[ccp4bb]: Skin formation in crystallization trials:

[mmeiyapp]

I am working on a protein with three domains. I have purified three different constructs – only one domain or two domains or all the three domains. All these proteins, I expressed as MBP fusion and purified well and runs as monomer in gel-filtration. Surprising all these proteins form skin in almost all conditions (PEG or salt) of Nextal and Hampton crystallization screen. These domains have some cysteines,and ddition of reducing agent (DTT or TCEP) during crystallization trials did not help and I still get skins. The proteins run as monomer in gel-filtration column even with out any reducing agents. Searching in Web did not give me clear answer. Any body has any such experience/suggestions.   Thanks muthu