Yes it happen in fusion proteins the MBP domain increase the expression of our gene of interest. There are some publication available where CBD increased the thermostability and expression of some enzymes. 

You can look for these publications. 

Good luck

Ashok Kumar, please re-read my question with care and answer more specifically if possible. The talk is solely about increasing expression levels by the presence of N-terminal MBP. So the question is why. What can be the mechanism of this?

Do you mean that you have tried other fusions (GST, etc) and  MBP most enhances the expression level of your protein?

Fusions in general can enhance the expression level of constructs with 5' mRNA structure or non-optimal codon usage at the start. 

Thanks for a good correction, Maria. I don't compare now with other fusions but rather compare with the target being expressed itself. I guess the picture can be different when GST-, MBP- His-tags are fused and this is case sensitive. But many publications report that once their target is connected to the N-terminal MBP they see robust expression. So, the question is why does it have this effect even when the very beginning of the 5' region is the same (differences in initiation efficacy does not matter I mean).

For those who are following, an interesting paper on the effect of MBP on expression of targets - PMID:21904041 (http://scripts.iucr.org/cgi-bin/paper?S1744309111022159).

Hi Nikolai! A very interesting issue. I think that the tag itself mainly plays role in enhancement of the correct folding of the fused protein (there are several hypotheses regarding this co-chaperon function of MBP-tag) thus it increases the pool of properly folded, soluble protein. Actually, the publication you have just mentioned shows that upon cleavage of the tag the solubility and stability of the fused protein is drastically reduced, once again pointing toward an important role of MBP in promoting the "compactness" or the fold of the proteins. It somehow resembles ubiquitin that has been shown to promote protein expression and solubility when fused to heterologously expressed proteins in E. coli (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2286746/).

So I guess it can be difficult to really assess if the tag itself promotes higher expression levels (although a luciferase assays or some sort of other reporter gene could be used) or we just can detect higher amounts of protein before it undergoes a turnover in E. coli because MBP stabilizes its conformation.

Hi Marcin! Nice to hear from you. I completely agree, especially with the second part - when we simply see the band in lysate it does not necessarily mean only improved expression itself (i.e. improved transcription/translation) but can rather indicate less  issues with degradation and/or survival of already expressed target, on which MBP can have its own positive/protecting effect if present in the N-terminus. For me it's very interesting that MBP has the most pronounced effect among many other fusion partners.