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TAP- and tapasin-dependent HLA-E surface expression correlates with the binding of an MHC class I leader peptide
V M Braud1, D S Allan, D Wilson
1Institute of Molecular Medicine, John Radcliffe Hospital, Oxford, OX3 9DS, UK. vbraud@worf.molbiol.ox.ac.uk
Insights
Surface expression of human leukocyte antigen-E (HLA-E) depends on binding peptides from major histocompatibility complex (MHC) class I molecules. This process involves the transporter associated with antigen processing (TAP) and calreticulin in the endoplasmic reticulum.
Area of Science:
- Immunology
- Molecular Biology
- Cell Biology
Background:
- The human leukocyte antigen-E (HLA-E) is a major histocompatibility complex (MHC) class Ib molecule with largely unknown cellular localization.
- Previous research demonstrated HLA-E's in vitro binding of peptides derived from the signal sequences of human MHC class I molecules.
Purpose of the Study:
- To investigate the cellular localization and regulation of HLA-E expression.
- To elucidate the molecular mechanisms governing HLA-E surface presentation and its dependence on MHC class I molecules.
Main Methods:
- Utilized a novel antibody for characterizing HLA-E expression at the cell surface.
- Investigated HLA-E interactions with endoplasmic reticulum-resident proteins, including the transporter associated with antigen processing (TAP) and calreticulin.
- Assessed the dependency of HLA-E expression on TAP and tapasin, and its dissociation from TAP upon peptide binding.
Main Results:
- Demonstrated cell surface expression of HLA-E, correlating with the presence of MHC class I molecules providing suitable leader sequence peptides.
- Revealed that HLA-E associates with TAP and calreticulin, and its expression is dependent on TAP and tapasin.
- Observed that HLA-E dissociates from TAP following the binding of MHC class I leader sequence peptides.
Conclusions:
- Surface expression of HLA-E is regulated by the binding of specific peptides derived from MHC class I leader sequences.
- The observed correlation between HLA-E and MHC class I surface expression suggests potential functional relevance.
- HLA-E binding peptides, originating from signal sequences, can be processed via the TAP complex for loading onto HLA-E molecules.
Background:
The human major histocompatibility complex (MHC) class lb molecule HLA-E is transcribed in most tissues but little is known about its localisation within the cell. We have recently shown that HLA-E binds signal-sequence-derived peptides from human MHC class I molecules in vitro.
Results:
Using a newly characterised antibody recognising HLA-E, we show that HLA-E is expressed at the cell surface. We demonstrate that HLA-E surface expression is correlated with the presence of MHC class I molecules which provide suitable leader sequence peptides capable of binding to HLA-E. Further studies on the interaction of HLA-E with molecules in the endoplasmic reticulum revealed that HLA-E associates with the transporter associated with antigen processing (TAP) and calreticulin, and that HLA-E expression is TAP-dependent and tapasin-dependent. In addition, HLA-E dissociates from TAP upon binding of MHC class I leader sequence peptides.
Conclusion:
These experiments establish that surface expression of HLA-E is regulated by the binding of a restricted pool of peptides from the leader sequence of MHC class I molecules. The correlation between HLA-E and MHC class I surface expression might be relevant to the function of HLA-E. Our results also show that, although these HLA-E binding peptides are derived from signal sequences, they may be released back into the cytosol and subsequently translocated by the TAP complex and loaded onto HLA-E molecules.