New concepts and aids to facilitate crystallization

•Binding proteins can be employed to assist in crystallization.•Crystallization chaperones revolutionized the structural biology of membrane proteins.•Conformation-sensitive chaperones provide information on specific functional protein states.•Protein engineering enables exploration of novel scaffolds as binding reagents.

Novel tools and technologies are required to obtain structural information of difficult to crystallize complex biological systems such as membrane proteins, multiprotein assemblies, transient conformational states and intrinsically disordered proteins. One promising approach is to select a high affinity and specificity-binding partner (crystallization chaperone), form a complex with the protein of interest and crystallize the complex. Often the chaperone reduces the conformational freedom of the target protein and additionally facilitates the formation of well-ordered crystals. This review provides an update on the recent successes in chaperone-assisted crystallography. We also stress the importance of synergistic approaches involving protein engineering, crystallization chaperones and crystallization additives. Recent examples demonstrate that investment in such approaches can be key to success.