Why is it that fusion protein expression vectors contain thr

Why is it that fusion protein expression vectors contain thrombin and factor Xa recognition sequences rather than trypsin or chymotrypsin recognition sequences? Why would it sometimes be desirable to remove the affinity tag from the fusion protein?

Solution

The fusion protein expression vectors contain thrombin and factor Xa recognition sequences because it is more convenient to cleave the protein of interest from its fusion protein. The recognition sequence for trypsin and chymotrypsin are not very specific and hence there is a chance that the protein of interest also will be cleaved during the removal of fusion protein.

It is desirable to remove the affinity tag from the fusion protein because it might interfere with the biological activity of the protein of interest or with its structure.