Crystallization of RNA-protein complexes: From synthesis and purification of individual components to crystals

A broad range of biological processes relies on complexes between RNA and proteins. Crystallization of RNA-protein complexes can yield invaluable information on structural organizations of key elements of cellular machinery. However, crystallization of RNA-protein complexes is often challenging and requires special approaches. Here we review the purification of RNA, RNA-binding proteins, and the formation and crystallization of RNA-protein complexes, using the crystallization of the P3 RNA domain of ribonuclease MRP, a multicomponent ribonucleoprotein complex involved in the metabolism of various RNA molecules, as an example. The RNA-protein complex was formed using gel-purified RNA, produced by run-off transcription with T7 RNA polymerase in vitro, and proteins that were overexpressed in Escherichia coli and purified to be RNase-free. The complex was crystallized using a sitting drop setup; initial screening for suitable crystallization conditions was performed using a sparse matrix approach.