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Proc Natl Acad Sci U S A August 8, 2006; 103 (32): 12087-92.
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Identification of IgF, a hinge-region-containing Ig class, and IgD in Xenopus tropicalis.

Zhao Y , Pan-Hammarström Q , Yu S , Wertz N , Zhang X , Li N , Butler JE , Hammarström L .

Only three Ig isotypes, IgM, IgX, and IgY, were previously known in amphibians. Here, we describe a heavy-chain isotype in Xenopus tropicalis, IgF (encoded by C(phi)), with only two constant region domains. IgF is similar to amphibian IgY in sequence, but the gene contains a hinge exon, making it the earliest example, in evolution, of an Ig isotype with a separately encoded genetic hinge. We also characterized a gene for the heavy chain of IgD, located immediately 3'' of C(mu), that shares features with the C(delta) gene in fish and mammals. The latter gene contains eight constant-region-encoding exons and, unlike the chimeric splicing of muC(H)1 onto the IgD heavy chain in teleost fish, it is expressed as a unique IgD heavy chain. The IgH locus of X. tropicalis shows a 5'' V(H)-D(H)-J(H)-C(mu)-C(delta)-C(chi)-C(upsilon)-C(phi) 3'' organization, suggesting that the mammalian and amphibian Ig heavy-chain loci share a common ancestor.

PubMed ID: 16877547
PMC ID: PMC1567701
Article link: Proc Natl Acad Sci U S A

Species referenced: Xenopus tropicalis
Genes referenced: hjv igh ighv4-31 ighx snrpb tpm1 XB5734592 [provisional] XB5798883 [provisional]

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References [+] :
Amemiya, Nucleotide sequence of a cDNA encoding a third distinct Xenopus immunoglobulin heavy chain isotype. 1989, Pubmed, Xenbase