GST contamination of GST fusion protein

[Paul J. Phelan]

To whom it concerns,
This question maybe have been asked before, but I am having trouble 
with separating GST from a preparation of GST-Importin alpha.  I have 
maybe 10-20% GST co-purifying with the fusion protein.  The pI of both 
proteins is the same, so ion exchange chromatography does not separate 
them, and even on a Sephacryl S-100 gel filtration column, the two 
proteins (87 kDa fusion, 27 kDa GST) eluted together.  Does anyone have 
a clever trick to remove GST from a GST fusion prep.?
My next attempt is going to be to try gel filtration again but with 0.3 
M NaCl and maybe some detergent to try to separate the proteins.

Any advice would be greatly appreciated, thanks

Paul Phelan Tufts University Biochemistry Dept. Boston