Dear Jan, There are a few things a would do in this case. The first is to check the processing to make sure the space group is really C2 and, although unlikely, not some other space group.
The second thing would be to try to place the first component. From your email it is not clear to me whether or not you were able to place the first component after the second component had been placed. In your case, I would give both components to phaser and ask phaser to first place component 2 and then component 1. It might be that the correct solution gets rejected because of clashes, so I would also try to trim the first component, or to increase the number of allowed clashes in Phaser. Although you expect two copies of your heterodimer, you may have a crystal with a high solvent content and only one dimer in the asymmetric unit. In this case the crystal packing should make sense i.e. continuous crystal contacts in all three dimensions. Best, Herman -----Ursprüngliche Nachricht----- Von: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] Im Auftrag von Jan Félix Gesendet: Freitag, 18. Oktober 2013 13:17 An: CCP4BB@JISCMAIL.AC.UK Betreff: [ccp4bb] Molecular Replacement using low sequence identity templates Dear all, I have a question regarding Molecular Replacement using low sequence identity templates. I have a 2.7 Angstrom dataset of a heterodimeric protein-protein complex (space group C 1 2 1, no twinning detected using xtriage). For the first component homologs are available, but for the other the best found template only has 20 % sequence similarity. Strangely, I cannot place the first component directly, but the second component can be placed (after trimming the template with chainsaw) using phaser with a TFZ score of 12 and a LLG of about 1200. Although at least 2 NCS copies of the heterodimer are expected based on the unit cell parameters, only 1 copy of the second component gets placed. If I try to place the first component based on the .sol file of the first MR solution, the TFZ score for the second placement is only about 3.5, but if I then try to place this second MR solution (2 components) as a whole I get a RFZ of 8, TFZ of 16 and a LLG of about 1000. However, none of the MR solutions I obtained seems to refine in PHENIX. Using autobuild does not lower the R/Rfree values which seem to get stuck at an R/Rfree of 0.40/0.47. I have tried trimming off loops, simulated annealing, DEN-refinement, morphing and MR-rosetta, but nothing seems to improve the model.. Also, in every MR solution only half of the asymmetric unit seems to be filled, but phaser fails to place more units..As I am seriously starting to doubt the actual content of the crystals, I tried Nearest Cell to search for similar space group, but without any hits. So here is my question. Is it possible to get TFZ/LLG values this high in C 1 2 1 with a completely incorrect model by chance, or can I assume that this MR solution points out that what I think is in the crystal is actually there? And secondly, as I am a bit stuck here, are there any new strategies I can try to tackle this problem? Off course, experimental phasing is an option, but the crystals grew slowly over e few months and I only had 1 drop with 1 crystal, so reproducing the crystals might be though.. Thanks for any tips and best regards, Jan
