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HLA-A

HLA-A belongs to the HLA class I heavy chain paralogues. This class I molecule is a heterodimer consisting of a heavy chain and a light chain (beta-2 microglobulin). The heavy chain is anchored in the membrane. Class I molecules play a central role in the immune system by presenting peptides derived from the endoplasmic reticulum lumen so that they can be recognized by cytotoxic T cells. They are expressed in nearly all cells. The heavy chain is approximately 45 kDa and its gene contains 8 exons. Exon 1 encodes the leader peptide, exons 2 and 3 encode the alpha1 and alpha2 domains, which both bind the peptide, exon 4 encodes the alpha3 domain, exon 5 encodes the transmembrane region, and exons 6 and 7 encode the cytoplasmic tail. Polymorphisms within exon 2 and exon 3 are responsible for the peptide binding specificity of each class one molecule. Typing for these polymorphisms is routinely done for bone marrow and kidney transplantation. More than 6000 HLA-A alleles have been described. The HLA system plays an important role in the occurrence and outcome of infectious diseases, including those caused by the malaria parasite, the human immunodeficiency virus (HIV), and the severe acute respiratory syndrome coronavirus (SARS-CoV). The structural spike and the nucleocapsid proteins of the novel coronavirus SARS-CoV-2, which causes coronavirus disease 2019 (COVID-19), are reported to contain multiple Class I epitopes with predicted HLA restrictions. Individual HLA genetic variation may help explain different immune responses to a virus across a population.
Protein class

Cancer-related genes, Disease related genes, Human disease related genes, Plasma proteins, Potential drug targets, Transporters

Predicted location

Intracellular, Membrane (different isoforms)

Single cell type specificity

Cell type enhanced (Proximal enterocytes, Ductal cells)

Immune cell specificity

Low immune cell specificity

Cell line specificity

Cell line enhanced (A-431, fHDF/TERT166, HBEC3-KT, RPMI-8226, U-266/70)

Interaction

Heterotrimer that consists of an alpha chain HLA-A, a beta chain B2M and a peptide (peptide-HLA-A-B2M) (PubMed:7504010, 7679507, 21943705, 19177349, 24395804, 26758806, 7504010, 7506728, 8805302, 7694806, 7935798, 9177355, 18275829, 22245737, 28250417, 11502003, 8906788, 19542454). Early in biogenesis, HLA-A-B2M dimer interacts with the components of the peptide-loading complex composed of TAPBP, TAP1-TAP2, TAPBPL, PDIA3/ERP57 and CALR (PubMed:21263072). Interacts with TAP1-TAP2 transporter via TAPBP; this interaction is obligatory for the loading of peptide epitopes delivered to the ER by TAP1-TAP2 transporter (PubMed:8805302, 8630735, 21263072). Interacts with TAPBPL; TAPBPL binds peptide-free HLA-A-B2M complexes or those loaded with low affinity peptides, likely facilitating peptide exchange for higher affinity peptides (PubMed:26869717). Only optimally assembled peptide-HLA-B2M trimer translocates to the surface of antigen-presenting cells, where it interacts with TCR and CD8 coreceptor on the surface of T cells. HLA-A (via polymorphic alpha-1 and alpha-2 domains) interacts with antigen-specific TCR (via CDR3 domains) (PubMed:22245737, 12796775, 18275829). One HLA-A molecule (mainly via nonpolymorphic alpha-3 domain) interacts with one CD8A homodimer (via CDR-like loop); this interaction insures peptide-HLA-A-B2M recognition by CD8-positive T cells only (PubMed:9177355, 2784196). Alleles A*23:01; A*24:02 and A*32:01 interact (via Bw4 motif) with KIR3DL1 on NK cells; this interaction is direct. (Microbial infection) Interacts with HHV-8 MIR1 protein. (Microbial infection) Interacts with HTLV-1 accessory protein p12I.

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