The quality control of MHC class I peptide loading

Abstract

The assembly of major histocompatibility complex (MHC) class I molecules is one of the more widely studied examples of protein folding in the endoplasmic reticulum (ER). It is also one of the most unusual cases of glycoprotein quality control involving the thiol oxidoreductase ERp57 and the lectin-like chaperones calnexin and calreticulin. The multistep assembly of MHC class I heavy chain with beta(2)-microglobulin and peptide is facilitated by these ER-resident proteins and further tailored by the involvement of a peptide transporter, aminopeptidases, and the chaperone-like molecule tapasin. Here we summarize recent progress in understanding the roles of these general and class I-specific ER proteins in facilitating the optimal assembly of MHC class I molecules with high affinity peptides for antigen presentation.

Figure 1. ER quality control and MHC class I assembly

(a) The ER glycoprotein quality control cycle. The composition of the N-linked glycan transferred to newly synthesized proteins is Glc₃Man₉GlcNAc₂. The first two terminal glucose residues are rapidly trimmed by the sequential action of Glucosidases I (Gls1) and II (GlsII) allowing the glycoprotein to enter the CNX/CRT cycle. The lectin-like chaperones specifically bind monoglucosylated glycans and promote protein folding in conjunction with ERp57. Following the release of the substrate from CNX, CRT and ERp57, the glycan is completely deglucosylated by Glucosidase II. If the protein has acquired its native state, it is removed from the cycle and exported to the Golgi. If not, the unfolded protein is recognized and reglucosylated by UGT, allowing it to re-engage with the chaperones. The cycle may continue until the protein has acquired its native structure. (b) MHC class I assembly in the ER. The folding of the MHC class I HC and formation of disulfide bonds in the α2 and α3 domains is assisted by CNX and ERp57. The HC then assembles with β₂m to generate an empty heterodimer that is highly unstable for most MHC class I alleles. The HC/β₂m dimer is stabilized by association with tapasin, ERp57, CRT, and TAP in the PLC, where it awaits peptide binding. TAP transports peptides generated from cytosolic proteins by the proteasome into the ER lumen where they are further trimmed by ERAAP/ERAP1 (mice and humans) and ERAP2 (humans only). Once high affinity peptides of the appropriate length are generated, they are loaded onto empty MHC class I molecule by the PLC. Peptide binding induces dissociation of MHC class I from the PLC and it subsequently traffics to the cell surface.

Figure 2. Components and interactions that define the MHC class I PLC

(a) A schematic of the PLC showing its components and defined intermolecular interactions. Only one unit of each protein is illustrated for simplicity not to indicate the stoichiometry. Some studies have suggested that the ratio of TAP1/2:tapasin:HC is 1:4:4 [4,54] whereas another has proposed that it is 1:2:1 [34]. (b) Overview of the MHC class I peptide binding groove and residues that are important for assembly or PLC interactions. The MHC class I HC is composed of three domains, the first two of which, α1, α2, comprise the peptide binding domain and include a disulfide bond. The third is an Ig-like domain and its disulfide bond is expected to be stable. Shown is the structure of HLA-B*4402 without the α3 domain or bound peptide. The peptide binding platform consists of two antiparallel α-helices overlaying an eight strand β-sheet structure. In this view, the peptide N-terminus binds at the bottom and the C-terminus binds at the top of the groove. Residues that are considered to be important for MHC class I assembly and/or PLC association are highlighted. Blue: Asn86 is the site of N-linked glycosylation and CRT binding. Orange: the α2–1 segment is encoded by residues 138–149. Red: the mutations in the MHC class I HC that have been shown to affect PLC association via tapasin are between 128–136. Green: a disulfide bond is present in the native structure of the α2 between Cys101 and Cys164.