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Dear Prashant, 

I have been working with a protein-protein complex expressed in mammalian cells, and that complex in very poorly soluble. Even with 500mM NaCl in the buffer, I cannot concentrate the complex to above 3 mg/mL. I tried an old school technique and precipitated my protein complex with ammonium sulphate (~80% saturation) on ice. When I recovered my complex, I was able to get almost 9mg/mL without any precipitation at all. The sample still crystallizes, but I believe now that the sulphate ion is shielding/masking part of the protein better than the NaCl did. Perhaps this would be worth a try for you as well?

Another thing you can try with a weakly soluble protein is to set up various ratios between the protein and the reservoir solution in your crystallization drop. 

One point I have not yet seen mentioned is that some proteins are hyper sensitive to changes in temperature. In my opinion, any protein sample should be kept cold, (on ice at the bench) UNLESS you have good reason and biophysical data to show otherwise. If you are concentrating at room temperature, try concentrating at 4C if you can, you might be pleasantly surprised. 

Good Luck!

Bryan Prince

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