Yo Thierry:
The periplasmic domain of the aspartate receptor, in the absence of
ligand, 1lih, is a dimer, but crystallizes as a monomer in the sense
that there is one monomer per asymmetric unit. There is a disulphide
bond between two Cys36 that maintains it as a dimer (and indeed
reduction of this bond inhibits crystallization). Each of two ligand
binding sites spans both monomers. So based on that, the biologically
relevant form is definitely a dimer, so you can't conclude otherwise
based on the fact that it crystallizes as one monomer per asymmetric
unit. Now if it were to crystallize in a space group lacking a
crystallographic 2-fold coincident with the natural dimer axis, that
might be a different story.
When you add aspartate, it crystallizes as a dimer with only one of
two potential binding sites occupied by the ligand.
Bill
William G. Scott
Contact info:
http://chemistry.ucsc.edu/~wgscott/
On Dec 1, 2008, at 2:47 PM, Fischmann, Thierry wrote:
Dear fellow crystallographers,
This is a question which is not CCP4-related.
Is anybody aware of a protein which is known to be a dimer in solution
(say by SEC), and yet crystallizes as a monomer? Wouldn't the high
concentration in the crystallization drop further favor dimerization?
In other words, if a protein crystallizes as a monomer, can I conclude
that it does not form biologically relevant dimers in solution?
Thank you in advance for your replies.
Thierry
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