T cell receptor
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The T cell receptor or TCR is responsible for recognizing antigen bound to MHC.
It is a heterodimer usually of an alpha and beta chain (though TCRs with gamma-delta chains exist as well). Each chain has three complimentarity determining regions (CDRs) and four surrounding framing regions. The CDR3 is the main CDR responsible for recognizing processed antigen though CDR1 of the alpha chain has been shown to interact with the N-terminal of the peptide while CDR1 of the beta chain interacts with the C-terminal. CDR2 is thought to recognize the MHC. The alpha chain is generated by VJ recombination while the beta chain is generated by VDJ recombination (both involve a somewhat random joining of gene segments). The transmembrane region of the TCR is composed of positively charged amino acids. It is thought that this allows the TCR to associate with coreceptors (like CD3) that have negatively charged transmembrane regions. These coreceptors are vital to propagating the signal from the TCR into the cell (the cytoplasmic tail of the TCR is extremely short, making it unlikely it participates in signalling).
CDR3 is the region at the intersection of the regions (V and J for the alpha chain, V D and J for the beta chain). It is the unique combination of the segments at this region along with palindromic and random nucleotide addition which accounts for the great diversity in specificity of the T cell receptor.
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