Major Histocompatibility Complex Class I Molecules Expressed with Monoglucosylated N-Linked Glycans Bind Calreticulin Independently of Their Assembly Status

The assembly of major histocompatibility complex (MHC) class I molecules with peptides in the endoplasmic reticulum (ER) is a critical step in the presentation of viral antigens to CD8+ T cells. This process is subject to quality control restrictions that prevent free class I heavy chains (HCs) and peptide-free HC-β2-microglobulin (β2m) dimers from exiting the ER. The lectin-like chaperone calreticulin associates with HC-β2m heterodimers prior to peptide binding, but its precise role in regulating the subsequent events of peptide association and ER to Golgi transport remains undefined. In vitro analysis of the assembly process has been limited by the specificity of calreticulin for monoglucosylated N-linked glycans, which are transient biosynthetic intermediates. To address this problem, we developed a novel expression system using Saccharomyces cerevisiae glycosylation mutants to produce class I HC bearing N-linked oligosaccharides with the specific structure Glc1Man9GlcNAc2. The monoglucosylated glycan proved to be both necessary and sufficient for in vitro binding of calreticulin to MHC class I molecules. Calreticulin bound as efficiently to peptide-loaded MHC class I complexes as it did to folding intermediates created in vitro, namely free class I HC and empty HC-β2m heterodimers. Thus, calreticulin is unable to discriminate between native and non-native MHC class I conformations and therefore unlikely to play a role in the recognition and release of peptide-loaded complexes from the ER. Furthermore, the recombinant expression system developed in this study can be used to produce a broad range of calreticulin substrates to elucidate its general mechanism of activity in vitro.