FAQ: My fusion protein is insoluble; is there anything I can do to get it expressed as soluble protein?

Expressing at a lower temperature is the first thing to try. One can go as low as 15°C by moving an incubator into the cold room. Of course, the cells grow very slowly at these temperatures, so grow the culture at 37°C and shift to the low temperature when adding IPTG. One also has to increase the time of induction to compensate for the slower growth – a rule of thumb is 2X for every 7°C. Other references for solubility problems include: 

Reviews on methods to make correctly-folded protein in E. coli:
Georgiou, G. et al. (1996) Current Opinion in Biotechnology 7:190–197. Sahdev S. et al. (2008) Mol. Cell Biochem 307:249-64 

Reviews on refolding:
Qoronfleh MW, et al. (2007) Protein Expr Purif  55(2):209-24.
Jungbauer A and Kaar W. (2007) J Biotechnol 128(3):587-96
Jungbauer A, et al. (2004) Curr Opin Biotechnol. 15(5):487-94
Vallejo, LF and Rinas,U (2004) Microb Cell Fact 3(1):11-22