one pure protein but has two clear bands in SDS-PAGE

[chengzhjun at hotmail.com]

We have got several proteins here encountering the same problems! Our recombinant  protein is on Pet22-b Vector and expressed in Bl21(DE3) E.coli strain. After purification by NTA-Ni column, we see two clear bands in the SDS-PAGE. The two bands have a similar concentration and they are very near to each other. In our protein solutions ,we have already added some protease inhibitor.
Anybody can help me to explain this phenomenon? Any suggestion is highly appreciated.
By the way,how to attach photos? I want to attach my SDS-PAGE result so that you can get a direct understanding of my problems.


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