Aglycosylated and phosphatidylinositol-anchored MHC class I molecules are associated with calnexin. Evidence implicating the class I-connecting peptide segment in calnexin association.
The endoplasmic reticulum resident protein calnexin interacts with several glycoproteins including class I MHC molecules. Calnexin is thought to retain free class I heavy chains and/or promote their folding and assembly with beta 2-microglobulin and peptide ligand. Whereas with other glycoproteins, Asn-linked glycans seem to be involved in calnexin association, with class I molecules the transmembrane region has been implicated. To critically define the structures on class I molecules that determine their interaction with calnexin, we have studied carbohydrate-deficient and transmembrane-variant class I molecules. Carbohydrate-deficient class I molecules were found to accumulate intracellularly in an open, non-beta 2-microglobulin-associated conformation. However, open as well as conformed class I molecules showed significant calnexin association whether they were aglycosylated or fully glycosylated. Thus, carbohydrate moieties may be necessary for efficient class I folding, but are not required for calnexin association. Calnexin was also found associated with a soluble class I molecule that has a truncated transmembrane segment, demonstrating that membrane attachment of class I is not required for interaction with calnexin. Finally, two isoforms of the class Ib molecule Q7b were compared. Unexpectedly, the glycosylphosphatidylinositol-anchored Q7b isoform was found associated with calnexin, whereas the soluble Q7b isoform was not calnexin associated. These comparisons of Q7b isoforms implicate the class I-connecting peptide segment and not the transmembrane region as a site of interaction with calnexin.
Carreno
B M
BM
Department of Genetics, Washington University School of Medicine, St. Louis, MO 63110, USA.
Schreiber
K L
KL
McKean
D J
DJ
Stroynowski
I
I
Hansen
T H
TH
D000595:Amino Acid Sequence; D000818:Animals; D015153:Blotting, Western; D002135:Calcium-Binding Proteins / Q000032:analysis* / Q000378:metabolism; D037281:Calnexin; D006023:Glycoproteins / Q000032:analysis*; D006031:Glycosylation; D015395:Histocompatibility Antigens Class I / Q000737:chemistry* / Q000378:metabolism; D007739:L Cells; D051379:Mice; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D010716:Phosphatidylinositols / Q000032:analysis*; D011233:Precipitin Tests; D011487:Protein Conformation
D002135:Calcium-Binding Proteins; D006023:Glycoproteins; D015395:Histocompatibility Antigens Class I; D010716:Phosphatidylinositols; D037281:Calnexin