Two-dimensional crystallization by dialysis for structural studies of membrane proteins by the cryo-EM method electron crystallography

Atomic-Scale Corrugations in Crystalline Polypeptoid Nanosheets Revealed by Three-Dimensional Cryogenic Electron Microscopy

Amphiphilic molecules that can crystallize often form molecularly thin nanosheets in aqueous solutions. The possibility of atomic-scale corrugations in these structures has not yet been recognized. We have studied the self-assembly of amphiphilic polypeptoids, a family of bio-inspired polymers that can self-assemble into various crystalline nanostructures. Atomic-scale structure of the crystals in these systems has been inferred using both X-ray diffraction and electron microscopy. Here we use cryogenic electron microscopy to determine the in-plane and out-of-plane structures of a crystalline nanosheet. Data were collected as a function of tilt angle and analyzed using a hybrid single-particle crystallographic approach. The analysis reveals that adjacent rows of peptoid chains, which are separated by 4.5 Å in the plane of the nanosheet, are offset by 6 Å in the direction perpendicular to the plane of the nanosheet. These atomic-scale corrugations lead to a doubling of the unit cell dimension from 4.5 to 9 Å. Our work provides an alternative interpretation for the observed Å X-ray diffraction peak often reported in polypeptoid crystals.

2D Electron Crystallography of Membrane Protein Single-, Double-, and Multi-Layered Ordered Arrays

The electron cryo-microscopy (cryo-EM) approach of 2D electron crystallography allows for structure determination of two-dimensional (2D) crystals of soluble and membrane proteins, employing identical principles and methods once 2D crystals are obtained. Two-dimensional crystallization trials of membrane proteins can result in multiple outcomes of ordered arrays, which may be suited for either 2D electron crystallography, helical analysis, or MicroED.The membrane protein 2D crystals used for 2D electron crystallography are either single- or double-layered ordered proteoliposome vesicles or sheet-like membranes. We have developed a cryo-EM grid preparation approach, which allows for the analysis of stacked 2D crystals that are neither suitable for MicroED nor for directly applying 2D electron crystallography. This new grid preparation approach, the peel-blot, uses the capillary force generated by submicron filter paper and mechanical means for the separation of stacked 2D crystals into single-layered 2D crystals, for which standard 2D electron crystallography can then be employed. The preparation of 2D crystals, the peel-blot grid preparation, and the structure determination by 2D electron crystallography are described here.

Inducing two-dimensional crystallization of membrane proteins by dialysis for electron crystallography

Electron crystallography is an electron cryo-microscopy (cryo-EM) method that is particularly suitable for structure-function studies of small membrane proteins, which are crystallized in two-dimensional (2D) arrays for subsequent cryo-EM data collection and image processing. This approach allows for structural analysis of membrane proteins in a close-to-native, phospholipid bilayer environment. The process of growing 2D crystals from purified membrane proteins by dialysis detergent removal is described in this chapter. A short section covers screening for and identifying 2D crystals by transmission electron microscopy, and in the last section, optimization of the purification to obtain crystals of higher quality is discussed.

Cellular distribution of copper to superoxide dismutase involves scaffolding by membranes

Efficient delivery of copper ions to specific intracellular targets requires copper chaperones that acquire metal cargo through unknown mechanisms. Here we demonstrate that the human and yeast copper chaperones (CCS) for superoxide dismutase 1 (SOD1), long thought to exclusively reside in the cytosol and mitochondrial intermembrane space, can engage negatively charged bilayers through a positively charged lipid-binding interface. The significance of this membrane-binding interface is established through SOD1 activity and genetic complementation studies in Saccharomyces cerevisiae, showing that recruitment of CCS to the membrane is required for activation of SOD1. Moreover, we show that a CCS:SOD1 complex binds to bilayers in vitro and that CCS can interact with human high affinity copper transporter 1. Shifting current paradigms, we propose that CCS-dependent copper acquisition and distribution largely occur at membrane interfaces and that this emerging role of the bilayer may reflect a general mechanistic aspect of cellular transition metal ion acquisition.