Moreover, the comparison of X-ray structures and 3D EM maps has proved to be an important tool for validating native conformations of several membrane proteins, especially when functional data contradicted predictions based on a crystal structure. In addition, 3D EM maps are better suited to investigate transporter activation because of the lipidic environment.