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. Author manuscript; available in PMC: 2013 Nov 1.
Published in final edited form as: Immunol Rev. 2012 Nov;250(1):102–119. doi: 10.1111/j.1600-065X.2012.01161.x

Fig. 7. Co-evolution of the elongated TCRCβ FG loop as well as CD3γ plus CD3δ genes from a single precursor (CD3P) in Gnathostamata.

Fig. 7

(Adapted from 57). (A) Sequence comparison of the TCRCβ FG loop regions among various species. The position of the F and G strands is defined based on the N15 TCR structure (37, 120). The bracketed region defines the elongated FG loop in mammalian species with well-conserved key residues (L219, W225, and P232) forming the hydrophobic core of the loop. The two cysteines contributing to the intra-chain and an inter-chain disulfide bonds are indicated by the filled and open circles, respectively. The conserved lysine residue in the transmembrane region of Cβ is also highlighted in yellow. (B) Schematic representation of evolutionary relationships between TCRβ and CD3 gene products. Possession of adaptive immunity with recombinatorial-based immune receptors is known for Agnathans and Gnathostomata. Gnathostomata possess a developed adaptive immune system supporting various VDJ recombinations for immunoglobulin (Ig) and TCR rearrangement, whereas Agnathans do not but contain variable lymphocyte receptors (VLR). Distinctions between mammals versus birds, amphibians, reptiles, and bony fish are described and shown schematically.