Allotype-specific glycosylation and cellular localization of HLA class I proteins

Presentation of self- and foreign peptide antigens by human leukocyte antigen (HLA) complexes at the cell surface is a key process in our immune response. The alpha-chain, the part of the HLA class I complex that contains the peptide binding groove, is one of the most polymorphic proteins in the human proteome. All HLA class I alpha-chains carry a conserved N-glycosylation site, but little is known about its nature and function. Here, we report an in-depth characterization of the N-glycosylation features in HLA class I molecules. In three cell lines we observe that different HLA-A alpha-chains carry similar glycosylation, distinctly different from the HLA-B, HLA-C and HLA-F alpha-chains. HLA-B alpha-chains carry mostly mature glycans, HLA-C and HLA-F alpha-chains carry predominantly high-mannose, whereas HLA-A molecules display the broadest variety of glycan characteristics. We hypothesized that these glycosylation features are directly linked to the cellular localization of the HLA complexes. Analyzing HLA class I complexes from plasma and inner membrane enriched fractions revealed confirmed that most HLA-B complexes can be found in the plasma membrane, most HLA-C and HLA-F molecules reside in the ER and Golgi membrane and HLA-A molecules are more equally distributed over all these cellular compartments. As peptide-binding and specificity is cellular compartment dependent, we corroborate from our data that standard measurements of HLA peptide-antigens from whole cell extracts likely do not exclusively capture the antigen repertoires presented at the cell surface, but also those still within the cell. Our data indicate that standard protein quantification of HLA alpha-chains does not correlate with cell surface expression levels, while analysis of glycopeptides provides allotype and compartment specific quantification.