Studying Membrane Protein Structures by MicroED.

Microcrystal electron diffraction (MicroED) enables atomic resolution structures to be determined from vanishingly small crystals. Soluble proteins typically grow crystals that are tens to hundreds of microns in size for X-ray crystallography. But membrane protein crystals often grow crystals that are too small for X-ray diffraction and yet too large for MicroED. These crystals are often formed in thick, viscous media that challenge traditional cryoEM grid preparation. Here, we describe two approaches for preparing membrane protein crystals for MicroED data collection: application of a crystal slurry directly to EM grids, and focused ion beam milling in a Scanning Electron Microscope (FIB-SEM). We summarize the case of preparing an ion channel, NaK, and the workflow of focused ion-beam milling. By milling away the excess media and crystalline material, crystals of any size may be prepared for MicroED. Finally, an energy filter may be used to help minimize inelastic scattering leading to lower noise on recorded images.