The N-terminal stem region of bovine and human beta1,4-galactosyltransferase I increases the in vitro folding efficiency of their catalytic domain from inclusion bodies

Many recombinant proteins overexpressed in Escherichia coli are generally misfolded, which then aggregate and accumulate as inclusion bodies. The catalytic domain (CD) of bovine and human beta1,4-galactosyltransferase (beta4Gal-T), expressed in E. coli, it also accumulates as inclusion bodies. We studied the effect of the fusion of the stem region (SR), as an N-terminal extension of the catalytic domain, on the in vitro folding efficiencies of the inclusion bodies. The stem region fused to the catalytic domain (SRCD) increases the folding efficiency of recombinant protein with native fold compared to the protein that contains only the CD. During in vitro folding, also promotes considerably the solubility of the misfolded proteins, which do not bind to UDP-agarose columns and exhibit no galactosyltransferase activity. In contrast, the misfolded proteins that consist of only the CD are insoluble and precipitate out of solution. It is concluded that a protein domain that is produced in a soluble form does not guarantee the presence of the protein molecules in a properly folded and active form. The stem domain has a positive effect on the in vitro folding efficiency of the catalytic domain of both human and bovine beta4Gal-T1, suggesting that the stem region acts as a chaperone during protein folding. Furthermore, investigation of the folding conditions of the sulphonated inclusion bodies resulted in identifying a condition in which the presence of PEG-4000 and L-arginine, compared to their absence, increased the yields of native CD and SRCD 7- and 3-fold, respectively.