Hi Hua,
Does your complex elute as a single peak from a gel filtration column or show a monomodal dynamic light scattering profile? If not, it may be worth looking into the method of complex formation. You can co-express the subunits, or purify each subunits separately, and then mix them at a particular molar ratio to form the complex. Purification with columns such as gel filtration after complex formation may be necessary for crystallization. Sometimes mixing the subunits at a high salt concentration, and then gradually lowering the salt concentration via dialysis may result in a more stable complex. If the subunits are small (e.g. 100-200 amino acid residues), refolding may be an option. Fusing the subunits and expressing them as a single polypeptide is another possible option.
I hope this helps.
Sincerely,
Wataru
On 2011/02/27, at 11:13, Hua Yuan wrote:
> Dear CCP4 community members,
>
> I've been trying to crystallize a protein complex that's very sensitive to ionic strength, i.e., lower salt (~0.3M) will cause precipitation of the complex but higher salt (~0.5 M) breaks the complex apart. The interaction that holds the complex is probably mainly ionic type.
> The crystals I got so far has only one component of the complex from which all the crystallization conditions have high salt such as 2M Ammonium Sulfate in them. Besides repeatly screening many crystallization conditions, I was wondering whether is any way to work around this problem. Your suggestions would be greatly appreciated!
>
> Thanks,
>
> Hua
>
>
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