Deglucosylation of N-linked glycans is an important step in the dissociation of calreticulin–class I–TAP complexes

Recent evidence indicates that newly synthesized major histocompatibility complex (MHC) class I proteins interact with calnexin, a transmembrane endoplasmic reticulum protein specific for certain glycoproteins bearing monoglucosylated glycans. Here, we studied the association of newly synthesized class I proteins with calreticulin, a soluble calnexin-related ER protein, in murine T cells. We found that, unlike calnexin–class I interactions, calreticulin assembly with class I proteins was markedly decreased in the absence of β(2) microglobulin expression and that calreticulin associated with a subset of class I glycoforms distinct from those assembled with calnexin but similar to those bound to TAP (transporter associated with antigen processing) proteins. Finally, these studies show that deglucosylation of N-linked glycans is important for dissociation of class I proteins from both calreticulin and TAP and that the vast majority of newly synthesized class I proteins associated with calreticulin are simultaneously assembled with TAP. The data demonstrate that calnexin and calreticulin chaperones assemble with distinct MHC class I assembly intermediates in the ER and show that glycan processing is functionally coupled to release of MHC class I proteins from peptide transport molecules.