The same question is answered in the link:

https://www.researchgate.net/post/How_can_I_express_recombinant_protein_with_molecular_weight_lower_than_10_KDa_in_Ecoli

Hi! protein size wouldn't create any problem for expression in e.coli. You can also try some tagged vectors and later you can cleave and purify your protein. There are many vectors containing GST, or His Tag, go for it

Good Luck.

Typically, an expression tag will provide better expression of small proteins. MBP allows for easy purification and then can be cleaved off. Additionally, if this is a transmembrane peptide or something that may be toxic to cells an MBP or GST tag will enhance solubility and give higher expression. 

DNA 2.0 is a company I have used that will help you create an optimized plasmid construct from expressing your protein.

Hi,

For the greatest chance of success I would use a synthetic gene that is optimised for expression in E. coli (at that size this should be a very reasonable price) and use SUMO as your expression tag (in particular I suggest you try pESUMO-ProKan from LifeSensors http://www.lifesensors.com/expression-systems/e-coli-vectors/sumopro-fusion-vectors/pe-sumopro-kan/p136#.VLAMW0TI8-Q).  SUMO is a wonderful tag to try - it generally increases expression levels AND solubility. You can cleave the SUMO tag off in a wide range of conditions using the SUMO protease (ULP-1 catalytic domain if you want to look up how to make it yourself) which leaves no cloning artefacts on your protein of interest.  It shouldn't be too difficult if you try it this way - although there's always lots of reasons why it may not work depending on the protein you're trying to express.

Best of luck

Andy

Thank you so much for yours answers. To be safe, I will try with GST tag, which is available in my lab. Some reports said that small proteins are easily degraded by proteases when expressed in E. coli. Is it true?

I have produced a couple proteins in the range of 10kD. These were not problematic, but indeed there are reports that say small proteins like this can be degraded quickly by E. coli. I'd use the GST to be safe.