TheIgHLocus of the Channel Catfish,Ictalurus punctatus, Contains Multiple Constant Region Gene Sequences: Different Genes Encode Heavy Chains of Membrane and Secreted IgD

Abstract

The δ-chain of catfish IgD was initially characterized as a unique chimeric molecule containing a rearranged VDJ spliced to Cμ1, seven C domain-encoding exons (δ1–δ7), and a transmembrane tail. The presence of cDNA forms showing splicing of δ7 to an exon encoding a secretory tail was interpreted to indicate that membrane (δm) and secreted (δs) forms were likely expressed from a single gene by alternative RNA processing. Subsequent cloning and sequence analyses have unexpectedly revealed the presence of three δ C region genes, each linked to a μ gene or pseudogene. The first (IGHD1) is located 1.6 kb 3′ of the functional Cμ (IGHM1). The second (IGHD3) is positioned immediately downstream of a pseudo Cμ (IGHM3P), ∼725 kb 5′ of IGHM1. These two δ genes are highly similar in sequence and each contains a tandem duplication of δ2-δ3-δ4. However, IGHD1 has a terminal exon encoding the transmembrane region, whereas IGHD3 has a single terminal exon encoding a secreted tail. The occurrence of IGHD3 immediately downstream of a μ pseudogene indicates that the putative δs product may not be expressed as a chimeric μδ molecule. Western blots and protein sequencing data indicate that an IGHD3-encoded protein is expressed in catfish serum. Thus, catfish δm transcripts appear to originate from IGHD1, whereas δs transcripts originate from IGHD3 rather than, as previously inferred, from a single expressed δ gene. The third δ (IGHD2) is associated with a pseudo Cμ (IGHM2P); its presence is inferred by Southern blot analyses.