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IUCrJ. 2019 Jan 24;6(Pt 2):197-205. doi: 10.1107/S2052252518017621. eCollection 2019 Mar 1.

Homochiral and racemic MicroED structures of a peptide repeat from the ice-nucleation protein InaZ.

Author information

1
Department of Chemistry and Biochemistry, UCLA-DOE Institute for Genomics and Proteomics, University of California Los Angeles, Los Angeles, CA 90095, USA.
2
Department of Biological Chemistry, UCLA-DOE Institute for Genomics and Proteomics, University of California Los Angeles, Los Angeles, CA 90095, USA.
3
Howard Hughes Medical Institute, Departments of Physiology and Biological Chemistry, University of California Los Angeles, Los Angeles, CA 90095, USA.

Abstract

The ice-nucleation protein InaZ from Pseudomonas syringae contains a large number of degenerate repeats that span more than a quarter of its sequence and include the segment GSTSTA. Ab initio structures of this repeat segment, resolved to 1.1 Å by microfocus X-ray crystallography and to 0.9 Å by the cryo-EM method MicroED, were determined from both racemic and homochiral crystals. The benefits of racemic protein crystals for structure determination by MicroED were evaluated and it was confirmed that the phase restriction introduced by crystal centrosymmetry increases the number of successful trials during the ab initio phasing of the electron diffraction data. Both homochiral and racemic GSTSTA form amyloid-like protofibrils with labile, corrugated antiparallel β-sheets that mate face to back. The racemic GSTSTA protofibril represents a new class of amyloid assembly in which all-left-handed sheets mate with their all-right-handed counterparts. This determination of racemic amyloid assemblies by MicroED reveals complex amyloid architectures and illustrates the racemic advantage in macromolecular crystallography, now with submicrometre-sized crystals.

KEYWORDS:

amyloid; co-crystals; electron crystallography; electron diffraction; ice nucleation; intermolecular interactions; racemic; structural biology

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