KMD Bioscience Co., Ltd.
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2019-04-24 Hits(1006)
1. Q: How to avoid inclusion bodies and improve soluble expression?
a. Proteins with high hydrophobicity or transmembrane domains. You should add fusion tags or add heat shock chaperones. You should induce for a shorter time at low temperature or change to poor media. Generate truncated forms of protein or use membrane rich strains.
b. Incorrect disulfide bond formation. You should add fusion partners, including thioredoxin, DsbA, DsbC. Clone in a vector containing secretion signal peptide to cell periplasm. Use gamiB (DE3) strains with oxidative cytoplasmic environment. Lower inducer concentration and induction temperature.
c. Incorrect folding. You should use a fusion partner. Co-express with molecular chaperones. Use strains with cold-adapted chaperones. Supplement media with chemical chaperones and cofactors. Reduce the inducer concentration and add fresh media. Induce for a shorter time at low temperature.
2. Q: Why is the actual band size different from the predicted?
a. Post-translational modification. Phosphorylation, glycosylation, etc which increases the size of the protein.
b. Post-translational cleavage. Many proteins are synthesized as pro-proteins, and then cleaved to give the active form.
c. Splice variants. Alternative splicing may create different sized proteins from the same gene.
d. Relative charge. The composition of amino acids have different relative charge which will affect the electrophoretic mobility.
e. Multimers such as dimerisation of a protein. This is usually prevented in reducing conditions, although strong interactions can result in the appearance of higher bands.
f. Protein structure such as disulfide bond, protein secondary structure or protein 3D structure formation.
g. Hydrophobic proteins, such as transmembrane proteins, may have difficulties in migrating into the gel, and thus resulting in different multi-banded patterns.
3．Q: How to express a protein with bioactivity? Why is the protein inactive?
For gaining a protein with bioactivity, you should choose a right expression system, a suitable expression vector, an appropriate purification method and a validation experiment. Otherwise, you can check the problems below:
a. Low solubility of the protein. You should fuse desired protein to a fusion partners and lower temperature.
b. Lack of essential post translational modification. You should change another expression system.
c. Incomplete folding. You should use a fusion partner and use strains with cold-adapted chaperones. Co-express with molecular chaperones at lower temperature. Monitor disulfide bond formation and allow further folding in vitro.
d. Mutations in cDNA. You should sequence plasmid before and after induction or use a recA− strain to ensure plasmid stability. Transform E. coli before each expression round.
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