Tackling the crystallographic structure determination of the COP9 signalosome

The COP9 signalosome (CSN) is an essential multi‐protein complex in eukaryotes. CSN is a master regulator of intracellular protein degradation, controlling the vast family of cullin–RING ubiquitin (E3) ligases (CRLs). Important in many cellular processes, CSN has prominent roles in DNA repair, cell‐cycle control and differentiation. The recent crystal structure of human CSN provides insight into its exquisite regulation and functionality [Lingaraju et al. (2014), Nature (London) , 512 , 161–165]. Structure determination was complicated by low‐resolution diffraction from crystals affected by twinning and rotational pseudo‐symmetry. Crystal instability and non‐isomorphism strongly influenced by flash‐cooling, radiation damage and difficulty in obtaining heavy‐atom derivatives, were overcome. Many different subunits of the same fold class were distinguished at low resolution aided by combinatorial selenomethionine labelling. As an example of how challenging projects can be approached, the structure determination of CSN is described as it unfolded using cluster‐compound MIRAS phasing, MR‐SAD with electron‐density models and cross‐crystal averaging exploiting non‐isomorphism among unit‐cell variants of the same crystal form.