CD8

CD8a molecule Crystallographic structure of the CD8 molecule.[1] Identifiers Symbol CD8A Alt. symbols CD8 Entrez 925 HUGO 1706 OMIM 186910 RefSeq NM_001768 UniProt P01732 Other data Locus Chr. 2 p12

CD8b molecule Identifiers 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.

Structure

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

Function

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]

References

External links

• http://www.tcells.org/scientific/CD8/ T-cell Group - Cardiff University]
• Mouse CD Antigen Chart
• Human CD Antigen Chart

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