This looks a bit strange..
If you have a hexamer in the asymmetric unit, in P3, then that means
all symmetry copies lie in the same plane. To generate the Patterson
peak, 2/3,1/3,0 the hexamer must be centred at 1/3,1/3, z
(with symmetry equivalents 0,-1/3,z and -1/3,0,z )
I would expect ypu to have a pseudo higher symmetry SG - does pointless
make any suggestions?
Eleanor
Jürgen Bosch wrote:
Hi Owen,
you should also make the following plot with your data:
y-axis relative intensity of off-origin peak versus x-axis resolution cutoff
used for calculation (30 Å - 4 Å in 2 Å steps).
You can have multiple cases of shifts and I would start with a perfect hexamer
first, take some random monomer and apply a perfect sixfold, move it along the
axis where it should be in your crystal lattice (things get more complicated if
you have a top/down hexamer, so keep it simple). Now if you shift your hexamer
to 2/3,1/3,0 your plot should yield a straight line and be independent of
resolution. Now start rotating the second hexamer relative to the first
clockwise with your sixfold, I would use increments of 3 degrees, which will
result in 19 models, then recalculate the off-origin peak heights and see if
they match up with your data. I should note here, if your real data does not
show a strong drop in peak height of the off-origin peak, then you most likely
don't have a slight rotational translation in your second hexamer.
One other important thing you should look at is the relative orientation of
your sixfold axis, is it truly perfectly aligned with one of the cell axis ? If
not fix this in your model, otherwise your calculations will be of academic
nature. For this particular case the use of GLRF is more helpful than MOLREP
(sorry Garib, but maybe Garib can come up with a solution to zoom into certain
peaks like you can do in GLRF).
When the tilt is fixed you should be able to figure out the rotational
translation in your second hexamer.
Enjoy your puzzle,
Jürgen
P.S. P3 is certain ? Check with pointless or by human brain visual inspection
(HBVI)
On May 15, 2010, at 11:53 PM, Owen Pornillos wrote:
Dear ccp4bb –
I have questions with regards to crystal disorder that gives rise to
translational pseudosymmetry.
We have a rotationally hexameric protein that crystallized in P3, with
one hexamer in the asu. The local 6-fold axis of the hexamer is non-
crystallographic, and is essentially parallel to the crystallographic
3-fold, which gave rise to translational pseudosymmetry. Intensities
for the (h,h+/-3n,l) reflections were on average about 8 times
stronger than the weak reflections, and the native patterson gave an
off-origin peak about 70-80% of origin (depending on the crystal) at
fractional coordinates (2/3,1/3,0). We are hypothesizing that the
break in local 6-fold symmetry is caused by small rigid-body
displacements of each subunit (as opposed to conformational changes in
the protein), and we are trying to estimate the magnitude of the
displacements in the crystal.
To do this, a perfectly symmetric hexamer with the local 6-fold axis
parallel to the crystallographic 3-fold was generated, and then shifts
were introduced to the atomic coordinates. The direction of the shift
was chosen randomly for each atom, and a single magnitude applied to
all atoms, which was then changed incrementally. Structure factors
were calculated from these models, and their pattersons were
examined. The magnitude of the off-origin peak could be reproduced
with an atomic shift of say, 1 Å. Because all of these calculations
were made with synthetic structure factors, this is not necessarily a
reliable estimate. The questions are, how far off are we, and in what
direction (i.e., are these shifts underestimates or overestimates)?
Is there a way to obtain a reliable estimate?
Thanks in advance,
Owen
-
Jürgen Bosch
Johns Hopkins Bloomberg School of Public Health
Department of Biochemistry & Molecular Biology
Johns Hopkins Malaria Research Institute
615 North Wolfe Street, W8708
Baltimore, MD 21205
Phone: +1-410-614-4742
Lab: +1-410-614-4894
Fax: +1-410-955-3655
http://web.mac.com/bosch_lab/
