Hi! I have been using the Champion pET-SUMO cloning system from Invitrogen to purify proteins in E. coli. I have noticed now with all of my proteins that during induction, there is overexpression not just of my fusion protein but also of a low molecular weight band around ~15 kDa. This protein always copurifies with my fusion protein during Ni-NTA chromatography. I have previously done mass spec on one of my expressions and found that this low molecular weight band is SUMO. I have performed sequencing on my BL21 clones and found that my genes of interest are in frame with clean transition from the SUMO sequence to the ATG codon that begins the sequence for my proteins. I even sent the sequencing files to Invitrogen and they saw no clear reason for any premature truncation of my protein. I have seen this double production of fusion protein and SUMO while doing a literature search but also plenty of cases where this hasn't happened.

Has anyone else had this issue before or know any possible solutions?

Thanks!!