Addressing the Protein Crystallization Bottleneck By Cocrystallization

The crystallization of macromolecules as an intermediate in the production of protein structures is often the rate-limiting factor. Achievements in the field of protein crystallization such as automation, availability of commercial crystallization kits, the application of developments in nanotechnology, and information mining of databases have significantly improved the throughput of crystallization screening. But this high throughput has not led to high output because of the crystallization bottleneck, and thus, there is a significant disparity between the pace of gene sequencing, protein production, and the determination of the gene product’s atomic structure. Development of complementary approaches that improve the crystallization potential of a protein would form a useful addition to the crystallographer’s toolbox. One such approach is to cocrystallize a target protein with another protein of known structure that specifically recognizes the target so that a more stable protein complex better suited for crystallization is formed. Such proteins are known as cocrystallization proteins (CCPs). In this review, the various mechanisms by which CCPs improve the odds of crystallizing a protein are explained. Additionally, we review some common CCPs such as Fabs (fragment antigen binding) and scFvs (single-chain fragment variable), advances, and current issues in the methodology of their generation and use, and discuss some recently developed novel CCPs such as ankyrin repeat proteins and affibodies.