8536d_ch05_105-136 8/22/02 2: 46 PM Page 109 mac46 mac46: 1256_deh: 8536d: Goldsby et al./Immunology 5e Organization and Expression of Immunoglobulin Genes CHAPTER 5 1 different restriction patterns by rearranged and unre- nucleotide sequence. When the nucleotide sequence was arranged loci. Extensive application of this approach has compared with the known amino acid sequence demonstrated that the Dreyer and Bennett two-gene chain variable region, an unusual discrepancy was observed. model-one gene encoding the variable region and another Although the first 97 amino acids of the A-chain variable re- encoding the constant region--applied to eavy and gion corresponded to the nucleotide codon sequence, the re- light-chain genes. nanning 13 carboxyl-terminal amino acids of the proteins ariable region did not. It turned out that many base pairs away a separate, 39-bp gene segment, called J for joining,en Multigene Organization of Ig Genes coded the remaining 13 amino acids of the A-chain variable region. Thus, a functional A variable-region gene contains As cloning and sequencing of the light- and heavy-chain two coding segments-a 5"'V segment and a 3 segment DNA was accomplished, even greater complexity was re- which are separated by a noncoding DNA sequence in unre- vealed than had been predicted by Dreyer and Bennett. The K arranged germ-line DNA rate multigene families situated on different chromosomes three Va gene segments, four Ja gene segments, and fouro and a light chains and the heavy chains are encoded by ser The A multigene family in the mouse germ line contain (Table 5-1. In germ-line DNA, each of these multigene fam- gene segments(Figure 5-3a). The JA4 is a pseudogene, a de ilies contains several coding sequences, called gene seg- fective gene that is incapable of encoding protein; such ments, separated by noncoding regions. During B-cell genes are indicated with the psi symbol (u). Interestingly, maturation, these gene segments are rearranged and brought J,4's constant region partner, CA4, is a perfectly functional together to form functional immunoglobulin genes. gene. The Va and the three functional Ja gene segments en- code the variable region of the light chain, and each of the Each Multigene Family Has Distinct Features three functional CA gene segments encodes the constant re gion of one of the three A-chain subtypes (Al, A2, and The K and A light-chain families contain V, J, and C gene seg- A3). In humans, the lambda locus is more complex. There ments; the rearranged V] segments encode the variable re- are 31 functional Va gene segments, 4 Jx segments, and gion of the light chains. The heavy-chain family contains V, 7 C, segments. In additional to the functional gene seg- D,J and C gene segments; the rearranged VD) gene seg- ments, the human lambda complex contains many VA, JA, ments encode the variable region of the heavy chain. In each and Cx pseudogenes. 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 K-CHAIN MULTIGENE FAMILY the heavy or light chain through the endoplasmic reticulum. The K-chain multigene family in the mouse contains approx The signal peptide is cleaved from the nascent light and heavy imately 85 Vx gene segments, each with an adjacent leader se chains before assembly of the finished immunoglobulin mol- quence a short distance upstream(ie, on the 5 side). There ecule. Thus, amino acids encoded by this leader sequence do are five Jk gene segments(one of which is a nonfunctional not appear in the im dobulin molecule pseudogene)and a single Ck gene segment( Figure 5-3b).As the A multigene family, the Vx and Jk gene segments en A-CHAIN MULTIGENE FAMILY code the variable region of the K light chain, and the Ck gene The first evidence that the light-chain variable region was ac- segment encodes the constant region. Since there is only one gene segment, there are no subt abtypes of k light cha tually encoded by two gene segments appeared when Tone. Comparison of parts a and b of Figure 5-3 shows that the gawa cloned the germ-line DNA that encodes the variable region of mouse A light chain and determined its complet arrangement of the gene segments is quite different in the K and A gene families. The k-chain multigene family in hu mans,which has an organization similar to that of the mouse, contains approximately 40 k gene segments, 5 Chromosomal locations of segments, and a single CK segment. TABLE 5.1 immunoglobulin genes in human and mouse HEAVY-CHAIN MULTIGENE FAMILY The organization of the immunoglobulin heavy-chain genes CHROMOSOME is similar to, but more complex than, that of the k and Gene Human Mouse A light-chain genes( Figure 5-3c). An additional gene gment encodes part of the heavy-chain variable region λ Light chain The existence of this gene segment was first proposed by Leroy Hood and his colleagues, who compared the K Light chain heavy-chain variable-region amino acid sequence with the Heavy chain VH and JH nucleotide sequences. The VH gene segment was found to encode amino acids l to 94 and the JH gene segmentdifferent restriction patterns by rearranged and unre￾arranged loci. Extensive application of this approach has demonstrated that the Dreyer and Bennett two-gene model—one gene encoding the variable region and another encoding the constant region—applied to both heavy and light-chain genes. Multigene Organization of Ig Genes As cloning and sequencing of the light- and heavy-chain DNA was accomplished, even greater complexity was re￾vealed than had been predicted by Dreyer and Bennett. The and light chains and the heavy chains are encoded by sepa￾rate multigene families situated on different chromosomes (Table 5-1). In germ-line DNA, each of these multigene fam￾ilies contains several coding sequences, called gene seg￾ments, separated by noncoding regions. During B-cell maturation, these gene segments are rearranged and brought together to form functional immunoglobulin genes. Each Multigene Family Has Distinct Features The and light-chain families contain V, J, and C gene seg￾ments; the rearranged VJ segments encode the variable re￾gion of the light chains. The heavy-chain family contains V, D, J, and C gene segments; the rearranged VDJ gene seg￾ments 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 mol￾ecule. Thus, amino acids encoded by this leader sequence do not appear in the immunoglobulin molecule. -CHAIN MULTIGENE FAMILY The first evidence that the light-chain variable region was ac￾tually encoded by two gene segments appeared when Tone￾gawa cloned the germ-line DNA that encodes the variable region of mouse light chain and determined its complete nucleotide sequence. When the nucleotide sequence was compared with the known amino acid sequence of the - chain variable region, an unusual discrepancy was observed. Although the first 97 amino acids of the -chain variable re￾gion corresponded to the nucleotide codon sequence, the re￾maining 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, en￾coded the remaining 13 amino acids of the -chain variable region. Thus, a functional 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 multigene family in the mouse germ line contains three V gene segments, four J gene segments, and four C gene segments (Figure 5-3a). The J4 is a pseudogene, a de￾fective gene that is incapable of encoding protein; such genes are indicated with the psi symbol (). Interestingly, J4’s constant region partner, C4, is a perfectly functional gene. The V and the three functional J gene segments en￾code the variable region of the light chain, and each of the three functional C gene segments encodes the constant re￾gion of one of the three -chain subtypes (1, 2, and 3). In humans, the lambda locus is more complex. There are 31 functional V gene segments, 4 J segments, and 7 C segments. In additional to the functional gene seg￾ments, the human lambda complex contains many V, J, and C pseudogenes. -CHAIN MULTIGENE FAMILY The -chain multigene family in the mouse contains approx￾imately 85 V gene segments, each with an adjacent leader se￾quence a short distance upstream (i.e., on the 5 side). There are five J gene segments (one of which is a nonfunctional pseudogene) and a single C gene segment (Figure 5-3b). As in the multigene family, the V and J gene segments en￾code the variable region of the light chain, and the C gene segment encodes the constant region. Since there is only one C gene segment, there are no subtypes of 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 and gene families. The -chain multigene family in hu￾mans, which has an organization similar to that of the mouse, contains approximately 40 V gene segments, 5 J segments, and a single C segment. HEAVY-CHAIN MULTIGENE FAMILY The organization of the immunoglobulin heavy-chain genes is similar to, but more complex than, that of the and 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 Organization and Expression of Immunoglobulin Genes CHAPTER 5 109 TABLE 5-1 Chromosomal locations of immunoglobulin genes in human and mouse CHROMOSOME Gene Human Mouse Light chain 22 16 Light chain 2 6 Heavy chain 14 12 8536d_ch05_105-136 8/22/02 2:46 PM Page 109 mac46 mac46:1256_des:8536d:Goldsby et al. / Immunology 5e: