Cryo-electron crystallography of small and mosaic 2-D crystals: an assessment of a procedure for high-resolution data retrieval

We describe a procedure to obtain structural data of biological macromolecules from small 2-D crystals using cryo-electron crystallography. The procedure has been applied to a membrane-embedded multi-protein complex (photosystem II). This complex, under certain conditions, forms 2-D crystals in its native membrane which are composed of many small mosaic patches and therefore represents an ideal test specimen. By averaging in reciprocal space over more than 50 crystalline patches, it was possible to calculate a projection map using data approaching 1.3 nm resolution while in the Fourier transforms of individual patches only reflections up to the 4th order were readily identifiable. The efficiency of the procedure was evaluated by examining the fidelity of (i) the rotational alignment of patches prior to merging, (ii) the space group assignment, (iii) phase and amplitude extraction, and (iv) the correction of the structure factors for the effects of the contrast transfer function (CTF).