Helical Membrane Protein Crystallization in the New Era of Electron Cryo-Microscopy

Helical assemblies of proteins, which consist of a two-dimensional lattice of identical subunits arranged with helical symmetry, are a common structural motif in nature. For membrane proteins, crystallization protocols can induce helical arrangements and take advantage of the symmetry found in these assemblies for the structural determination of target proteins. Modern advances in the field of electron cryo-microscopy (cryo-EM), in particular the advent of direct electron detectors, have opened the potential for structure determination of membrane proteins in such assemblies at high resolution. The nature of the symmetry in helical crystals of membrane proteins means that a single image potentially contains enough information for three-dimensional structural determination. With the current direct electron detectors, we have never been closer to making this a reality. Here, we present a protocol detailing the preparation of helical crystals, with an emphasis on further cryo-EM analysis and structural determination of the sarco(endo)plasmic reticulum Ca²⁺-ATPase in the presence of regulatory subunits such as phospholamban.