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[ccp4bb]: X-ray absoprtion edges used for MAD

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Dear All

I am hoping that somebody amongst you might be able to furnish me with some
information ...

We have recently solved the structure of the PDZ1 domain of Na+/H+ exchanger
regulatory factor
using the dispersive signal from the LIII edge of Mercury (see Webster at
al. (2001) Acta Cryst D57, 
714-716 and our J. Mol. Biol. paper that is currently in press). We were
unable to obtain satisfactory
expression of our protein from selenomethionine auxotrophs and only obtained
a single mercury
derivative in spite of an extensive heavy atom screen from which the SIR
phases were insufficient
to solve the structure.

In the end then, we decided to try a MAD experiment using our lone Mercury
derivative and obtained a
beautiful anomalous signal at three different wavelengths on beamline F2 at
CHESS. An analysis of our
data with SOLVE yielded excellent phases and a model consisting of over 80%
of the protein was built
by ARP/WARP in the first electron density map calculated with the new

I was wondering whether anybody had done a survey of elements other than
Selenium that have been
successfully used for structure determination with MAD, since it seems that
a lot of time can be saved
if even a single, suitable heavy-atom derivative of a protein can be
obtained for such an experiment. I
know that there are plenty of tables of wavelengths and dispersive
differences for different elements, but
I would be very interested to see if anybody had compiled statistics for
which elements had actually
worked for MAD structure determinations. Such a survey might beneficially
bias our choice of which
heavy-atoms are worth screening first, especially if the biological
labelling of proteins is not an option
due to time constraints or technical problems at the level of expression

With thanks, in anticipation of your assistance


Gordon Webster
Division of Experimental Medicine
Beth Israel Deaconess Medical Center
Harvard Institutes of Medicine
Boston USA

email: gwebster@caregroup.harvard.edu