HLA class I histocompatibility antigen, alpha chain E (HLA-E) also known as MHC class I antigen E is a protein that in humans is encoded by the HLA-Egene. The human HLA-E is a non-classical MHC class I molecule that is characterized by a limited polymorphism and a lower cell surface expression than its classical paralogues. The functional homolog in mice is called Qa-1b, officially known as H2-T23.
Like other MHC class I molecules, HLA-E is a heterodimer consisting of an α heavy chain and a light chain (β-2 microglobulin). The heavy chain is approximately 45 kDa and anchored in the membrane. The HLA-E gene contains 8 exons. Exon one encodes the signal peptide, exons 2 and 3 encode the α1 and α2 domains, which both bind the peptide, exon 4 encodes the α3 domain, exon 5 encodes the transmembrane domain, and exons 6 and 7 encode the cytoplasmic tail.
HLA-E has a very specialized role in cell recognition by natural killer cells (NK cells). HLA-E binds a restricted subset of peptides derived from signal peptides of classical MHC class I molecules, namely HLA-A, B, C, G. These peptides are released from the membrane of the endoplasmic reticulum (ER) by the signal peptide peptidase and trimmed by the cytosolic proteasome. Upon transport into the ER lumen by the transporter associated with antigen processing (TAP), these peptides bind to a peptide binding groove on the HLA-E molecule. This allows HLA-E to assemble correctly and to be expressed on the cell surface. NK cells recognize the HLA-E+peptide complex using the heterodimeric inhibitory receptor CD94/NKG2A/B/C. When CD94/NKG2A or CD94/NKG2B is engaged, it produces an inhibitory effect on the cytotoxic activity of the NK cell to prevent cell lysis. However, binding of HLA-E to CD94/NKG2C results in NK cell activation. This interaction has been shown to trigger expansion of NK cell subsets in antiviral responses.
^Mizuno S, Trapani JA, Koller BH, Dupont B, Yang SY (Jun 1988). "Isolation and nucleotide sequence of a cDNA clone encoding a novel HLA class I gene". Journal of Immunology. 140 (11): 4024–30. PMID3131426.
^ abBraud VM, Allan DS, O'Callaghan CA, Söderström K, D'Andrea A, Ogg GS, Lazetic S, Young NT, Bell JI, Phillips JH, Lanier LL, McMichael AJ (Feb 1998). "HLA-E binds to natural killer cell receptors CD94/NKG2A, B and C". Nature. 391 (6669): 795–9. PMID9486650. doi:10.1038/35869.
^Braud V, Jones EY, McMichael A (May 1997). "The human major histocompatibility complex class Ib molecule HLA-E binds signal sequence-derived peptides with primary anchor residues at positions 2 and 9". European Journal of Immunology. 27 (5): 1164–9. PMID9174606. doi:10.1002/eji.1830270517.
^Lemberg MK, Bland FA, Weihofen A, Braud VM, Martoglio B (Dec 2001). "Intramembrane proteolysis of signal peptides: an essential step in the generation of HLA-E epitopes". Journal of Immunology. 167 (11): 6441–6. PMID11714810. doi:10.4049/jimmunol.167.11.6441.
^Bland FA, Lemberg MK, McMichael AJ, Martoglio B, Braud VM (Sep 2003). "Requirement of the proteasome for the trimming of signal peptide-derived epitopes presented by the nonclassical major histocompatibility complex class I molecule HLA-E". The Journal of Biological Chemistry. 278 (36): 33747–52. PMID12821659. doi:10.1074/jbc.M305593200.
^Braud VM, Allan DS, Wilson D, McMichael AJ (Jan 1998). "TAP- and tapasin-dependent HLA-E surface expression correlates with the binding of an MHC class I leader peptide". Current Biology. 8 (1): 1–10. PMID9427624. doi:10.1016/S0960-9822(98)70014-4.
Jensen PE, Sullivan BA, Reed-Loisel LM, Weber DA (Jun 2004). "Qa-1, a nonclassical class I histocompatibility molecule with roles in innate and adaptive immunity". Immunologic Research. 29 (1–3): 81–92. PMID15181272. doi:10.1385/IR:29:1-3:081.