In situ protein micro-crystal fabrication by cryo-FIB for electron diffraction

MicroED (micro electron diffraction) is an emerging technique to use cryo-electron microscope to study the crystal structure of macromolecule from its micro/nano-crystals, which are not suitable for conventional X-ray crystallography. However, this technique has been prevented for its wide application by the limited availability of producing good micro-/nano-crystals and the inappropriate transfer of crystals. Here, we developed a complete workflow to prepare suitable crystals efficiently for MicroED experiment. This workflow includes in situ on-grid crystallization, single-side blotting, cryo-focus ion beam (cryo-FIB) fabrication, and cryo-electron diffraction of crystal cryo-lamella. This workflow enables us to apply MicroED to study many small macromolecular crystals with the size of 2 ~ 10 m, which is too large for MicroED but quite small for conventional X-ray crystallography. We have applied this method to solve 2.5[A] crystal structure of lysozyme from its micro-crystal within the size of 10x10x10 m3. Our work will greatly expand the availability space of crystals suitable for MicroED and fill up the gap between MicroED and X-ray crystallography.