Characterization of four novel ϵ chain mRNA and a comparative analysis of genes for immunoglobulin E in rodents and man

Abstract

The nucleotide sequence of the 3′ region of the E chain gene for human IgE is presented. A comparison of the entire region from 5′ of exon C1 to the M2 exon of the mouse, rat and human ϵ chain genes shows that the overall structure of the ϵ chain gene have changed only minimally during the 60-70 million years of evolutionary separation between rodents and man. We have previously shown that a number of rearrangements larger than 10 bp have relatively recently occurred in the C4/M1 intron of the rat or the mouse E chain genes. A majority of these rearrangements were found within or in close proximity to repetitive sequences of Z-DNA-forming potential (CA dinucleotide repeats). The C4/M1 intron has evolved very rapidly, to such an extent that no apparent homology can be detected between rodents and man. Only remnants of the repetitive sequences are present in man, supporting the theory that repetitive sequences having Z-DNA-forming properties may play a role in the evolution of the eucaryote genome by promoting recombinations, leading to a rapid evolutionary drift of sequences in close proximity to these repeats. We report here the characterization of the membrane domains of human IgE and four novel mRNA transcribed from the human E chain locus. The primary structures have been determined by polymerase chain reaction cloning and nucleotide sequence analysis. All five mRNA contain the C3 domain and the membrane exon 2 (M2). Due to frame shifts caused by novel splice sites or novel splice-site combinations, the proteins encoded by three out of these four novel mRNA differ in their carboxy-terminal end from the classical secreted or membrane-bound immunoglobulins. Northern blot analysis shows significant levels of at least three out of these four novel mRNA in an IgE-producing human cell line. One of the mRNA encodes a transmembrane-like structure which has characters in common with the transmembrane region of the CD3 components of the T cell receptor complex (CD3 γ,δ and ϵ). This indicates that IgE-producing B cells possibly have two separate signal-transducing systems. A comparison of the classical membrane anchoring domain of the human ϵ chain with a panel of immunoglobulin membrane domains from fish to higher mammals is presented. A tyrosine and a glutamine residue is found to be conserved between all cytoplasmic domains of all post-switch immunoglobulin classes indicating a functional conservation of these amino acid residues. Suggested functions for these residues are their participation in signal transduction or in linkage to the cytoskeleton for internalization of antibody-antigen complexes.