Crystallographic ab initio protein structure solution below atomic resolution

A general approach to address the 'phase problem' in protein crystallography is described, allowing protein structures to be directly solved from 2 Å resolution diffraction data without using heavy atom doping or relying on a preexisting structure model for molecular replacement. Ab initio macromolecular phasing has been so far limited to small proteins diffracting at atomic resolution (beyond 1.2 Å) unless heavy atoms are present. We describe a general ab initio phasing method for 2 Å data, based on combination of localizing model fragments such as small á-helices with Phaser and density modification with SHELXE. We implemented this approach in the program Arcimboldo to solve a 222-amino-acid structure at 1.95 Å.