A role for UDP-glucose glycoprotein glucosyltransferase in expression and quality control of MHC class I molecules

Abstract

UDP-glucose:glycoprotein glucosyltransferase 1 (UGT1) serves as a folding sensor in the calnexin/calreticulin glycoprotein quality control cycle. UGT1 recognizes disordered or hydrophobic patches near asparagine-linked nonglucosylated glycans in partially misfolded glycoproteins and reglucosylates them, returning folding intermediates to the cycle. In this study, we examine the contribution of the UGT1-regulated quality control mechanism to MHC I antigen presentation. Using UGT1-deficient mouse embryonic fibroblasts reconstituted or not with UGT1, we show that, although formation of the peptide loading complex is unaffected by the absence of UGT1, the surface level of MHC class I molecules is reduced, MHC class I maturation and assembly are delayed, and peptide selection is impaired. Most strikingly, we show using purified soluble components that UGT1 preferentially recognizes and reglucosylates MHC class I molecules associated with a suboptimal peptide. Our data suggest that, in addition to the extensively studied tapasin-mediated quality control mechanism, UGT1 adds a new level of control in the MHC class I antigen presentation pathway.antigen processing | chaperones M ajor histocompatibility complex (MHC) class I-mediated antigen presentation is critical for adaptive immune responses to intracellular pathogens. MHC class I assembly and peptide loading constitute a specialized case of glycoprotein folding, using general chaperones and enzymes involved in the endoplasmic reticulum (ER) quality control machinery as well as MHC class I-specific cofactors. Like other glycoproteins, MHC class I heavy chains (HCs) are modified during translocation by asparagine (N)-linked glycosylation with the glycan Glc 3 Man 9 GlcNAc 2 . After the first two glucose residues are trimmed by glucosidases I (GlsI) and II (GlsII), the monoglucosylated (Glc 1 Man 9 GlcNAc 2 ) glycan interacts with the lectin chaperone calnexin (CNX) for oxidative folding (1). The maturing HCs are partially stabilized by β 2 -microglobulin (β 2 m) association, and the HC/β 2 m dimers are recruited by the second lectin chaperone calreticulin (CRT) into the peptide loading complex (PLC) via their monoglucosylated glycans (2, 3). The PLC is composed of the HC/β 2 m dimer, CRT, the transporter associated with antigen processing (TAP), and a disulfide-linked tapasin-ERp57 conjugate (reviewed in ref. 4). It facilitates peptide loading onto MHC class I molecules and functions in quality control by retaining empty and suboptimally loaded MHC class I molecules in the ER.The glycosylation status of ER MHC class I molecules is highly regulated. Monoglucosylation is not detectable in free human MHC class I HCs (5), whereas PLC-associated HCs are virtually all monoglucosylated (3, 5). Inhibiting deglucosylation of the monoglucosylated glycan prolongs the interaction of MHC class I molecules with the PLC (3, 6), and, in vitro, GlsII can trim the monoglucosylated glycans of free HCs but not PLC-associated HCs (3). This suggests that only when MHC class I molecules dis...

The murine cytomegalovirus immunoevasin gp40 binds MHC class I molecules to retain them in the early secretory pathway

“…Class I molecules loaded with suboptimal peptide ligands are retained in the early secretory pathway of wild-type cells by two different mechanisms (Springer, 2015): first, by the class-I-specific chaperone tapasin (Paulsson et al, 2002), and second, by the lectin calreticulin (Howe et al, 2009), probably in concert with the UDPglucose:glycoprotein glucosyltransferase (UGT1) (Zhang et al, 2011). Both tapasin and calreticulin are members of the class I peptide-loading complex (PLC).…”

Peptide-independent stabilization of MHC class I molecules breaches cellular quality control*

“…MHC-I molecules are again monitored for high-affinity peptide occupancy by a system that is related to the standard glycoprotein quality control (Ellgaard and Helenius, 2001). It recognizes suboptimally loaded MHC-I, most likely through UDP-glucose:glycoprotein glucosyltransferase (UGT1), retains them in the ER-Golgi intermediate compartment (ERGIC) and the cis-Golgi, prevents them from becoming endoglycosidase H resistant, and eventually returns them to the ER with the help of calreticulin (Hsu et al, 1991;Paulsson et al, 2006;Garstka et al, 2007;Purcell and Elliott, 2008;Wearsch and Cresswell, 2008;Howe et al, 2009;Zhang et al, 2011). This second system is responsible for the intracellular retention of tapasin-dependent MHC-I allotypes in tapasin-deficient cells (Peh et al, 1998;Garstka et al, 2011).…”

Single-particle electron microscopy structure of UDP-glucose:glycoprotein glucosyltransferase suggests a selectivity mechanism for misfolded proteins

“…In addition to its role as a sensor of protein folding, UGGT contributes to the maturation of the major histocompatibility complex of class 1 molecules (MHC1), ensuring that a highaffinity peptide has been loaded onto the MHC1 heavy chains (28,29). The MHC1 is composed of MHC-encoded heavy chains, which are glycosylated, and a ␤ 2 -microglobulin domain.…”