Y sil y enh

FIGURE 8-7 Germline organization of human TCR loci. The human TCR p, a, y, and 8 chain loci are shown, as indicated. Exons and introns are not drawn to scale, and nonfunctional pseudogenes are not shown. Each C gene is shown as a single box but is composed of several exons, as illustrated for Cp1. Gene segments are indicated as follows: L, leader (usually called signal sequence); V, variable; D, diversity; J, joining; C, constant; enh, enhancer; sil, silencer (sequences that regulate TCR gene transcription).

contained within the TCR a locus (Fig. 8-7). Each germline TCR locus includes V and J gene segments, the latter being just upstream of C region exons in each locus. In addition, TCR p and TCR 8 loci also have D segments, like the Ig heavy chain locus. At the 5' end of each of the TCR loci, there is a cluster of several V gene segments, arranged in a very similar way to the Ig V gene segments. Upstream of each TCR V gene is an exon that encodes a leader peptide, and upstream of each leader exon is a promoter for each V gene.

In each TCR locus, similar to the Ig loci, C region genes are located just 3' of the J segments. There are two C genes in each of the human TCR p (Cp) and TCR y (Cy) loci and only one C gene each in the TCR a (Ca) and TCR 8 (C8) loci. Each TCR C region gene is composed of four exons encoding the extracellular C region Ig-like domain, a short hinge region, the transmembrane segment, and the cytoplasmic tail. The TCR p and TCR 8 chain loci resemble the Ig heavy chain locus and contain D segments between V genes and J segments. Each human TCR C gene has its own associated 5' cluster of J segments. In the TCR a or y chains (which are analogous to Ig light chains), the V domain is encoded by the V and J exons; and in the TCR p and 8 proteins, the V domain is encoded by the V, D, and J gene segments.

The relationship of the TCR gene segments and the corresponding portions of TCR proteins that they encode is shown in Figure 8-6B. As in Ig molecules, the TCR V

and C domains assume an Ig fold tertiary structure, and thus the TCR is a member of the Ig superfamily of proteins.

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