Effects of domain composition on catalytic activity of human UDP-glucose:glycoprotein glucosyltransferases

Uridine diphosphate (UDP)-glucose:glycoprotein glucosyltransferase (UGGT) 1 is a soluble protein residing in the endoplasmic reticulum (ER) and partially in ER-Golgi intermediate compartment. Characteristically, it is able to recognize incompletely folded proteins and re-glucosylate their high-mannose-type glycans. By virtue of this, UGGT1 acts as a folding sensor in the glycoprotein quality control system in the ER. On the other hand, human UGGT2 (HUGT2) has been believed to be an inactive homolog of human UGGT1 (HUGT1), whereas our recent study discovered its activity as UGGT. Although the activity of HUGT2 is significantly lower than HUGT1, C-terminal catalytic region, accounting for approximately 20% of the full-length enzyme, shares high amino acid sequence identity (>85%). In this study, we aimed to clarify the contribution of the noncatalytic domains by comparing activities of truncated forms of recombinant HUGT1/HUGT2 and HUGT1/HUGT2 chimeras with full-length enzymes. Our results obtained by using synthetic substrate indicate that the C-terminal catalytic regions of HUGTs are functional as UGGT. While the activity of HUGT1, but not that of HUGT2, was enhanced by the presence of N-terminal domains, activities of catalytic domains are similar between two homologs.