Some tips to improve the protein solubility
1. Reduce the rate of recombinant protein expression
a. lower culture temperature
b. using a weak promoter
c. using low copy number plasmid expression vector
d. reduce the concentration of inducer.

2. Change the medium :
a. Add factor in the medium to help the protein folding.
b. Add buffer to maintain the pH stable.
c. Add 1% glucose to inhibite lac promoter.
d. Add sorbitol and other factors can stabilize the native structure of the prokaryotic protein.
e. Add ethanol, thiols or disulfides.

3. Co-expression with molecular chaperones or folding enzymes. Commonly used molecular chaperones are: GroES-GroEL，DnaK-DnaJ-GrpE，ClpB. Commonly used folding enzymes are : peptidyl prolyl cis/trans isomerases (PPI's), disulfide oxidoreductase (DsbA) and disulfide isomerase (DsbC), protein disulfide isomerase (PDI).

4. secretion expression. the target protein is secreted into the periplasmic space.

5. The use of specific strains. AD494, which has a mutation in thioredoxin reductase (trxB) . Origami, a double mutant in thioredoxin reductase (trxB) and glutathione reductase (gor).

6. soluble fusion protein partner.

7. Only express a fragment of the target protein. > 70 kDa proteins in E. coli is difficult to express.

8. vitro unfolding, refolding.