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Article History

Received: 27 June 2016

Accepted: 8 July 2016

First Online: 18 July 2016

Change Date: 4 August 2016

Change Type: Corrigendum

Change Details: In the version of this article initially published online, the section on 'Cryo-EM sample preparation and data acquisition' in the Online Methods contained errors. The units for defocus values were incorrectly reported as millimeters. The correct unit should be micrometers. Images were recorded automatically instead of manually as initially stated. This section of the Online Methods has been rewritten to improve clarity, and these errors have been corrected for the print, PDF and HTML versions of this article.

Competing interests

: The authors declare no competing financial interests.