The extracellular IgV-like domain of<unk> -α interacts with the α3 portion of the Class I MHC molecule. 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<unk>  surface protein are called<unk> + T cells. The main recognition site is a flexible loop at the α3 domain of an MHC molecule. This was discovered by doing mutational analyses. The flexible α3 domain is located between residues 223 and 229 in the genome. In addition to aiding with cytotoxic T cell antigen interactions the<unk>  co-receptor also plays a role in T cell signaling. The cytoplasmic tails of the<unk>  co-receptor interact with Lck (lymphocyte-specific protein tyrosine kinase). Once the T cell receptor binds its specific antigen Lck phosphorylates the cytoplasmic CD3 and ζ-chains of the TCR complex which initiates a cascade of phosphorylation eventually leading to activation of transcription factors like NFAT, NF-κB, and AP-1 which affect the expression of certain genes.
CD8
CD4
CD74
CD137
CD22
CD23
CDV3 
CD44