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 > >
