Characterization and molecular cloning of the mRNA for the heavy (epsilon) chain of rat immunoglobulin E.

We report a study of the mRNA for the heavy (epsilon) chain of rat IgE. Cytoplasmic RNA was prepared from the two rat immunocytomas IR2 and IR162 and fractionated by sucrose gradient centrifugation. An enriched fraction containing approximately 5% mRNA for the epsilon chain was obtained in this way. When translated in vitro, it produced a 59,000-dalton polypeptide, which in the presence of a membrane fraction yielded a 90,000-dalton polypeptide, presumably through posttranslational modification. Both polypeptides were precipitated by rabbit antisera that were monospecific for rat epsilon chains. The epsilon chain mRNA was estimated to be approximately 2200 nucleotides long and constitutes a minute fraction in the total mRNA both in the IR2 and the IR162 tumors, unlike the mRNA for light chains. Double-stranded cDNA copies prepared frm the RNA fraction, which was enriched for epsilon chain mRNA, were inserted into the Pst I cleavage site of the pBR322 vector. Twenty clones with inserts exceeding 1000 base pairs were used for selection of mRNA from the IR2 tumor. By in vitro translation of the selected mRNA, one clone was identified that yielded a polypeptide with the same size as the unprocessed epsilon chain. The nucleotide sequence was determined for part of the inserted cDNA in this candidate clone and was found to be homologous to a sequence in the constant region (C) of the human epsilon chain. In this communication we report a sequence from the C epsilon 3 domain of the rat IgE. When compared to the corresponding sequence of human IgE, 55% of the amino acids in the rat sequence were found to be conserved.