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Cryo-EM structure of haemoglobin at 3.2 A determined with the Volta phase plate

By Maryam Khoshouei, Mazdak Radjainia, Wolfgang Baumeister, Radostin Danev

Posted 15 Nov 2016
bioRxiv DOI: 10.1101/087841 (published DOI: 10.1038/ncomms16099)

With the advent of direct electron detectors, the perspectives of cryo-electron microscopy (cryo-EM) have changed in a profound way (ref. 1). These cameras are superior to previous detectors in coping with the intrinsically low contrast of radiation-sensitive organic materials embedded in amorphous ice, and so they have enabled the structure determination of several macromolecular assemblies to atomic or near-atomic resolution. According to one theoretical estimation, a few thousand images should suffice for calculating the structure of proteins as small as 17 kDa at 3 A resolution (ref. 2). In practice, however, we are still far away from this theoretical ideal. Thus far, protein complexes that have been successfully reconstructed to high-resolution by single particle analysis (SPA) have molecular weights of ~100 kDa or larger (ref. 3). Here, we report the use of Volta phase plate in determining the structure of human haemoglobin (64 kDa) at 3.2 A. Our results demonstrate that this method can be applied to complexes that are significantly smaller than those previously studied by conventional defocus-based approaches. Cryo-EM is now close to becoming a fast and cost-effective alternative to crystallography for high-resolution protein structure determination.

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