I express and purify my protein by Ni-NTA beads in 50mM Tris 8.0 and 150mM NaCl and get enough protein. But when I want to use ion exchange column to purify it, proteins get precipitating even only 3-fold dilution using buffer A. If I use less dilution fold, proteins couldn't bind to column, and if I use low salt buffer to purify protein in Ni-NTA step, it will also get precipitating. SO HOW CAN I treat my protein so that I can purify it with ion exchange column? THANK YOU!