Allotype-Specific Glycosylation and Cellular Localization of Human Leukocyte Antigen Class I Proteins

Presentation of antigens
by human leukocyte antigen (HLA) complexes
at the cell surface is a key process in the immune response. The α-chain,
containing the peptide-binding groove, is one of the most polymorphic
proteins in the proteome. All HLA class I α-chains carry a conserved
N-glycosylation site, but little is known about its nature and function.
Here, we report an in-depth characterization of N-glycosylation features
of HLA class I molecules. We observe that different HLA-A α-chains
carry similar glycosylation, distinctly different from the HLA-B,
HLA-C, and HLA-F α-chains. Although HLA-A displays the broadest
variety of glycan characteristics, HLA-B α-chains carry mostly
mature glycans, and HLA-C and HLA-F α-chains carry predominantly
high-mannose glycans. We expected these glycosylation features to
be directly linked to cellular localization of the HLA complexes.
Indeed, analyzing HLA class I complexes from crude plasma and inner
membrane-enriched fractions confirmed that most HLA-B complexes can
be found at the plasma membrane, while most HLA-C and HLA-F molecules
reside in the endoplasmic reticulum and Golgi membrane, and HLA-A
molecules are more equally distributed over these cellular compartments.
This allotype-specific cellular distribution of HLA molecules should
be taken into account when analyzing peptide antigen presentation
by immunopeptidomics.