A hetero-micro-seeding strategy for readily crystallizing closely related protein variants

A generic cross-seeding approach to protein crystallization

Obtaining diffraction-quality crystals is often the rate-limiting step during structure determination of biological macromolecules by X-ray crystallography. To address this problem, we investigated a cross-seeding approach with a mixture integrating a heterogeneous set of protein crystal fragments to be used as generic seeds. The fragments are nanometre-sized templates chosen to promote crystal nucleation of protein samples unrelated to the proteins forming the seeds. An atypical crystal form of the human serine hydrolase retinoblastoma binding protein 9 was obtained by adding the mixture to the protein sample before performing standard crystallization assays. The structure was solved by X-ray crystallography at 1.4 Å resolution. Follow-up experiments showed that crystal fragments of α-amylase were critical components in this particular result. The limitations and future applications of our experimental developments are discussed.

ROY Reclaims Its Crown: New Ways To Increase Polymorphic Diversity

Chemical compounds that exist in multiple crystalline forms are said to exhibit polymorphism. Polymorphs have the same composition, but their structures and properties can vary markedly. In many fields, conditions for crystallizing compounds of interest are screened exhaustively to generate as many polymorphs as possible, from which the most advantageous form can be selected. We report new ways to search for polymorphs and increase polymorphic diversity, based on crystallization induced by suitably designed mixed-crystal seeds. The potential of the strategy has been demonstrated by using it to produce new polymorphs of the benchmark compound ROY as single crystals structurally characterized by X-ray diffraction. This allows ROY to reclaim its crown as the most polymorphic compound in the Cambridge Structural Database. More generally, the methods promise to become valuable tools for polymorphic screening in all fields where crystalline solids are used.