Crystal 'unengineering': reducing the crystallisability of sulfolobus solfataricus Hjc

The protein Hjc from the thermophilic archaeon Sulfolobus solfataricus (Ss) presented many challenges to both structure solution and formation of stable complexes with its substrate, the DNA four-way or Holliday junction. As the challenges were caused by an uncharacteristically high propensity for rapid and promiscuous crystallisation, we investigated the molecular cause of this behaviour, corrected it by mutagenesis, and solved the X-ray crystal structures of the two mutants. An active site mutant SsHjcA32A crystallised in space group I2₃ (a 144.2Å; 68% solvent), and a deletion of a key crystal contact site, SsHjcδ62-63 crystallised in space group P2₁ (a 64.60, b 61.83, _c 55.25Å; β≤95.74°; 28% solvent). Characterisation and comparative analysis of the structures are presented along with discussion of the pitfalls of the use of protein engineering to alter crystallisability while maintaining biological function.