Dear Stefan,
We recently had a case where a protein mutant formed tetramers with
internal 21212 symmetry. It turned out that these internal symmetry axis
would align perfectly with the crystallographic symmetry axes, but with
some translations. All processing programs including XDS and pointless
would insists that the space group was P21212, as was wildtype. However,
molecular replacement did not work, resulting in large gaps in the
packing, or if one would push very hard, some well defined molecules and
some poorly defined molecules.
You may have the same problem. The tetramers of octamers (I cannot
deduce them from your picture) will have a perfect 4-fold or 422
symmetry, with will leave a very strong imprint on your diffraction
data, like in our case. We solved to problem by going back to P1, and
from the P1 solution deduce the real space group (which turned out to be
P21). In your case I would process the data in P1 and do an MR search
with the tetramers (or octamers) you show in your picture. The number of
octamers to be found should be reasonable.
Best,
Herman
________________________________
From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On
Behalf Of Stefan Gajewski
Sent: Wednesday, December 14, 2011 6:24 PM
To: CCP4BB@JISCMAIL.AC.UK
Subject: Re: [ccp4bb] very strange lattice: high anisotropy, 78%
solvent content and maybe merohedral twinning
Thanks to everyone so far.
Here is a snapshot looking down the long dimension so you see
the (absence) of contacts in a and b.
As I posted earlier, there is no mainchain atom in my protein
unaccounted for in that region of the lattice. the few disordered
elements are far away from that close gap.
And yes, I use NCS restraints.
Stefan
