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Updated: Jun 25, 2026

Demonstration of Heterologous Complexes formed by Golgi-Resident Type III Membrane Proteins using Split Luciferase Complementation Assay
Published on: September 10, 2020
The recognition motif of the glycoprotein-folding sensor enzyme UDP-Glc:glycoprotein glucosyltransferase
Kiichiro Totani1, Yoshito Ihara, Takashi Tsujimoto
1RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
Abstract:
The folding of glycoproteins is primarily mediated by a quality control system in the ER, in which UDP-Glc:glycoprotein glucosyltransferase (UGGT) serves as a "folding sensor". In this system, client glycoproteins are delivered to UGGT after the trimming of their innermost glucose residue by glucosidase II, which releases them from the lectin chaperones calnexin (CNX) and calreticulin (CRT). UGGT is inactive against folded proteins, allowing them to proceed to the Golgi apparatus for further processing to complex- or hybrid-type glycoforms. On the other hand, this enzyme efficiently glucosylates incompletely folded glycoproteins to monoglucosylated structures, providing them with an opportunity to interact with CNX/CRT. In order to clarify the mode of this enzyme's substrate recognition, we conducted a structure-activity relationship study using a series of synthetic probes. The inhibitory activities of various glycans suggest that UGGT has a strong affinity for the core pentasaccharide (Man3GlcNAc2) of high-mannose-type glycans. Our comparison of the reactivity of acceptors that have been modified by various aglycons supports the hypothesis that UGGT recognizes the hydrophobic region of client glycoproteins. Moreover, we discovered fluorescently labeled substrates that will be valuable for highly sensitive detection of UGGT activity.
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