Each Multigene Family Has Distinct Features

The k and X light-chain families contain V, J, and C gene segments; the rearranged VJ segments encode the variable region of the light chains. The heavy-chain family contains V, D, J, and C gene segments; the rearranged VDJ gene segments encode the variable region of the heavy chain. In each gene family, C gene segments encode the constant regions. Each V gene segment is preceded at its 5' end by a small exon that encodes a short signal or leader (L) peptide that guides the heavy or light chain through the endoplasmic reticulum. The signal peptide is cleaved from the nascent light and heavy chains before assembly of the finished immunoglobulin molecule. Thus, amino acids encoded by this leader sequence do not appear in the immunoglobulin molecule.

X-CHAIN MULTIGENE FAMILY

The first evidence that the light-chain variable region was actually encoded by two gene segments appeared when Tone-gawa cloned the germ-line DNA that encodes the variable region of mouse X light chain and determined its complete

Chromosomal locations of

TABLE 5-1

immunoglobulin genes in

human and mouse

CHROMOSOME

Gene

Human

Mouse

X Light chain

22

16

k Light chain

2

6

Heavy chain

14

12

nucleotide sequence. When the nucleotide sequence was compared with the known amino acid sequence of the X-chain variable region, an unusual discrepancy was observed. Although the first 97 amino acids of the X-chain variable region corresponded to the nucleotide codon sequence, the remaining 13 carboxyl-terminal amino acids of the protein's variable region did not. It turned out that many base pairs away a separate, 39-bp gene segment, called J for joining, encoded the remaining 13 amino acids of the X-chain variable region. Thus, a functional X variable-region gene contains two coding segments—a 5' V segment and a 3' J segment— which are separated by a noncoding DNA sequence in unre-arranged germ-line DNA.

The X multigene family in the mouse germ line contains three VX gene segments, four JX gene segments, and four CX gene segments (Figure 5-3a). The JX4 is a pseudogene, a defective gene that is incapable of encoding protein; such genes are indicated with the psi symbol Interestingly, JX4's constant region partner, CX4, is a perfectly functional gene. The VX and the three functional JX gene segments encode the variable region of the light chain, and each of the three functional CX gene segments encodes the constant region of one of the three X-chain subtypes (X1, X2, and X3). In humans, the lambda locus is more complex. There are 31 functional VX gene segments, 4 JX segments, and 7 CX segments. In additional to the functional gene segments, the human lambda complex contains many VX, JX, and CX pseudogenes.

k-CHAIN MULTIGENE FAMILY

The K-chain multigene family in the mouse contains approximately 85 VK gene segments, each with an adjacent leader sequence a short distance upstream (i.e., on the 5' side). There are five JK gene segments (one of which is a nonfunctional pseudogene) and a single CK gene segment (Figure 5-3b). As in the X multigene family, the VK and JK gene segments encode the variable region of the k light chain, and the CK gene segment encodes the constant region. Since there is only one CK gene segment, there are no subtypes of k light chains. Comparison of parts a and b of Figure 5-3 shows that the arrangement of the gene segments is quite different in the k and X gene families. The K-chain multigene family in humans, which has an organization similar to that of the mouse, contains approximately 40 VK gene segments, 5 JK segments, and a single CK segment.

HEAVY-CHAIN MULTIGENE FAMILY

The organization of the immunoglobulin heavy-chain genes is similar to, but more complex than, that of the k and X light-chain genes (Figure 5-3c). An additional gene segment encodes part of the heavy-chain variable region. The existence of this gene segment was first proposed by Leroy Hood and his colleagues, who compared the heavy-chain variable-region amino acid sequence with the VH and JH nucleotide sequences. The VH gene segment was found to encode amino acids 1 to 94 and the JH gene segment

VISUALIZING CONCEPTS

VISUALIZING CONCEPTS

LVX2 Jx2 CX2 JX4 CX4 L VX1 -

kb kb kb kb

Jx3 Cx3 JX1 Cxi

L VK1 L Vk2 L VKn

23 kb

LVh1 LVHn DH1 dH13 JH1 JH4 ^ C5

55 kb

34 21 15 14 12

kb kb kb kb kb

FIGURE 5-3

Organization of immunoglobulin germ-line gene segments in the mouse: (a) \ light chain, (b) k light chain, and (c) heavy chain. The \ and k light chains are encoded by V, J, and C gene segments. The heavy chain is encoded by V, D, J, and C gene segments. The distances in kilobases (kb) separating the various gene segments in mouse germ-line DNA are shown below each chain diagram.

was found to encode amino acids 98 to 113; however, neither of these gene segments carried the information to encode amino acids 95 to 97. When the nucleotide sequence was determined for a rearranged myeloma DNA and compared with the germ-line DNA sequence, an additional nucleotide sequence was observed between the VH and JH gene segments. This nucleotide sequence corresponded to amino acids 95 to 97 of the heavy chain.

From these results, Hood and his colleagues proposed that a third germ-line gene segment must join with the VH and JH gene segments to encode the entire variable region of the heavy chain. This gene segment, which encoded amino acids within the third complementarity-determining region (CDR3), was designated D for diversity, because of its contribution to the generation of antibody diversity. Tonegawa and his colleagues located the D gene segments within mouse germ-line DNA with a cDNA probe complementary to the D region, which hybridized with a stretch of DNA lying between the VH and JH gene segments.

The heavy-chain multigene family on human chromosome 14 has been shown by direct sequencing of DNA to contain 51 VH gene segments located upstream from a cluster of 27 functional DH gene segments. As with the light-chain genes, each VH gene segment is preceded by a leader sequence a short distance upstream. Downstream from the Dh gene segments are six functional JH gene segments, followed by a series of CH gene segments. Each CH gene segment encodes the constant region of an immunoglobulin heavy-chain isotype. The CH gene segments consist of coding exons and noncoding introns. Each exon encodes a separate domain of the heavy-chain constant region. A similar heavy-chain gene organization is found in the mouse.

The conservation of important biological effector functions of the antibody molecule is maintained by the limited number of heavy-chain constant-region genes. In humans and mice, the CH gene segments are arranged sequentially in the order C^, C8, C7, C£, Ca (see Figure 5-3c). This sequential arrangement is no accident; it is generally related to the sequential expression of the immunoglobulin classes in the course of B-cell development and the initial IgM response of a B cell to its first encounter with an antigen.

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  • Arthur Banks
    How many j segments of heavy chain multigene?
    8 years ago

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