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CD8a molecule
Crystallographic structure of the CD8 molecule.[1]
Symbol CD8A
Alt. symbols CD8
Entrez 925
HUGO 1706
OMIM 186910
RefSeq NM_001768
UniProt P01732
Other data
Locus Chr. 2 p12
CD8b molecule
Symbol CD8B
Alt. symbols CD8B1
Entrez 926
HUGO 1707
OMIM 186730
RefSeq NM_172099
UniProt P10966
Other data
Locus Chr. 2 p12

CD8 (cluster of differentiation 8) is a transmembrane glycoprotein that serves as a co-receptor for the T cell receptor (TCR). Like the TCR, CD8 binds to a major histocompatibility complex (MHC) molecule, but is specific for the class I MHC protein.[2] There are two isoforms of the protein, alpha and beta, each encoded by a different gene. In humans, both genes are located on chromosome 2 in position 2p12.


Tissue distribution

The CD8 co-receptor is predominantly expressed on the surface of cytotoxic T cells, but can also be found on natural killer cells.


To function, CD8 forms a dimer, consisting of a pair of CD8 chains. The most common form of CD8 is composed of a CD8-α and CD8-β chain, both members of the immunoglobulin superfamily with an immunoglobulin variable (IgV)-like extracellular domain connected to the membrane by a thin stalk, and an intracellular tail. Less-common homodimers of the CD8-α chain are also expressed on some cells. The molecular weight of CD8 is about 13,463.2 Da. Structure of the CD8 molecule was determined by Leahy, D.J., Axel, R., and Hendrickson, W.A. by X-ray Diffraction at a 2.6A resolution.[1] The structure was determined to have an immunoglobulin-like beta-sandwich folding and 114 amino acid residues. 2% of the protein is wound into α-helices and 46% into β-sheets, with the remaining 52% of the molecules remaining in the loop portions.

Schematic representation of the heterodimeric CD8 co-receptor


The extracellular IgV-like domain of CD8-α interacts with to the α3 portion of the Class I MHC molecule.[3] This affinity keeps the T cell receptor of the cytotoxic T cell and the target cell bound closely together during antigen-specific activation. Cytotoxic T cells with CD8 surface protein are called CD8+ T cells. The main recognition site is a flexible loop at the a3 domain of an MHC molecule. This was discovered by doing mutational analyses. The flexible a3 domain is located between residues 223 and 229 in the genome.[1]


  1. ^ a b c PDB 1cd8; Leahy DJ, Axel R, Hendrickson WA (March 1992). "Crystal structure of a soluble form of the human T cell coreceptor CD8 at 2.6 A resolution". Cell 68 (6): 1145–62. doi:10.1016/0092-8674(92)90085-Q. PMID 1547508.  
  2. ^ Gao G, Jakobsen B (2000). "Molecular interactions of coreceptor CD8 and MHC class I: the molecular basis for functional coordination with the T-cell receptor". Immunol Today 21 (12): 630–6. doi:10.1016/S0167-5699(00)01750-3. PMID 11114424.  
  3. ^ Devine L, Sun J, Barr M, Kavathas P (1999). "Orientation of the Ig domains of CD8 alpha beta relative to MHC class I". J Immunol 162 (2): 846–51. PMID 9916707.  

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